XXII. STARS VARIABLE IN LONG PERIODS…

XXII. STARS VARIABLE IN LONG PERIODS

“Long periods” of variability range from 120 to 610 days. None more protracted have been definitely ascertained, and those that are shorter belong, with rare exceptions, to stars differently characterised. The distribution of the periods we are at present concerned with is exhibited graphically in Fig. 31. They number 208, and are taken from Chandler’s Third Catalogue, only three, which exceed 500 days, being for convenience omitted. Periods of several years have besides been ascribed to a few stars, but on insufficient grounds. No true conformity to them is maintained. Phases that are unusually slow are also extremely uncertain in development. So far, the 610-day cycle of S Cassiopeiæ is the longest that can be depended upon to recur. A cursory reference to our diagram will show how largely, among long periods, those between 280 and 300 days preponderate. Accidents of discovery connected with the length of the year cannot well have produced this preference, which seems to be genuine, and not merely apparent. On the other hand, the indentations of the curve are assuredly casual, and will be smoothed down with the multiplication of objects.

The typical long-period variable is Mira Ceti. It was the first detected; it rises to the brightest maxima; it presents the most vivid and distinctive spectrum. More than 300 of its cycles have been watched, more or less attentively; yet familiarity has not diminished wonder at the “Wonderful” star.[673] Its modes of procedure are as much an enigma to the spectroscopists of Lick and Potsdam as they were to Fabricius and Holwarda. An instructive comment upon them is the omission, from Chandler’s Third Catalogue, of the modifying terms appended to the mean period of 332 days in his Second. It amounts to an abandonment of the attempt to predict, with even approximate accuracy, the capricious changes of the Frisian pastor’s Stella insolita. Argelander’s laborious efforts for their regulation have thus proved futile. He considered an oscillation extending over 80 years, and comprising 88 periods, to be fairly well established, and found indications of another of 160 years;[674] but their supposed effects have ceased to be apparent. Guthnick’s “long inequality,” covering 200 cycles, will doubtless prove equally illusory. No method is indeed securely traceable in the accelerations or retardations of the maxima, and they digress to the extent of fully two months. Long and short periods can neither be perceived to alternate nor to occur in series; still opposite deviations balance each other; there is no progressive alteration in the length of the cycle.

The highest maximum and the lowest minimum recorded for Mira were both observed by Sir William Herschel. He found the variable nearly equal to Aldebaran on 6th November 1779,[675] while four years later it was invisible with a telescope showing stars of the tenth magnitude. Of late it has not been known to descend below 9·5, and it sometimes stops short at 8·0 magnitude. Its greatest brightness is even more inconstant. No more than 5·6 magnitude was attained in November 1868,[676] or one-fortieth the lustre of the phase viewed by the Bath organist, and maxima higher than the third magnitude are uncommon. The course of change likewise fluctuates, but in general the rise is considerably more rapid than the decline, and the high-level status is maintained for about two months, the low-level for at least three, without striking alterations. Yet change is always in progress. The light-curve has no flat stretches.[677] No connection is apparent between the acuteness of the light crises in this star and the times of their occurrence.[678] They do not tend to become abortive when hurried, nor is delay accompanied by intensification. Argelander entirely failed to correlate irregularities of period with discrepancies in the amplitude of change. As the fruit of tercentennial experience it has, however, been learned that long-period variables are no transitory phenomena. Mira, at any rate, exhibits no symptoms of decadence since the maximum which surprised Fabricius in August 1596.

Its spectrum gives evidence of powerful disturbance, but none of duplicity. Motion-shifts depending upon orbital revolution are imperceptible. The periodicity of the star must be explained otherwise than by attributing to it a binary character. The task of doing so is indeed most arduous. Once in eleven months the brightness augments some hundreds of times, and concomitant spectral modifications afford assurance that these annual outbursts are accompanied by atmospheric ignition. What occasions them? We are ignorant; yet the issue may be narrowed by the following consideration. If external action of any kind were concerned in their production we should expect the incandescence to be coronal or chromospheric—to affect primarily the outer layers of the gaseous envelope. But in fact the innermost strata are those set aglow, while the overlying vapours remain comparatively cool. The masking of a bright hydrogen line by calcium absorption places the subsistence of this relation beyond question. So far, then, the evidence favours the view that variability of the Mira type arises spontaneously, rather than through outside influence.

On the 13th of December 1885 Mr. J. E. Gore was struck with an unfamiliar reddish star of the sixth magnitude in the Club of Orion. No map included it, and until it reappeared a year later after an interval of quasi-extinction, there was no telling whether it should be reckoned as a Nova or a variable. Dr. Copeland recorded for it “a very beautiful banded spectrum of the third type, seven dark bands being readily distinguished with the prism.” The intervening spaces appeared “full of bright lines, especially in the green and blue.” Two of these were certainly emitted by hydrogen, and others probably by helium, since a vivid D_{3} was observed by Von Konkoly ten days later, the adjacent sodium pair being, as usual, dark. Fig. 32 exhibits the light-curve of U Orionis during the maximum half of its period, as delineated by M. Porro at Turin 1889–90. Its form is by no means invariable. In general, the brightest phase is reached much more abruptly than it was in 1890. The main rise, however, is always prompt, and the decline gradual, although the minima appear to be well defined. They have not, indeed, owing to their faintness, been much observed. The widest amplitude of the star’s change is from 5·3 to about 12·5 magnitude; but these limits are seldom attained. Nor is the assigned period of 375 days conformed to with any exactitude. As in the case of Mira, prediction has to be qualified with a large allowance for unexplained disturbance.

At high maxima χ Cygni radiates about 6500 times more powerfully than at low minima. The star, that is to say, has a range of fully nine and a half magnitudes, from the fourth to near the fourteenth. But in some of its cycles it fails to ascend beyond 6·5 magnitude; for it resembles the other stars of this class in having no fixed measure of change.[679] Its period is now 406 days; it has lengthened, on an average, by a quarter of an hour at each recurrence since Kirch, in July 1686, missed from its wonted place the star located by Bayer in collo Cygni;[680] nor is there yet any sure sign of a compensatory reversal. The nature of the secular perturbation thus betrayed can scarcely be imagined. Argelander noticed besides deviations from the mean period up to forty days, and sought, with imperfect success, to analyse and regularise the inequalities upon which they depended.[681] The increase of light in this variable occupies 171 days, or considerably less than half the period. Its most brilliant phases are brief, while fainter maxima are sometimes prolonged for a couple of months. In 1847 the star remained visible to the naked eye during 97 days, although the usual time of “lucidity” is, by Argelander’s estimate, only 52 days.[682] The scarlet blaze of its light is often very striking.

R Hydræ is an accelerating variable. In 1708 the interval from one maximum to the next was 500 days; it had shortened to 437 in 1870, and to 425 in 1891. The highest maxima are of 3·5 magnitude, the lowest more than six times less bright. The minima, on the other hand, occur with fair uniformity at 9·7 magnitude. Strongly red in all its phases, R Hydræ displays a gorgeous colonnaded spectrum lit up with bright hydrogen lines.

The variability of L_{2} Puppis was discovered by Gould at Cordoba in 1872. The range in magnitude—3·5 to 6·3—is moderate, the period—137 days—comparatively short. Wide departures from it, however, are not infrequent, and it is almost equally divided between the ebb and the flow of luminosity.[683] The colour of this star suggests a conflagration, and its spectrum resembles and is no less effective than that of Mira.[684] An exceptionally large proper motion, for a member of its class, has been determined for it by Professor Porter of Cincinnati.

An analogous object is met with in W Puppis. Here the rise occupies 62 days, the decline only 58, the visual limits of variation being the eighth and eleventh,[685] the photographic, the ninth and twelfth magnitudes. The discrepancy is naturally accounted for by the non-actinic quality of light conspicuously red to the eye. The detection of bright hydrogen lines in a third-type spectrum gave Mrs. Fleming in 1895[686] the clue to the character of this star. The fluctuations inferred to take place from that unfailing symptom were looked for and quickly found. The light-curve of W Puppis is very regular, and takes a much sharper bend at minimum than at maximum.

In S Ursæ Majoris we meet a much older acquaintance. Its periodicity, discovered by Pogson in 1853, was established by a record of its magnitude made by Lalande in 1790. It is of a highly perturbed nature. The maximum brightness varies between 6·7 and 8·2 magnitude; the minima are uncertain to the extent of perhaps three magnitudes, some unusually faint at 13·3 magnitude having been watched by Baxendell. The actual length of the cycle is about 226 days; it is modified by a recurring inequality with a range of nearly eight days, but much more extensively by irregular deviations. These seem to be connected with two curious inflections of the light-curve. About six weeks before maximum the rise is arrested, sometimes for a few days, sometimes for as many weeks. A corresponding stay in the decrease of light usually precedes each minimum.[687] Upon the duration of these halts evidently depends the retarded or hurried accomplishment of the phases. Noteworthy besides is the occasional equalisation of the times occupied in waxing and waning. This is apparently a consequence of the partial abolition in certain cycles of the pause before minimum. That of 1875 was distinguished by a steady maximum lasting from 23rd February to 13th April, and followed by a decrease quicker than the preceding increase.[688] Similarly, the late Sir Cuthbert Peek’s diagram for 1894 (copied in Fig. 33) shows a flat maximum antecedent to a precipitate decline. Again, four years later, the light during two months scarcely varied from the eighth magnitude;[689] and it may be remarked that, as in the sun, long maxima are low maxima. The mean light-curve of S Ursæ, from observations made at Harvard College, 1889–99, is depicted in Fig. 34. Only the general course of change can be followed by its means; the effects of temporary obstructions or disturbances are necessarily eliminated.

Deep red, and often hazy when faint, this star has been seen nearly white at maximum—a not uncommon kind of colour-fluctuation. It gives a poorly-developed spectrum of the third type.

T Ursæ Majoris has a total range of nearly six magnitudes, and a period of 302 days; but its variations make no approach to uniformity. The maximum brightness is largely uncertain; the minima are sometimes, though rarely, as low as 13·5 magnitude. The luminous tide flows, at certain epochs, with extreme rapidity. Between 5th January and 22nd February 1901, for instance, the star increased from 12·4 to 7·7 magnitude. In forty-six days it acquired a seventy-six-fold brilliancy, and the augmentation was accompanied by a blanching of its rays. Of their dull ruddy hue scarcely a tinge survived at full light. Fig. 35 reproduces the mean curve drawn at Harvard, which is, of course, much more symmetrical than any of the individual curves serving as its basis. The maxima and the minima appear from it to be about equally sharp.

The curve of T Cassiopeiæ, on the other hand, cannot be smoothed into shapeliness. It is represented in Fig. 36. The secondary maximum occasioning the hump on the upward branch is never absent, and protracts the cycle to 445 days, considerably more than half of which (240 days) are occupied by the abnormally impeded phase of increase. The period is affected by a compensatory inequality.

The variability of R Leonis was detected by Koch in 1782; yet six score years of scrutiny have only sufficed to render more manifest the almost hopeless intricacy of the laws to which it is subject. Since 1890 the maxima have been persistently accelerated, computations, founded on a nominal period of 312 days, being left in the lurch in November 1896 by forty-three days. That there is a large periodic inequality admits of no doubt; “but the observations of the last few years,” Dr. Chandler remarked in 1896, “show that it is complicated with other unknown terms,” the neglect of which, pending the development of their nature, seemed to him safer than the attempt to use them for purposes of prediction in ignorance of their value. That is to say, the phases can be registered as they occur, but defy accurate anticipation. The glowing colour and brilliant spectrum of the star make it an object of singular beauty and interest. Its total range is from 5·2 to 10 magnitude, but the oscillations are often of less amplitude.

The variability of V Delphini was discovered by Mrs. Fleming in 1891 by the shining of bright hydrogen lines amid the flutings of its spectrum.[690] It is of enormous extent. Between maximum and minimum-light there is a difference of close upon ten magnitudes. Observed as of 7·5 magnitude on 1st October 1899 with the forty-inch Yerkes refractor, the star had on the ensuing 20th July sunk to invisibility, and must therefore have been below the seventeenth magnitude.[691] The period is 540 days.

A strange anomaly in the light-change of R Lyncis was placed on record at Sir Cuthbert Peek’s observatory in 1898.[692] From a smouldering minimum the star had risen by 2nd March to 10·6 magnitude; when, suddenly reversing its course, it dropped in eighteen days to the thirteenth magnitude, but finally resumed the interrupted process of brightening, and mounted at the customary rate to a maximum of 7·5 magnitude on 11th August. Such apparent caprices constitute indeed a baffling enigma, but should, for that reason, be the more steadily kept in view in dealing with the general question of stellar variability. The high and low phases of R Lyncis are alike definitely marked. The period assigned to them is 380 days.

The variation of R Cygni exceeds eight magnitudes. In rising from one extreme to the opposite it gains a 2500-fold increase of light. The maxima, however, as usual in this class of variables, are very unequal, some being seven times more brilliant than others. Mr. Espin believed in 1888 that a regular alternation of high and low phases might be counted upon;[693] but their subsequent disordered succession belied the inference. The star is among those in which dimness is occasionally attended by a curious diffuseness of aspect. Thus in February and March 1894, having previously sunk out of sight with a 6⅓-inch refractor, it came again into view, at the Rousdon Observatory, in the shape of a small bluish nebula, resembling a faint comet.[694] This object, which had contracted on 24th March into a needle-point of light of the twelfth magnitude, resumed its nebular appearance in February 1895 and March 1896. The mean period of R Cygni is 426 days; but the interval between the maxima of November 1890 and February 1892 amounted to 457.[695] No more than 150 days are, as a rule, spent in the rise.

The unique form of light-curve represented in Fig. 37 is assigned by Professor Pickering to T Andromedæ. Derived from photographic data, it awaits visual confirmation, yet can scarcely be widely erroneous.[696] The uniform progression it depicts need not, indeed, be accepted as a rigid reality; it must be encroached upon by sundry kinds of disturbance, and the maxima, however brief, cannot be instantaneous; at every full tide there is an interval of “slack water.” Nevertheless, if even the mean curve prove to be linear, the circumstance will be of great interest, and the star, meanwhile, merits close attention. Owing to its redness the photographic curve is transposed downward on the scale to the extent of one and a half magnitudes,[697] so that the variable shows four times brighter to the eye than to the sensitive plate. Hence changes of colour, should they at any time supervene, would necessarily produce large distortions in the automatically registered course of light-fluctuation. The systematic comparison of visual and photographic determinations of magnitude might, indeed, be used as an effectual means of testing the permanence in hue of long-period variables. While it remained constant, the curves of light-change, obtained retinally and chemically, should flow parallel to one another; they would merge together if the star blanched, and diverge still farther if it reddened. The variability of T Andromedæ was discovered by Dr. Anderson of Edinburgh in 1893. The period seems to have suddenly shortened from 281 to 265 days in 1895.

Variables of the fourth spectral type are mostly crimson-tinted, and have protracted periods. Indeed, these two characteristics show some kind of mutual dependence,[698] Chandler’s rule, “the redder the star the longer the period,” being, on the whole, conformed to. Apposite examples are furnished by S Cephei, U and V Cygni. The period of S Cephei averages 484 days, but is subject to an alternate lengthening and shortening.[699] More than half of it, or about 257 days, is occupied in the ascent from minimum to maximum, and this exceptional arrangement is consistently maintained. The light-curve is highly irregular. Smoothed out by striking a decennial balance, it took the form shown in Fig. 38 from the Harvard College representation. But its dissymmetry is greatly modified from that of the tracing given by the observations of any single period. Usually there is a rather swift increase and decrease, followed by intervals of approximate constancy, at maximum, of about a hundred, at minimum, of fifty days.[700] Not unfrequently, however, the curve has a sharp apex, and its downward flow is interrupted by a secondary rise. Such “stand-stills” (as Mr. Maxwell Reed calls them) are a familiar feature of long-period variability. Seventh-magnitude brightness is never fully attained by S Cephei, and it occasionally drops below the thirteenth magnitude. Like many very red stars, it has accesses of bad definition. They occur, very remarkably, not at low light, but near maxima, when “a ruddy haze” seems to envelop a definite disc.[701]

In 463 days, almost equally divided between the gain and the loss of brightness, U Cygni varies from 7·0 to 11·6 magnitude; yet with no approach to regularity. The maxima are sometimes fainter than the eighth magnitude; the minima fluctuate, it is thought cyclically,[702] from 9·4 to 11·6 magnitude. The periodicity, too, is complicated by an outstanding disturbance.

The period of V Cygni is 418 days, and the rise, which occupies 220 days, is disproportionately slow. Lindemann registered a steady decline in the maximum-brightness of the star, from 6·8 magnitude in 1882 to 8·4 in 1891;[703] in some cycles it does not exceed 9·5 magnitude; while the minima occur, with tolerable uniformity, at the low level of 13·5 magnitude. The maxima are succeeded, at intervals of two months, by subordinate phases of recovery. The colour of V Cygni is especially intense. The carmine of its beams corresponds to a powerful stoppage of the complementary blue and violet radiations, by which a splendid preponderance is secured to the red end of the spectrum. No bright lines have been certainly recognised in the dispersed light of either U or Y Cygni; they are, as we have seen, prominent in U Hydræ, a fourth-type variable of no settled periodicity.

A fundamental distinction is apparent between the two chief kinds of stellar variability. Stars with “short” periods are—in a few cases demonstrably, in the rest presumably—close pairs, their mode of circulation prescribing, in some unknown way, their laws of light-change. The strict accuracy of its fulfilment hence results as if by mechanical constraint. Mira-variables, on the contrary, give no signs of duplicity; and the marked irregularity of their phases affirms their origin through a complex interaction of physical disturbances. That these are internal and constitutional, there is the best reason to believe. Spectroscopic symptoms are fairly decisive on the point. They have as yet, however, been very partially observed. A bare gleaning of facts has been gathered, and we want a full harvest as a foundation for safe inferences. A spectroscopic study throughout their cycles of variable carbon-stars would, for instance, be most valuable. The behaviour of the bright lines shown by them might even prove crucial, first, as regards the position in their atmospheres of the emitting strata, next, as to the seat of the recurring commotions. The spectra of U and V Cygni and of R Leporis may be cited as among those claiming systematic and prolonged observation.

The reality of the diffuse aspect intermittently presented by certain variables could readily be tested by examining them at such times with a reflector. Refractors, owing to their imperfect colour-correction, often produce abnormal images of objects peculiarly tinted. Nevertheless, if this were the true and only explanation of the effects in question, we should expect to find them develop under uniform conditions, and they appear instead incalculably, and as if by caprice. An instructive example is furnished by V Cygni. On 19th July 1882, six weeks after a maximum, Lindemann[704] saw the variable at Pulkowa as an indistinct coppery disc. But at the same interval, before the high maximum of 31st August 1882, it showed not a trace of nebulosity, although intensely red. On 8th October 1883 it appeared almost blood-coloured and very diffuse, while nine days later its image was point-like, stellar, and precise. Analogous observations have been made by Mr. Grover, Sir Cuthbert Peek’s assistant, on R, S, and T Cassiopeiæ, R and S Ursæ Majoris, and several other objects of their class; and by Mr. Knott on U Geminorum, which, as a white star giving a continuous spectrum, ought to come regularly to focus. For not a few stars, such as R Coronæ and S Herculis, dim, bluish nebulosities are substituted at low minima; and not uncommonly, even on nights of excellent definition, variables in moderately high phases appear sharp, though very red, and as if projected on a background of glowing haze. Then again, they present a clearly outlined disc, or a “large, woolly, ill-defined image resembling a small but bright planetary nebula.”[705] Most of these diversities defy anticipation; they can be associated with no particular stages of variation, and some of the reddest stars, R Leonis and R Leporis among the number, appear to be exempt from them. Yet their literal interpretation as indicative of physical alterations in the bodies affected by them would lead to consequences of outrageous improbability. Provisionally, at any rate, the wiser course is to refer them to a combination of atmospheric and instrumental causes. With these, no doubt, a genuine change of luminous quality concurs, whereby semi-extinct stars, being thrown out of focus, assume a nebular disguise. It is noteworthy that the records of a series of observations on the minima of twenty-two long-period variables, executed with the Yerkes forty-inch refractor in 1900,[706] include no mention of unusual phenomena. Attention, however, seems to have been directed entirely to the determination of magnitudes, nor had any of the stars on the list (U Geminorum excepted) been previously remarked for optical peculiarities.

A clue to the labyrinth of stellar variations is likely to be afforded by the continued investigation of solar periodicity. Comparisons of the spot-curve with the light-curves of Mira, χ Cygni, T Ursæ, or almost any of their congeners, bring a conviction that the phenomena differ in degree rather than in kind. The plottings of solar and stellar disturbances show the same character of dissymmetry, and the same order of irregularity. In both classes of representation, high summits are usually sharp, low summits blunt. In both, the course of change is now halting, now hurried. Hesitations, subordinate ascents, and subordinate subsidences before completing the phase, are common features. There is, indeed, no mode of departure from uniformity traceable in the solar cycle that cannot be strictly paralleled in the caprices of stellar emission. The analogy has been rounded out by the discovery that the sun, at spot-maxima, is essentially a bright-line star. Its spectrum then shares, in a just perceptible degree, the blazing quality that distinguishes the spectra of Mira-variables. This is a further and an irrefragable proof of the correspondence of the epochs. Light-maxima in the stars match spot-maxima in the sun. In each case a development of internal energy gives rise to enhanced incandescence, accompanied, in the single specimen of a sub-variable star within reach of detailed observation, by rendings of the photospheric envelope, and outbursts of chromospheric flames. Looking a little closer, we can discern the probability that the cyclical variations of all these bodies depend essentially upon a rise and fall of activity in the vertical circulation by which radiation is maintained. The rate of conveyance of heated matter from within outward must be a determining factor of photospheric brilliancy, transcendent lustre implying unusual celerity of transport. This is the vital process of suns, the checking of which must immediately become sensible in their diminished output of light. Here, if anywhere, will be found the secret of stellar variability.

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XXIII. PECULIAR AND IRREGULAR VARIABLES

The stars varying in periods comprised between thirty and a hundred days are not numerous, and they are often peculiar. Among them are to be found such remarkable objects as R Sagittæ, R Scuti, U Geminorum, and S^2 Cygni. Of R Sagittæ, with its double period and reversing minima, something has already been said. The possibility that it is in reality a “short-period variable” on a magnified time-scale is suggested by its resemblance to β Lyræ, and emphasises the question as to its spectroscopic duplicity. Its irregularities, though considerable, do not appear to transcend the limit of what might be explicable in a gravitational system.

This, however, cannot be said of R Scuti; and since the two stars are analogous in their mode of variation, a rationale clearly impossible for one must be regarded, for that reason alone, as highly improbable for the other. The fluctuations of R Scuti, first observed by Edward Pigott in 1795, extend from 4·4 to about 9·7 magnitude, and have a nominal period of seventy-one days. But they cannot be even empirically embraced in any formula. As Mr. Flanery remarked in 1896, “No set of elements yet devised will fit this star long.”[707] Each in turn has to be rejected as unserviceable. Thus, on 6th May 1897, the star showed a complete inversion of phase;[708] it was at a low minimum instead of at the computed maximum. It then rose to an unforeseen brightness of 5·4 on 11th June, diminished to half-light during eighteen days, and finally, remounting the slope it had just descended, shone duly at the predicted maximum of 17th July. Again, it varied less than a magnitude for two months after the maximum of 29th August 1896, but underwent a precipitate decline at the end of the stationary spell. The light-curve, from Mr. Flanery’s observations July to October 1895, is given in Fig. 39. It is that of a star which refuses to be bound by the shackles of any definite theory. Faint and brighter minima alternate, as Argelander long ago perceived; and they perhaps, now and again, exchange relative values, like those of R Sagittæ. If, then, we double the period, and call it 142 instead of 71 days, the star might rank, despite its vagaries, as an analogue of β Lyræ and R Sagittæ. For a subordinate minimum, placed midway between two maxima, is a feature common to all three, though the other circumstances of variation are in each star widely different. Such resemblances in the midst of diversity are extremely perplexing to students of stellar light-change. The similarity of some of the phenomena suggests a uniform principle of explanation; but the attempt to extend its application serves only to undermine the credit it originally possessed. The eclipse rationale, for example, suits β Lyræ passably well, and might be accommodated to the less equable phases of R Sagittæ, but is wholly incompatible with the disordered fluctuations of R Scuti. This, however, in view of their fundamental resemblance to those of the accurately variable star in the Lyre, raises the question whether eclipses can be regarded as occurring in the one case, when they assuredly do not occur in the other. Over and over again this difficulty presents itself. No theory seems elastic enough to bear the strain put upon it by the variety of the facts. Each member of a group of related stars adds its quota to the burthen of explanation to be borne; until finally the breaking point is reached, and a collapse ensues, leaving the ground encumbered with the débris of the original speculation.

R Scuti might usefully be made the subject of detailed spectrographic investigation. Bright lines shine in the blue and violet sections of its light; but they have not been identified, and the flutings associated with them appear ill-pronounced, or even subject to effacement.

As a curiosity of the skies, R Scuti is much outdone by a small star in Gemini, the abnormal behaviour of which was noticed by Hind in 1855. Habitually tranquil at 13·1 magnitude, U Geminorum rises with amazing celerity to near the ninth once in two, three, or four months. A leap upward of nearly four magnitudes is often accomplished in a single day, and that without preliminary fluctuations. The descent is always much slower, but along a very changeable curve. Two types of maximum are shown in Fig. 40. In one, the episode of brightening occupies fifteen to twenty days, in the other it is terminated in nine or ten. And, as a rule, they alternate one with the other. Nothing, indeed, is certain about this star except its uncertainty. “Predictions in regard to it,” Mr. Parkhurst concludes from his experience, “can be better made after the fact.”[709] The greatest light varies from 8·9 to 9·7 magnitude;[710] the least to a rather larger extent. Thus on 28th February and 26th March 1897 the star must have been below fourteenth magnitude, since Father Hagen lost sight of it with the twelve-inch refractor of the Georgetown College Observatory.[711] No relation is perceptible between the amount and the duration of change; long and short maxima are indifferently high and low. They are fickle, too, in their time-connections. The period—if it can be called a period—may be as short as 71, or as long as 126 days. Their unpunctuality apart, the changes undergone by U Geminorum bear a strong resemblance to those of cluster-variables. There are the same relatively prolonged intervals of repose, followed by vehement spasms of activity, beginning abruptly, dying out gradually. It will be of much interest to inquire whether the rays of objects so singularly and so similarly affected approximate to uniformity in quality. Those of U Geminorum are in colour dull bluish white; they give, according to Pickering and Copeland, an ordinary continuous spectrum. Still it is possible that peculiarities might be revealed by special scrutiny with powerful instruments.

U Geminorum ranked as a unique object until S^2 Cygni was discovered. Miss Louisa D. Wells in 1896 traced the fluctuations of the latter on the Harvard plates from 7·2 to below 11·2 magnitude; and the shortness of their apparent period of forty days combined so unusually with their wide range,[712] that they immediately became a cynosure for observers in that branch. The more closely they were watched, the more nearly they were found to conform to those of Hind’s variable. In both stars, stationary minima are interrupted, at intervals not wholly irregular, by sudden ascents of three or four magnitudes; and as in U Geminorum, so in S^2 Cygni, long and short maxima are coupled together in pairs, yet by no invariable law. Breaks in the alternate succession have been in each case recorded. In March and April 1897, for instance, S^2 Cygni rose to consecutive short maxima,[713] and a pair of long maxima again marred the rhythmical flow early in 1900. The curve for a double period is reproduced, from a drawing by Messrs. Parkhurst and Daniel, in Fig. 41. They comment on “the sharp turns in the curve at the beginning and end of maximum” as being “peculiar to this type of variable,”[714] which they judiciously assimilate to that prevalent in clusters. The mean period of about fifty-seven days attributed to S^2 Cygni has a wide margin of uncertainty. The interval from one maximum to the next may be curtailed to thirty-six, or expanded to sixty-three days, and that without traceable plan or method. Revolution in an orbit is hence absolutely excluded from among the possible causes of light-change. An hypothesis proposed by M. Hartwig of Bamberg[715] merely illustrates the baffling nature of the problem set by this star. He suggests that the maxima are occasioned by the brief kindling, at periastron passage, of a satellite pursuing a highly eccentric path. The irregularities of the period, he thinks, may be accounted for by a rapid gyration of the line of apsides. The blazing of “new” stars is, in his view, an analogous phenomenon;[716] but new stars exhaust their energy in a single display, while the variables in Gemini and Cygnus seem to have a limitless power of recuperation. The light of S^2 Cygni is tinged with blue; nothing has been published regarding its spectrum.

Although narrow in range (4·6 to 5·4 magnitude), the variations of υ Herculis are unsurpassed in singularity. Recognised by Schmidt in 1872,[717] they have proved to be “irregularly periodic”; their tendency to preserve definite time-relations appears to be continually resisted and sometimes overborne by countervailing influences. Occasionally they conform approximately to a forty-day cycle, then break loose, and become for a time utterly lawless. The minima are attended by extraordinary fluctuations; the maxima are normally tranquil. The star was marked “red” in the Copenhagen Catalogue, but is now pale yellow, and shows a helium spectrum. Scant attention has of late been bestowed upon it.

A southern star noticed by Dr. A. W. Roberts[718] in 1891 to vary from 6·8 to 8·0 magnitude in a period of 38½ days, is remarkable for its phenomenally quick rise. Only 5½ days are needed for the tripling of its light, while thirty-three elapse during the corresponding subsidence. Hence, if the average ratio for Cepheid variables of the times of increase and decrease held good for U Carinæ, its period would at once be abridged to eighteen days, and there could be no mistake about its membership of a class to which it is affiliated by the type of its variations.

The periodicity of R Lyræ, detected by Baxendell in 1856, often much perturbed, is never wholly effaced. Argelander found it to be comprised within forty-eight days, which Schönfeld reduced to forty-six; yet in 1872 Schmidt considered it uncertain between the limits of thirty and sixty days.[719] Pannekoek’s introduction of a periodic term for its regularisation[720] can be reckoned only a temporary expedient. The oscillation is of small amplitude, from 4·0 to 4·7 magnitude, but two striking outbursts of light, witnessed by Sawyer in November 1884,[721] imply essential instability. The simultaneous development of emission-rays might possibly have been observed had a prismatic eye-piece been at hand; but no spectroscopic examination was feasible at the critical moments. Ordinarily, the star, which is deeply tinted with orange, gives a superb colonnaded spectrum unmarked by bright lines.

Periodic cannot be sharply distinguished from irregular variables. Stars of an intermediate character are quite common. Some degree of precision in change may even be temporarily maintained by objects eventually found to be eminently unmethodical in their modes of procedure. Such are α Herculis, α Orionis, and β Pegasi. Each assumed on first acquaintance a false air of submissiveness to a time-law, which each very soon laid aside. Their fluctuations rather exceed half a magnitude, and are included in an indeterminate number of months; their progress can in no wise be anticipated. Similarly, stars credited on historical grounds with extremely long periods have of late paid not the smallest regard to them. An instructive example is met with in R Cephei. Catalogued by Hevelius in the seventeenth century, and by Groombridge in 1807, as of the fifth magnitude,[722] it thereafter lost light, and in 1840 had sunk to the tenth magnitude. Collating all the available data, Pogson in 1856 assigned to the diminished star a period of seventy-three years, and predicted its restoration to naked-eye visibility in 1880.[723] But the prediction remains unfulfilled; the obscurity of R Cephei seems likely to be permanent.

At Potsdam in 1898 MM. Müller and Kempf noticed a star in Perseus[724] as variable in an unprecedented fashion.[725] After an indefinite term of constancy at 6·3 magnitude, it began in 1892 to decline at the very slow rate of one-eighth of a magnitude yearly, and continued to do so for six years. The counter-process was comparatively rapid. In twenty months the object had regained its former status, so that the complete oscillation occupied 7⅔ years. This time, however, there was no long stationary maximum. Already by the end of 1899 fading had made some progress; but it remains to be seen whether any true periodicity can be established.

A period of five years, ascribed by Mr. Espin to 63 Cygni, has been rejected on further experience of fluctuations distinctively capricious. Many, perhaps most red stars, are unstable to the extent of half a magnitude; and 63 Cygni is a very red star. The fact that it is one and a half magnitudes fainter chemically than visually, supplies a kind of measure for the intensity of its colour.

The empirical rule that irregularity gains more and more the upper hand with increasing length of period[726] is illustrated by S Persei. Indeed the order of succession in the changes of this object is by no means satisfactorily ascertained. Safarik[727] and Hagen[728] hold them to be rudely periodical in about two and a third years; but most other observers prefer to consider them as entirely irregular.[729] Their range though wide, from near the seventh to the thirteenth magnitude, is seldom completely traversed. Experience alone can decide whether the rudimentary method traceable in these variations during fourteen years previous to 1894 will continue to regulate them in the future. Phases so unpunctual are liable to effacement. Quite possibly, the actual instability of S Persei represents a more or less transitory state, which may be succeeded by one of approximate constancy in shining.

The most illustrious of casually variable stars is η Carinæ, formerly designated η Argûs. Futile attempts have been made to accommodate it with periods. It has none. It is, in the full sense of the term, irregular. Its changes are perhaps modified by influences of an unimaginable nature connected with the vast surrounding nebula. But those influences undoubtedly act upon a body of inherently peculiar constitution. The spectrum of η Carinæ is of a kind associated in every other known instance with absolute whiteness. It resembles that of P Cygni;[730] many hydrogen and helium lines are brightened in it, yet the star shows the colour of a Mira-variable. Whether this was always so or not, we have no means of deciding. The first note of a distinctive hue in the southern wonder-star was made by Piazzi Smyth, 1st January 1845, when he announced from the Cape a fresh increase in its light. For a month back, he wrote, it had been brighter than Canopus, and very red.[731] Then in 1850 Gilliss found it to outmatch Mars in depth of tint; and Thome described it in 1887 as of a “dull scarlet,” passing into “bright orange” during a slight temporary rise. The history of this star is familiar to most of our readers.[732] It need not here be repeated. One fact in addition to those currently stated may, however, be mentioned. In his star-maps of 1603, Bayer marked η Argûs as of the second magnitude, probably on the authority of Petrus Theodorus of Embden, who navigated the Indian seas 1594 to 1596. The variable was then equally bright in the sixteenth and in the eighteenth centuries, and its comparative insignificance when Halley placed it in the fourth rank was due to a merely transient decline. Whether the splendour of its beams has ever before been so completely shorn away as it is now, might be questioned. Excesses entail exhaustion, and the flaring maximum which culminated in 1843 was followed by a reactive sinking towards the ashes of extinction. Since 1886, as the observations of Finlay, Innes, and Roberts testify, the star has wavered between 7·0 and 7·7 magnitude, and no sign of its speedy restitution to brilliancy is perceptible. Its future is beyond divination. The present minimum may be indefinitely prolonged, but further change is more likely in the case of so ruddy an object. Another great outburst cannot indeed be reckoned upon even for a remote age. A “temporary” character may so far belong to η Carinæ that its biography will include but one absolute maximum. The star is sensibly devoid of proper motion.[733] Its distance from the earth must accordingly be prodigious.

The capricious disappearances of R Coronæ surprised Pigott in 1795. Usually of about the sixth magnitude, the star is liable at any moment, without note of warning, to drop to the thirteenth. The intervals of maximum lustre sometimes last for years. One extended from 1817 to 1824, another from 1843 to 1845.[734] But in the last-named year, and again in 1852, R Coronæ vanished from view with Argelander’s comet-seeker, regaining brightness on each occasion slowly, and, as it were, with difficulty. Of late its descents have been less profound. Schmidt observed a minimum at twelfth magnitude in August 1883;[735] Sawyer recorded on 13th October 1885 one arrested at 7·4 magnitude.[736] Having been visible to the naked eye nearly throughout 1893, the star sank to the ninth magnitude about 7th March 1894, and after an intermediate partial recovery, to 10·25 on 1st August.[737] By the end of the year the phase of instability seemed to have terminated. These lawless fluctuations, taken in connection with the extraordinary spectral changes ascribed to it, render this object one exceptionally inviting to careful study.

An analogue to it, but with a much narrower range of mutability, is the lucid white star ε Aurigæ. Ordinarily of the third magnitude, it fades at long and uncertain intervals to one-quarter of this brightness. One such diminution was observed by Heis in 1847; another by Schmidt in 1875.[738] There is nothing in the quality of the light to account for them. The spectrum is modelled on that of Procyon, only with an increase of definiteness, and marked differences of relative intensity in the lines.[739]

The vicissitudes of T Tauri derive special interest from their presumable connection with those of a group of nebulæ. Discovered by Hind 11th October 1852, it dwindled during fifteen years from tenth to twelfth magnitude pari passu with the fading of the adjacent “temporary” nebula, but attained in March 1868 a second and higher maximum, coincidently with the brightening of “Struve’s nebula,” another member of the collection. Again it declined, and was left unnoticed from 1877 until Burnham and Barnard, directing the Lick thirty-six inch to its place in October 1890, perceived it as the faint nucleus of a small condensed nebula,[740] which four and a half years later survived only as a feeble glow round the almost extinct variable. The glow was resolved by the Yerkes refractor into a little wisp of nebulosity, attached brush-wise to the star;[741] and partial impressions of it came out on plates exposed by Professor Keeler for four hours with the Crossley reflector, 6th and 29th December 1899.[742] “Can it be,” Professor Barnard asks in surprise, “that the star becomes essentially a nebula as it sinks in light?” The question goes to the root of cosmic relations, and it is raised under more than one aspect by investigations of stellar variability. The associations and transformations of T Tauri are hence of profound significance, and should be diligently supervised until they can be linked together by some rational principle of causation.

Irregular variability has a wide and indefinite reach. It includes changes almost instantaneous, and changes well-nigh millennial in their development. The light of certain stars has undergone a slow secular decline. A noteworthy instance is that of Θ Eridani, identical, as Dr. Anderson has conclusively shown,[743] with Ptolemy’s “Last in the River” (the Arabic Achernar), the title and honour of which have been usurped by the more southerly, and now far brighter α Eridani. Al-Sûfi in the tenth century reckoned Θ among the thirteen brightest stars visible in Irak; and it was still of the first magnitude in 1437, the epoch of Ulugh Beigh’s Catalogue. Yet it had sunk to the third when Halley visited St. Helena in 1677, and of the third it still remains. Two other stars which have undeniably faded with the lapse of centuries are β Leonis and δ Ursæ Majoris,[744] and their fading may even now be imperceptibly progressing. Nor is their eventual restoration by any means assured. Accessions of lustre are rarer, and often transitory. On 6th August 1868, 83 Ursæ Majoris, a sixth-magnitude star near Mizar, was seen by Birmingham to be the equal of δ Ursæ; though for that night only. The next, it had visibly gone off, and before long the whole of its added splendour had departed. Its amount was very considerable. The star attained, during its unexplained rise, to threefold its customary brilliancy. One of Burnham’s close pairs, z Virginis, underwent in 1866 a similar phase. This was before it was known to be double, so that our curiosity as to whether both the nearly equal components shared in the brightening remains ungratified. Red stars not infrequently drop abruptly to a lower rank. Thus the Danish astronomer Torwald Köhl had for years observed B.D. + 20° 1083, in the constellation Taurus, as of 7·7 magnitude, when on 22nd January 1898 he was taken aback to find it not much above the ninth.[745] Sixty per cent of its rays had been, as a consequence of some inexplicable collapse of energy, subtracted or suppressed. A step towards the bourne from which there is no returning may be taken in such cases. Stars vanish, but they seldom or never reappear. Yet renovation plays its part no less than decay. Waning stars have their correlatives in waxing stars. Alcor, the Rider, and Benetnasch, the third Horse of the Wain, are among these. Pollux, too, seems to have bettered its position, and Alcyone is far more predominant than of yore in the Atlantid family. Night’s robe will still be profusely spangled, even though a few of its gems grow dim.

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XXIV. TEMPORARY STARS

A temporary star is a variable that rises sheer from profound obscurity to a single maximum. The maximum may be prolonged or multiple, but it must be essentially one. The occurrence of a second independent outburst would at once relegate the object to the category of irregular variables. The distinction is perhaps arbitrary, but we can only investigate by dividing. It will be best to plunge at once in medias res with some account of an apparition which attained to epochal importance through the efficacy of photographic methods of research.

Nova Aurigæ sprang into conspicuousness with the stealthy speed of Jack’s Beanstalk. On 8th December 1891 Dr. Max Wolf took, at Heidelberg, a photograph of the sky round χ Aurigæ, showing stars to the ninth magnitude. The Nova was not among them; the spot destined for its occupation was vacant. Forty-eight hours later, as a Harvard negative attested, a fifth magnitude star filled the blank. By 20th January 1892, twelve records of this stranger’s presence were included in the same series; but inadvertently, for the documents were stored up unread, and it was only through their subsequent examination that a maximum of 4·4 magnitude was inferred to have taken place on 20th December. During nearly two months, then, a new star, readily visible to the naked eye, shone unnoticed in the heavens. It was finally perceived by an amateur, Dr. Thomas D. Anderson of Edinburgh, and an anonymous post-card, by which, on 1st February 1892, he conveyed the news to Dr. Copeland, formed the starting-point of widespread astronomical activity. The spectrum of the Nova was promptly photographed at Tulse Hill, South Kensington, Stonyhurst, Potsdam, and Mount Hamilton; eager study was devoted to its implications; and they were of a most unexpected kind. For no previous stellar apparition had been analytically recorded; and the similar spectral phenomena, doubtless present in Nova Coronæ and Nova Cygni, eluded definite determination with the eye. But when the turn of Nova Aurigæ came, the spectrographic method was effectively at hand, and the peculiarities of its light could be rendered obvious, salient, and permanent. Some of them were indeed visually manifest. The spectrum was at once seen to blaze with bright lines, many of them greatly widened (see Plate XVII.); and nearly all came out photographically as attended by strong dark companions on their more refrangible sides. The entire hydrogen series, from crimson C to the last of its ultra-violet associates, was thus doubled, no less than the pre-eminent calcium pair, the sodium D, and a considerable number of lines since identified as originating from helium. An exceptional feature was the predominance of “green” helium; D_{3} and the rest of the lines belonging to the “yellow” set were comparatively faint; while λ 4922, λ 5016, and their fundamental, λ 6678, shone lustrously. Their superiority, never before observed in the spectrum of a heavenly body, results in the laboratory from heightening the exhaustion of the emitting gas; but this condition seemed to be excluded in Nova Aurigæ by the distended aspect of rays unmistakably proceeding from a more than ordinarily condensed atmospheric stratum. Unprecedented, likewise, was the simultaneous brightening of the three D-lines. No stellar spectrum previously observed had shown the action of sodium otherwise than by absorption. Both characteristics, it is true, may have been present in earlier Novæ, but they first arrested attention in Anderson’s star.

These novelties, curious though they were, sank nevertheless into insignificance compared with one dominant trait. This was the large opposite displacements of the bright and dark sets of lines. Contrary motions of prodigious velocity appeared to be indicated, and for a time their prevalence was taken to be incontrovertibly attested. The outburst, by a consensus of opinion, resulted from the approach, and integrated the light of two components, one a bright-line star receding from the earth at the rate of about 230 miles a second, the other a dark-line star hurrying towards it with a speed of 320 miles. Soon, however, incongruities began to develop, and they grew and multiplied as time went on. To begin with, the spectral shifts underwent no alteration; the movements indicated by them—if they did truly indicate movements—persisted without abatement as the bodies animated by them withdrew from each other’s vicinity to a distance greatly exceeding that of Neptune from the sun. Now velocities, to continue uniform, must be inherent; that is to say, they cannot represent the merely temporary effects of gravitational pull, since orbital acceleration is strictly balanced by retardation. Evidently, then, the components of Nova Aurigæ did not simply fall together; they should have been fabulously massive to have produced, by their mutual attraction from infinity, a speed which continued at the rate of 550 miles a second three months after the periastral rush-past, the date of which presumably coincided with the first rise to brilliancy on 10th December. Professor Seeliger of Munich[746] calculated that 15,000 times the sun’s gravitative power, at the very least, must have been at work if the orbits traversed were parabolic; and the extravagance of the estimate sufficed, and was designed to compel its rejection. Hyperbolic motion was accordingly resorted to; the encountering bodies brought, it was supposed, their own velocities with them from the farthest bounds of space; and they were of so high an order that the increments due to mutual gravity left them sensibly unaltered. A pair of “runaway” stars, one moving towards, the other from the earth, must, it seemed, have accidentally passed each other almost within grazing distance. They were primitively unrelated; their quasi-collision could never be repeated; they were as unlikely as any two stars in the heavens to be similar in constitution. Yet their spectra affirmed their close affinity; both were of pure helium type; one might be called the negative copy of the other. Nor was this all. Anomalies still more glaring presently disclosed themselves. Too obviously, on the adopted hypothesis, one pair of meeting stars could not suffice to explain the phenomena. Vogel stipulated for a triple encounter;[747] Campbell found evidence of the interaction of four luminous masses.[748] The more attentively, in fact, the spectrum was examined, the more complex it appeared. The bright lines were not simple emanations, but groups of differently refrangible rays; the dark lines were intersected by bright threads, variable in number and position. Several distinct sets of lines, each with its separate amount of shift, and each hence associated with a differently moving mass, thus stood out independently. “On the hypothesis of four bodies,” Professor Campbell wrote, “the principal system of bright lines was not displaced appreciably, and the star yielding it was practically at rest with reference to the solar system. Another system was displaced towards the red, a distance corresponding to a velocity of recession of about 315 miles a second. The system of fine bright lines, and likewise the system of dark lines, were displaced towards the violet, a distance corresponding to a velocity of approach of about 400 miles a second.” The analysed light of the Nova, on this showing, consisted of four superposed spectra disconnected in their origin. The case, however, was presented with diffidence; no conclusive force was claimed for it. The spectrum, indeed, if multiple at all, was more than quadruplicate. Victor Schumann pointed out[749] that not less than six, if as many as two stars were engaged in the outburst, and planets ad libitum were thrown into the mêlée by Vogel.[750] The collision-theory, in short, collapsed under the weight of the facts it had to carry; speculation was plainly off the track; a new principle of explanation had to be sought. It was difficult to find; yet some probable though partial truths had been laid hold of. Sir William Huggins, for instance, pointed out that the complexities of the spectrum might be in part due to “reversals” in the atmosphere of a single star.[751] Such effects of the stratification of glowing vapours are common in the sun, and may be artificially produced in a diversity of forms. Their presence in Nova Aurigæ was clearly recognisable. The opinion, too, expressed by Father Sidgreaves might safely be adopted that the spectrum of the new star “was, on the whole, what the solar chromospheric spectrum might be expected to show on a grander scale of disturbance.”[752] Finally, M. Seeliger’s general theory of stellar conflagrations,[753] as arising from the passage of compact bodies through “cosmical clouds,” or nebulæ, though strained to meet superfluous requirements, had fundamentally much to recommend it. Entire originality could not be claimed for it; Mr. Monck, and possibly others besides, had earlier proposed a similar view; but it was by Seeliger independently developed, and independently applied to the circumstances of the latest event. These were indeed particularly embarrassing to theorists. They seemed to imply the continuous progress of opposite radial movements of enormous velocity, and the problem of bringing them into play, whether by the rushing of inflamed gases or by the bodily transport of luminous globes, was, when considered in all its complicated bearings, formidable, if not desperate. Yet it had to be faced, for at that time spectral displacements were explicable only as effects of motion; they should perforce be interpreted on the radial velocity principle. This is no longer absolutely prescribed; the same phenomena have been found to bear other meanings—meanings not yet thoroughly intelligible, but promising, when they become so, to provide the keys to many enigmas.

We may now trace the further course of the apparition. During nearly three months it retained most of its brightness, despite wide fluctuations; then on 6th March a precipitate decline set in, bringing the object on 26th April to the limit of visibility with the great Lick refractor. In Fig. 42 the course of change so far is graphically portrayed. It was naturally believed to have reached its term; the Nova seemed definitively extinct; but in this, as in other respects, its behaviour defied anticipation. Observations resumed 17th August 1892, after its conjunction with the sun, showed, in the place of the vanished star, a stellar nebula of the tenth magnitude.[754] The recovered light was entirely altered in quality. No stranger disclosure has been made by the prismatic method than that of the spectral transformation of Nova Aurigæ between March and August. The dark lines of the former spectrum had become effaced; the bright lines were no longer chromospheric but nebular. Their character, moreover, denoted disturbance of a very peculiar kind. They were wide and hazy, and some at least could be resolved into groups projected upon a feeble luminous background.[755] Plainly the state of things producing the complex “reversals” noted in the spring had become intensified in the reappearance of the autumn. Yet the multiple bands composing the new spectrum corresponded most faithfully, in number and mean position, with the rays of a planetary nebula. The representation was essentially perfect. Out of nineteen Nova-lines measured by Professor Campbell, only one of subordinate importance appeared foreign to nebular light.[756] Both the Wolf-Rayet blue bands were present, as in certain planetaries; while the distinctive, though enigmatical, nebular line at λ 373, noticed as absent from the Tulse Hill photographs of the original spectrum, emerged as it altered, and was recorded by Von Gothard at Herény in November 1892.[757] All the metallic lines, all the helium lines (except a trace of λ 4472) had died out; only those of hydrogen survived; and hydrogen glows universally. The metamorphosis could hardly have been more complete. Professor Campbell might well say that the relation of the later to the earlier spectrum was “not apparent.”[758] It was certainly most obscure. At least, however, the conviction was acquired that the light “emanated from one source;” although subsequent experience obliges us to regard as fictitious the rapid approaching movement attributed to that source on what then seemed incontrovertible evidence. The presumed velocity reached a maximum of 190 miles a second in September, but in November 1892 had decreased to about half that value. And the slackening continued until, at the end of two years, the shifted lines occupied almost normal positions. Subsidence of physical agitation might safely be associated with the change. It was attended by spectral modifications of a different kind. In the transformed Nova bright lines at λ 436 and λ 575 were at first prominent. They also occur in nebulæ, but by exception and inconspicuously. Eventually they faded out, and the spectrum remained typically nebular save for the anomalous breadth of the lines.[759]

As a star, Nova Aurigæ had been decidedly yellow; in its nebular stage it assumed a greenish tint. Its chromatic peculiarities accounted for the haziness of its aspect with many refractors. Outstanding rays, left unfocussed when the rest were united, created a spurious disc of which no trace could be perceived with a reflector.[760] The revived Nova maintained tenth-magnitude lustre with trifling fluctuations for upwards of four years; then it began once more slowly to decline, and in the latter part of 1897 looked “more like a most minute and faint planetary nebula than a star.”[761] In 1897 it had sunk below the thirteenth magnitude; in 1900 it touched the fourteenth.[762] A partial revival was noted at the Lick and Yerkes observatories in 1901, the peculiarity of its focus continuing to indicate the nebular quality of its light.[763]

Nova Aurigæ was neither the first nor the last temporary star to don, in fading, a garb of nebular light. Nova Cygni passed through an identical phase, and the example has since been frequently imitated. Chiefly, indeed, the methods of investigation brought to bear upon it, and the novel considerations to which, in consequence, it gave rise, rendered the apparition of 1892 memorable. Its dominant features were those of a model “blaze star”; but they were for the first time decisively ascertained. An evanescent phenomenon was rendered virtually permanent, and for dubious visual impressions, definite transcripts of fact were substituted. They are likely to be multiplied in the future; and it has meantime become abundantly clear that no special theory, fitting only the circumstances of Nova Aurigæ, can be true.[764] Whatever rationale of them may be ultimately adopted, it must be one capable of general application.

The photographic era of investigation began with Nova Aurigæ, that of photographic discovery with Nova Normæ. The former object was still visible when the latter came into notice. Virtually a replica of the earlier spectrum imprinted itself upon a Draper Memorial plate exposed at Arequipa by Professor Bailey 10th July, and examined by Mrs. Fleming at Harvard College 26th October 1893.[765] It included about a dozen bright lines, each shadowed by a dark one on its more refrangible side. In position, in relative shift, in character, the system of duplicate rays matched with singular fidelity that scrutinised and wondered at in the previous year. There was then nothing casual about its production. The spectral displacements and the augmented refrangibility of the dark lines belonged to the essence of the phenomenon. Portentous velocities, specially directed, and inherent in specially constituted masses, could have had nothing to do with them. The appearance of Nova Normæ, then, disposed of what was left of the encountering star-theory.

The conflagration was of a strictly temporary nature. Absence from a chart-plate proved the star to have been fainter than the fourteenth magnitude, 27th May 1893; and a spectrographic plate, taken 21st June, added the information that it had not at the later date reached tenth-magnitude brightness. Its rise to the seventh, on or before 10th July, was probably effected in a few hours; but though fortunately registered at an early stage in its career, it was not recognised for nearly four months, by which time it had sunk to one-sixth of its primitive lustre. The same marvellous change witnessed in Nova Aurigæ attended its decline. On 13th February 1894 Professor Campbell[766] succeeded in observing the star with the Lick thirty-six inch, in spite of its low meridian altitude of barely 2½ degrees. The spectrum was nebular. It consisted, to the eye, of the well-known trio of green rays, together with the golden line (λ 575), also emitted by the transformed Nova of August 1892. Measurements of them afforded no evidence of displacement; their wave-lengths were as usual. Towards the middle of 1895 Nova Normæ became telescopically invisible. It was already provided with a successor. Just as it glimmered out, a star in the constellation Argo lit up. Nova Carinæ was detected by the same means as, though with somewhat more delay than, Nova Normæ. Two impressions of its spectrum, which chanced to be secured, on 14th April and 15th June 1895 respectively, disclosed precisely the same arrangement of coupled lines, the dark set above the bright, so remarkable in earlier examples.[767] During those two months this had not appreciably varied, but a significant change of a different kind had come about. A vivid ray at λ 4700 (approximately), scarcely visible 14th April, was as bright as the hydrogen lines on 15th June. Its identity with the Wolf-Rayet azure band—the leader line of the Rydberg hydrogen series—may safely be assumed, and its kindling was most likely the prelude to a complete nebular transformation. But of this we have no certain knowledge, since the later history of Nova Carinæ remained unwritten. The conflagration was brief; it had burnt itself out before Mrs. Fleming’s examination of the plates transmitted from Arequipa gave the alarm of its occurrence. Hence no express observations were feasible.

The year 1895 was prolific of temporary stars. One was retrospectively announced to have appeared in the constellation Perseus in 1887.[768] A bright-line spectrum, dimly self-imprinted upon a Draper Memorial plate exposed at that period, was at first thought to signify the rise to maximum of an ordinary Mira-variable. But the originating object soon vanished, to all seeming, definitively, and its brief incandescence asserted itself, by its non-recurrence, as that of a new star. There was a further note of distinction. The fifth line of hydrogen (Hε), never apparent in an ordinary variable, glimmered in the peculiar spectrum of the star of 1887, which included besides an unknown line at λ 4060.

The last record obtained of Nova Carinæ preceded the first of Nova Centauri by just a fortnight. They differed, however, widely in character. The spectrum singled out by Mrs. Fleming, 12th December 1895, from a crowded spectrographic picture taken at Arequipa 18th July in the same year, resembled that of an exceptional nebula, 30 Doradûs. It seemed abortively or imperfectly stellar.[769] The source, too, from which it emanated was situated in a nebulous environment. With the fading of its brightness—which probably never greatly exceeded the seventh magnitude—an outlying portion of a known nebula (N.G.C. 5253), momentarily effaced by the blaze, as a fire is “put out” by the sun, shimmered into view, like a halo round the dying star.[770] It still survives, while not a trace can be seen of its quondam inmate. By the time that Nova Centauri came to be recognised, it was unfortunately already far advanced on its return to obscurity. It had sunk below the eleventh magnitude on 22nd December, when Professor Campbell secured the first of three observations of its spectrum. He described it as continuous, though peculiar, for the blue section was of disproportionate strength, and the yellow showed inequalities, as if through the superposition of bright lines.[771] A parting glimpse of the strange star was caught by Professor Hussey at Lick on 16th July 1896. It was then excessively faint, and immersed in nebulosity. The analogy between this apparition and that which illuminated the great Andromeda nebula in 1885 was unmistakable, and both diverged significantly from the type of Nova Aurigæ.

It was reverted to by Nova Sagittarii. This star was photographed at Arequipa 8th March 1898, and identified by Mrs. Fleming as “new” a year later.[772] Yet it was at maximum fully the equal of Nova Aurigæ, and might have been seen at a glance by any one familiar with sky scenery. It waned, however, very speedily. It was of only 8·2 magnitude on 19th April, when a spectrographic record chanced to be secured, and had declined to the eleventh before its deliberate investigation became possible. Plate XVI. Fig. 4, reproduces two spectral photographs of Nova Sagittarii, obtained within forty-eight hours of each other. Six or seven hydrogen lines are shown in them, broad and bright, but without the dark companions usually visible in such spectra. The second band from the right is the more refrangible of the Wolf-Rayet blue radiations (λ 4643); the K of calcium is absent, but six sharp rays, identified with chemically unclaimed lines in the spectrum of Nova Aurigæ, came out distinctly on the negatives. Although taken at so brief an interval, the records disagreed in some particulars. Thus the unknown line at λ 4060, emitted by the first Nova Persei (1887), was strongly absorbed in Nova Sagittarii on 19th April, but had vanished on 21st April; while the chief nebular line, invisible at the earlier, shone at the later date. The initiated change did not stop here. In March and April 1899 a purely nebular spectrum was derived from the semi-extinct object by Professors Campbell and Wright.[773] The regular cycle had been run through; a planetary nebula replaced the faded star. In one important respect, nevertheless, Nova Sagittarii had departed, so far as the extant data could inform us, from the pattern set by Nova Aurigæ. The spectrum, as already mentioned, was single, not duplicated by absorption. But there is good reason to believe that this difference did not originally exist; it supervened with the declension of light. In “temporary” spectra the bright lines always seem to survive the dark; the chiaroscuro effect is produced only near the period of maximum, and this period had terminated before the star of 1898 was spectrographically registered. Earlier impressions, we may be sure, would have displayed the coupled lines symptomatic of the enormous disturbances attending these extraordinary outbursts.

Their frequency, established by the camera, is a fact of subversive import, but has already ceased to excite surprise. The recognition of the autographs of new stars has become a regular part of the business conducted at such a “Solomon’s House” as the Harvard College Observatory. In the Annual Reports thence issued the mention of new stars has grown to be habitual and familiar. Of Nova Sagittarii the last had only just been heard, when Nova Aquilæ was announced to have appeared.[774] This was about fifteen months after date. The star made its début on a Draper Memorial chart-plate 21st April 1899 as an object of the seventh magnitude, and displayed the quality of its light in a spectrograph taken 3rd July. It was characteristically that of a Nova, although nebular lines had begun to come in, and the transformation was completed in the course of September. During a year, Nova Aquilæ wore the aspect of a planetary nebula, sinking gradually from the tenth to the twelfth magnitude. It was last observed by Professors Campbell and Wright with the Lick thirty-six-inch refractor and a 60° prism spectroscope. “The visible spectrum,” the former wrote,[775] “consisted of extremely faint continuous light in the green, and of three bright bands in the positions of the three principal nebular lines. The relative intensities of the three bands agreed approximately with the corresponding intensities in the well-known nebular spectra. The bands were not monochromatic, but on the contrary were very broad, perhaps fully twice as broad as the bands in the nebular spectrum of Nova Aurigæ in August 1892.”

It may be useful to tabulate the results, in the discovery of new stars, of the photographic surveys executed at Harvard College and at Arequipa, its southern dependency.

(i.) Nova Persei. Appeared 1887; ninth magnitude; hydrogen lines and λ 4060 bright. Temporary character recognised 1895.

(ii.) Nova Normæ. Spectrum photographed 10th July 1893; character detected 26th October 1893. Seventh magnitude; showed twelve bright lines coupled with more refrangible dark ones; spectrum nebular 13th February 1894.

(iii.) Nova Carinæ. Spectrum photographed 21st April 1895; examination followed in October. Hydrogen lines bright with more refrangible dark companions. Faded from eighth to eleventh magnitude between April and July.

(iv.) Nova Centauri. Spectrum photographed 18th July 1895; irregularly continuous; resembled that of 30 Doradûs. Character detected 12th December 1895. Maximum magnitude 7·2. Situated within a nebula.

(v.) Nova Sagittarii. Photographed 8th March 1898, when of 4·7 magnitude; detected March 1899. Spectrum photographed 19th and 21st April 1898. Hydrogen lines bright; no dark companions; λ 4060 dark. Spectrum nebular, 13th March 1899.

(vi.) Nova Aquilæ. Photographed 21st April 1899, when of seventh magnitude. Bright-line spectrum photographed 3rd July 1899; recognition ensued in July 1900. Spectrum nebular, October 1898.

In the course of seven years, then, five temporary stars were photographically discovered, and it is safe to assert that all would have escaped direct visual notice. Apparitions of the kind are, we hence learn, hardly to be counted as exceptional; their occurrence cannot depend upon rare contingencies, but must enter, to some extent, into the regular economy of nature. The conditions precedent are doubtless widely prevalent, and they are the same for outbursts of all magnitudes indifferently. The famous Novæ of history—the stars of Hipparchus, of Tycho Brahe, of Kepler—were due to just such prepared accidents as result, more frequently and less obtrusively, in the appearance of an extra dark dot on a chart-plate, or in the emergence, on a prismatic negative, of a “peculiar” spectrum among a crowd of normal ones.

But this obscure kind of manifestation was far transcended by the “New Star of the New Century” (to quote Father Sidgreaves’s designation of Nova Persei, No. 2). This object worthily commemorated the turning of a leaf in the book of ages. The sidereal heavens had harboured no such brilliant “guest” since Kepler’s star shone in Ophiuchus. Dr. Anderson, the discoverer of Nova Aurigæ, was still more lucky with Nova Persei, for he caught it on the rise. It was, however, already nearly the equal of Algol when he sighted it in the early morning of 22nd February 1901; while twenty-eight hours previously, as a photograph taken by Mr. Stanley Williams attested, it must have been fainter than the twelfth magnitude. Confirmatory evidence was derived from the Harvard College series, two specimens of which are reproduced in Fig. 43 by the kind permission of Professor Pickering. In the earlier photograph, taken with an exposure of sixty-six minutes 19th February, no trace of anything unusual is perceptible; in the second, to which, seven days later, a shorter exposure was given, the Nova is of dominating importance. It continued to gain light rapidly for about a day and a half after its detection at Edinburgh. On the evening of 23rd February, it was observed through drifting clouds at Harvard College to be brighter and bluer than Capella.[776] In thirty hours it had increased by two and a half magnitudes; during that one night it took rank as the premier star of the northern hemisphere. But its supremacy was quite transient. Increase was at once followed by decrease; there can have been no appreciable pause at maximum. Already on 24th February, the star had lost fully one-third of its light; the inevitable downward course was entered upon, and was pursued, although with singular intermittences. But the most remarkable characteristic of Nova Persei was its spectral variability. It showed, to begin with, a spectrum of the ordinary Orion type—a continuous prismatic strip, scarcely encroached upon by narrow lines of hydrogen and helium absorption. Some of these, it is true, proved on close examination to have their lower edges slightly brightened, but there was no other sign of disturbance. On the following night it was noticed that the K of calcium, previously imperceptible, stood out markedly obscure, after which an extraordinary change took place. During the twenty-four hours that followed the maximum of 23rd February, the character of the spectrum was completely transformed. Hydrogen now blazed in it; and the range of broad bright lines representing its emissions was duplicated, just as in Nova Aurigæ, by a range of more refrangible shadow-bands. These were strongly displaced towards the violet, and their displacement appears to have increased progressively for some days. It finally corresponded, if interpreted on the Doppler principle, to an approaching velocity of about 1000 miles a second, while the bright lines, so far as their distended condition allowed a judgment on the point to be formed, retained pretty nearly their usual places. The Doppler principle, however, had plainly a very restricted application to the case of Nova Persei. Dr. Vogel[777] used it, with excellent judgment, to determine the star’s true radial motion from measurements of fine dark reversals of H and K, which seemed entirely exempt from alterations of a physical kind. He found it to be about twelve miles a second in a direction away from the sun. He was less successful in his endeavour to explain the conspicuous shifting of the dark hydrogen lines on the pressure-theory as adapted by Wilsing. For pressure does indeed alter the refrangibility of the rays emitted by vaporous strata submitted to it, but the alteration is always in the same sense. The refrangibility of light is diminished by condensation; the resulting spectral displacements are towards the red. Nevertheless, those of the absorption lines in Nova Persei were towards the blue. Nor was the masking arrangement devised for the purpose of smoothing away this fundamental contradiction, one that would work. The star itself, through the gradual unfolding of its peculiarities, emphatically disavowed it. A Stonyhurst spectrograph of 7th March showed the dark hydrogen lines well separated from the bright;[778] they were unmasked, yet none the less shifted. Their altered positions were thus seen to be those of the rays in their entirety, and not merely of outlying wings left visible, while their central parts were concealed. Subsequently to 7th March these lines thinned off, and in about a fortnight disappeared finally. The spectrum then remained in much the same condition as that of Nova Sagittarii at the date of the earliest record of it; the distinctive chiaroscuro effect was gone; the bright lines no longer cast shadows.

Sections of the light-curve of Nova Persei are delineated in Figs. 44 and 45. They seem to tell of a long succession of efforts towards recovery, constantly defeated by the invincible advance of exhaustion. These variations in magnitude were associated with remarkable spectral modifications. The light of this wonderful object was, from the first, of extremely unstable quality. The emitting and absorbing vaporous layers were evidently in a state of continual flux. Their turmoil was betrayed by changes of intensity, width, and wave-length in both dark and bright lines, most of them taking place unconnectedly, so to speak, and capriciously. By degrees, however, their confusion reduced itself to some kind of partial order. During the month of March, Nova Persei assumed the character of a variable fluctuating extensively in a period of three days, and the spectrum was found to vary quite decidedly in correspondence with the light. The phenomena of its alternations are illustrated in the subjoined reproductions of photographs taken by Father Sidgreaves (see Plate XVI. Fig. 5). The second and fourth are distinctive of minimum epochs; the first and third characterise much higher grades of brightness. Among the features of the minimum spectrum are the fading of the continuous radiance, the displacement upward of the first ultra-violet member of the hydrogen series (Hζ), and the development of a blue band at λ 463. The two types of spectrum continued to succeed each other with approximate regularity from 19th March to 3rd May. Numerous anomalies, indeed, presented themselves. They were described in detail by Fathers Sidgreaves[779] and Cortie,[780] by Lockyer, Pickering[781] and Hale,[782] but no clue to their physical interpretation has been found. Only those affecting one narrow region of the spectrum can here be further adverted to. The appearance of the D-lines in Nova Persei was most remarkable. Special attention was paid to them at the Yerkes Observatory, and they are delineated in Plate XVI. Fig. 3, from a photograph taken on an “Erythro” plate with the forty-inch refractor of that establishment. The relations of the various lines shown in it need some brief explanation. The broad bright band is due to the radiation of sodium; the fine dark lines projected upon it are obvious reversals by a cooler and rarer overlying stratum of the same metallic vapour. These threads of absorption are slightly displaced towards the red; they indicate about the same rate of recession from the earth that was derived by Vogel from the similar reversals of H and K. The fact, then, admits of no doubt that the kindled mass was travelling away from us with a small velocity.[783] This remained constant—so far as our information goes—from first to last; but the position of bright D underwent a marked alteration. The broad bright band, traversed almost centrally in the early days of March by hair-like reversals, shifted in the course of a month so notably towards the blue that the fiducial lines (as they might be called) lay in April close to its less refrangible edge; while the absorption line concealing D_{3} had meantime become diffused towards the violet. We have no inkling of a possible cause for these changes. They were steadily progressive, and so disclaimed any immediate connection with the periodical variations of the Hζ line. According to Father Sidgreaves’s view, indeed, these last originated from no change of refrangibility, but from the brightening of a cyanogen band situated just above the hydrogen line it illusorily modified. The sodium band, however, actually moved upward; the emitting molecules quickened their vibrations through some unknown kind of influence. Only the less refrangible members of the helium-series glowed in this marvellous spectrum. None higher up than the blue “Orion” line at λ 4472 could be seen; while the blood-red λ 6678, and D_{3} (when sodium-absorption thinned off) shone intensely.

The colour of Nova Persei changed from white to red a few days after its maximum on 23rd February, and red it remained for some months. The tinge, indeed, lightened to clear orange in its spasms of recovery, but deepened to a purplish glow at each subsequent decline. It faded completely in July 1901, when the spectrum of the Nova resembled that of a planetary nebula.[784] Plate XVIII. Fig. 2 illustrates, from the Lick observations, the predominance in it, on 11th August, of the leading nebulium-radiation. This was in accordance with precedent; but the wildest flights of imagination were outrun by what ensued.

On 22nd and 23rd August 1901, Professor Max Wolf took long-exposure photographs of the Nova with the sixteen-inch Bruce objectives lately mounted as a twin-telescope at Königstuhl. Both plates showed the presence of detached nebulous masses to the south-east of the star;[785] and these proved to be only the brightest parts of a vast spiral formation photographed by Mr. Ritchey at the Yerkes Observatory on 20th September, under the form represented in Plate XVIII. Fig. 1.[786] Its intimate structural relationship to the star is patent; but there was more to come. Renewed impressions obtained 7th and 13th November with the same instrument—a twenty-four-inch reflector of his own construction—showed Mr. Ritchey that the nebula was expanding with portentous speed;[787] and an identical discovery was made by Mr. Perrine from a comparison of the Yerkes photograph of 20th September with a Crossley picture secured by a duplicated exposure 7th and 8th November.[788] The movements indicated were at the rate of about one minute of arc in five weeks; and they were maintained until the dimly shining spires affected by them faded into chemical invisibility. Moreover, the spinning of the nebulous web was found to have been an initial accompaniment of the stellar outburst. Two close coils of it were discerned by Mr. Perrine on scrutiny of a negative exposed for ten minutes 29th March 1901.[789] And since they were closer in the ratio of their expansion between September and November, it became evident that the process had gone on unchecked during seven or eight months; while the tracing of it backward showed it to have actually commenced almost simultaneously with the kindling of the conflagration. That is to say, the rate of flow of the nebulous streams indicated their issue from the Nova about 17th February, or five days previously to its visible manifestation.

Speculation regarding this unique phenomenon has naturally been active; and an explanatory hypothesis of considerable plausibility was hit off independently by Professor Kapteyn[790] and Mr. W. E. Wilson.[791] It affirms the nebula to have been pre-existent, and to remain unchanged. But since we see it by the reflected light of the Nova, its various spires and condensations have come successively into view as the flare of the explosion travelled outward in widening circles. Hence an illusory effect of radial expansion was produced, while, in point of fact, the temporarily illuminated cosmic folds were as immovable as aligned snow-peaks, in turn set aglow by the rising sun. The parallax corresponding to this rationale is 0″·011; it implies that the Nova actually blazed in the third year of James I. (1605), and rose to a culminating splendour eight thousand times that of our sun. In this, at least, there is no improbability. The star is devoid of sensible parallax or proper motion; and its distance from the earth cannot, accordingly, be expressed in “numbers that have name.”

A strictly temporary character may safely be ascribed to its nebulous appurtenances; they were either evolved from its mass, or rendered luminous through its influence. Professor Max Wolf suggested the propagation of electric waves of the Hertzian type as the cause of the far-spreading excitement attested by the gleaming annuli. Professor Very[792] inclined to regard them as constituted by material corpuscles, such as give rise to cathode-rays, ejected from the star under the stress of light-pressure or electrical repulsion. There was, however, as Mr. Louis Bell[793] remarked, no evidence of acceleration, consequently none of the continued action of a repulsive force; while the supposition that the nebula round Nova Persei had been photographed by reflected light was, in his opinion, discredited by the absence of polarisation-effects, as well as by the long persistence of strongly nebulous patches close to the star, which, unless they had been self-luminous, should have vanished with its fading. Their light, accordingly, is surmised by Mr. Bell to have been of an auroral nature. It developed as a secondary consequence of electro-magnetic strains, propagated through space with the velocity of light, from a sun-like centre of disturbance.

What cannot be gainsaid is that the apparition, with its strange attendant circumstances, ranks as the most interesting on record. Even the classic Nova Aurigæ was far outdone by the surprising diversity of the phases, both spectral and luminous, exhibited by its brilliant successor. Their study afforded much new knowledge, and opened wide tracts for future experiment and research. A critical point of difference between the observations made on the two occasions was the timeliness of those on the latter star. They were in full swing while it was still mounting towards the summit of its splendour. As a consequence, the order of its spectral phenomena was clearly ascertained and proved wholly unexpected. At the outset, the light was of ordinary quality; waning had already set in before the displacements and distension of its linear ingredients announced the prevalence of disturbance. This fact is full of meaning; let us weigh it well. We learn from it that the sudden development of luminous energy was essentially of photospheric origin. Gaseous eruptions and explosions followed, and produced the duplicated spectrum characteristic of temporary stars. But they were merely incidental to the primary event, which occurred independently of them. This disclosure narrows the field of speculation as to the nature of that primary event, and is so far enlightening. Further, we now know that the symptoms associated with prodigious radial velocities were not inherent, but consequential. The obscure body abruptly kindled to vivid incandescence gave no evidence of rapid motion; it seemed a leisurely traveller through space.

We may now attempt to arrange and generalise the items of information lately gained about Novæ. Three have appeared in nebulæ or clusters—Nova Scorpii in 1860, Nova Andromedæ in 1885, and Nova Centauri in 1895. Set apart by their nature no less than by their situation, they were evidently transient adjuncts to the formations in which they were immersed, and probably imitated their luminous peculiarities. Our knowledge of them, however, is partial and unsatisfactory. Passing on to “blaze stars” proper, we note the following circumstances as common to the class.

1. They have their habitat in the Milky Way. Nova Coronæ alone had any considerable galactic latitude.

2. None have any sensible parallax or proper motion. They must then be vastly remote.

3. They rise from and relapse into approximate obscurity. The one known star distinguished by a temporary flare was Nova Coronæ. Several, nevertheless, continue perceptible in their effete state.

4. The bright lines of Novæ are coupled with dark lines of shorter wave-lengths. This mode of juxtaposition is invariable.

5. The spectra of Novæ resemble, in their early stages, that of the solar chromosphere, later that of nebulæ, the emissions of hydrogen and helium ultimately yielding their predominance to the green rays of nebulium. The Wolf-Rayet blue bands generally make an intermediate appearance.

Now what may we legitimately conclude from these varied phenomena? Very little, unfortunately, of a positive nature; we must be content, in the main, with negative inferences. Yet it is no small advantage to clear the ways of thought by abolishing untenable hypotheses. It may then safely be stated that the remarkable spectral shiftings in temporary stars are not effects of translatory motion; they supply no argument for the duplicity of the light-source. Neither do they originate through pressure, which tends to damp down vibrations, not to accelerate them; and it is chiefly a shortening of wave-lengths that has to be explained. Staggered by this difficulty, Dr. Wilsing suggested[794] the alternative view that Novæ are not incandescent, but “luminescent” bodies. The distinction, first made by E. Wiedemann,[795] is valid and valuable, although next to nothing is known about the essential conditions upon which it rests. All that can be said is that they involve the production of light to a great extent without heat. But the supposition that they are found in temporary stars is an extremely hazardous one. It is countenanced only by the one fact that emissions due to luminescence are, or may be, accompanied by corresponding absorptions of greater refrangibility, and so present a colourable imitation of the perplexing chiaroscuro spectrum displayed during stellar outbursts.

On the whole, the most promising theory of their occurrence is that stars in the Milky Way occasionally get entangled in the diffused nebulosities with which that region abounds, and blaze through the resistance offered to their motion, just as meteors kindle to brief splendour in shooting athwart our cloud of “circumfluous air.” We must, it is true, be content for the present to accept it in principle; attempts to elaborate it in detail can only, until much fresh knowledge has been acquired, result in failure. Even M. Seeliger’s[796] ingenuity did not avail to conduct him to a successful issue. He demanded too much from the star-and-nebula hypothesis—demanded, indeed, more than the conditions (as we now know) actually required; and it hence incurred unmerited discredit. Admitted provisionally, it will perhaps serve as a guide to ultimate truth. An important discussion of the possibilities connected with it, and of the manner in which they might serve the purposes of spectroscopic interpretation, was published by Mr. J. Halm of the Edinburgh Observatory in July 1901.[797] Proceeding from the hypothesis that a Nova becomes visible when “a dark body impinges upon, and penetrates into a mass of nebular material,” he constructed a vorticose system of radiating and absorbing gases travelling with the great intruded globe, and skilfully adapted in all its parts to give rise to the observed phenomena. His arguments are not, scarcely indeed could be, in all respects convincing; yet they form a contribution of stimulating quality to the general doctrine of temporary stars. This has wide bearings. It should include, not only a rationale of the actual conflagrations, but also some definite and consistent view as to the previous state and history of the bodies subjected to them. They are glibly designated “dark stars,” but embarrassments supervene when we attempt to give precision to our conceptions of what constitutes a “dark star.” We will, nevertheless, essay the task in the next chapter.

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XXV. DARK STARS

Dark stars are of sun-like dimensions, but of planetary obscurity. They constitute a realm scarcely penetrable by direct investigation, and therefore eminently inviting to speculative thought. They are indefinitely numerous; they may be, in many cases, enormously massive and bulky; they are probably endowed, in general, with very high velocities: some are perhaps among our nearest neighbours in space. They exist both in a solitary and in an associated condition. The dynamical relations of unattached dark stars, although of the deepest interest, from their bearing upon large questions of sidereal construction, are unknown, and seem likely long to remain so; but in mixed systems of shining and obscure bodies they are readily open to investigation. Such systems occur quite freely, as we learn from observations of three distinct kinds. Their unseen members variously betray their presence: (i.) by perturbing the orbital movements of visually double stars; (ii.) by occasioning periodic changes of velocity in visually single stars; (iii.) by occulting their bright primaries, and so causing them to be affected with a special type of variability. In the first case, the effects can be discerned telescopically, in the second they come out spectrographically, in the third they are both photometric and spectrographic. Solitary dark globes, on the other hand, are brought within our ken in only one fashion. Given a certain combination of circumstances of a barely conjecturable nature, they leap into light as Novæ, and in this capacity attain various degrees of notoriety.

The existence of an “invisible” department in astronomy was divined by Laplace; it was demonstrated by Bessel through his discovery, with the mind’s eye, of the semi-obscure satellites of Sirius and Procyon. Its importance cannot well be exaggerated. Unseen bodies may, for aught we can tell, predominate in mass over the sum-total of those that shine; they supply possibly the chief part of the motive power of the universe. They appear, at any rate, to be profusely distributed throughout its compass. One star in six, by a fair estimate, is a spectroscopic binary; and spectroscopic binaries commonly disclose their nature, not by the doubling, but by the shifting of their spectral lines. This is as much as to say that one of the revolving masses radiates imperfectly or not at all. Again, temporary stars presumably rise from crowded ranks. For every one that blazes into view, multitudes must remain undistinguished. The raw material of such outbursts is doubtless lavishly provided, while the accidents actually occasioning them must be few. Stellar apparitions thus derive extraordinary importance from their implication of an unfathomable background stored with effete or undeveloped suns, the presence of which evades immediate cognisance.

“Dark stars” need not be absolutely obscure. Some, at any rate, may be feebly luminous, though powerfully attractive, like the companions of the Dog stars. These, if situated much nearer to their primaries, would escape telescopic recognition, and could challenge spectrographic notice only indirectly, through the motion-displacements due to their influence on the brilliant spectra superposed upon, and effacing their own. In other cases, stars are swayed from their computed paths by the power of entirely rayless satellites. The fourth member of the system of ζ Cancri, and the hypothetical third component of 70 Ophiuchi appear to be dark in the strictest sense. Most probably an indefinite number of gradations connect the planetary and sun-like states. Thus in some Algol pairs the occulting body is to our appreciation obscure; in others it gives some, though very little light; while in a third variety the companions stand on the same level of lustre. Algol itself, U Cephei, and Y Cygni specifically exemplify these differences. They manifest themselves as well in non-eclipsing spectroscopic binaries. A minority, headed by β Aurigæ, consist of globes fully luminous; a few, such as α Virginis and ο Leonis, show traces of a second spectrum; but most are real crypto-doubles, only one member of which emits any perceptible light. With these the heavens may be said to swarm.

A number of interesting questions are suggested by the vast numbers and singular relationships of dark stars. What is their history? What their destiny? What their function in the universal scheme of things? The view commonly taken of them is that they are antiquated suns—suns in a state of senile decay—suns that, having used up their radiative vitality, are relegated to a lower cosmic plane. Such there must be, unless regenerative machinery be at work to supply, in some unimagined way, the perennial waste patent to our senses. Suns wear out, yet do not incur annihilation. They must survive their faculty of shining. Immense, but strictly limited reservoirs of energy, they are constructed, not for its economical storage, but for its rapid expenditure. Its exhaustion, which, to our apprehension, is simply a question of time, will leave them gigantic earths, solid, inert globes, with only their gravitative force unimpaired. That quondam-suns of this description pursue their unnoticed journeys through space, cannot reasonably be denied; they undoubtedly swell the hosts of dark stars; yet there are indications that these are not mainly of the effete quality—that they do not merely represent the caput mortuum resulting from one inexorable evolutionary process.

The frequent association of obscure with brilliantly luminous stars is very remarkable. Algol and its companion make a typical instance. They are assuredly of contemporaneous, and, judging by the usual criteria, of comparatively recent origin. The closeness of the system formed by them denotes, on the theory of tidal evolution, that it has, during no very long time, been subject to such modifying influence; and Algol itself bears all the marks of stellar juvenility. Another specially instructive example among non-eclipsing binaries is afforded by Castor. Here a great Sirian sun has an obscure and a brilliant attendant circulating respectively in less than three days and in several hundred years. Now the remote component must have separated from the parent sphere long before the close satellite came into being; yet it is at the acme of vitality, while that of the junior star is, so far as radiative symptoms can tell, already spent. Nor is there any warrant for assuming that they differ considerably in mass. Indeed, whatever disparity there may be is just as likely as not to be in the wrong direction. A doubt thus arises whether the shining of suns depends solely upon temperature; for gradations of cooling cannot possibly account for the abrupt contrasts of luminosity met with in mutually revolving globes. Irresistibly the hypothesis presents itself that, among the varieties of the cosmos, there may be a multitude of bodies conditioned in most respects like suns, but which have never attained to the possession of a photosphere, and have consequently remained imperfect radiators. The formation of the dazzling cloud-shell by which suns are visibly bounded, is a baffling enigma. It must involve an intricate combination of circumstances, not always perhaps realised. Should any of them fail, a “dark star” would result—a body intensely hot, yet destitute of the apparatus requisite for the effective diffusion of light and heat.

We can now see that the obscurity of temporary stars previous to their sudden illumination does not necessarily imply that they were effete. Their galactic preference also argues the contrary. The Milky Way is apparently a region where development has not gone far. Things are there more nearly in their primitive state than elsewhere in the sidereal world. Growing and flourishing suns seem to congregate in that vast gathering-ground, rather than those that have seen better days. Hence, if stellar apparitions signified the rekindling of stars sunk to extinction through age, the Milky Way is precisely the part of the heavens in which we should least look for their occurrence. That they frequent it almost exclusively, helps to convince us that their antecedent dimness was not due to decrepitude. This inference is confirmed by the character of their spectra. The light suddenly acquired has the quality considered to mark an early stage of sidereal development. The blaze of a helium star can with difficulty be supposed to proceed from a highly-condensed or semi-solidified body.

Dark stars must then, on this showing, be divided into two classes, the first consisting of suns on the retired list, the second, of those incapacitated by nature for active service. Their discrimination in practice can scarcely be effected, with our present resources, otherwise than tentatively or by conjecture. Still it may not be in all cases hopeless. To take one instance. The system of 61 Cygni is held by Wilsing of Potsdam to include one or more unseen members. If so, their place—since the associated group bears many marks of antiquity—should be found among lapsed suns. And this suggests the further thought that, in a few more ages, the entire family may have ceased from radiation, and will continue, in the rayless void, to fulfil their mutual rounds and pursue their way towards an unknown goal. That many such blind systems exist, inaccessibly to observation, is a conjecture which can neither be verified nor refuted.

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XXVI. THE GENERAL QUESTION OF STELLAR VARIABILITY

The variability of stars is a “wood of error.” There seems no exit from it; the tracks that invite our ideas to enter upon them turn out circuitous and misleading. Yet its exploration need not therefore be abandoned as hopeless; and indeed lines of approach are converging upon it from many directions in a manner that promises well for the future elucidation of much that is still mysterious. In the meantime, we can at least attempt to arrange the known facts in some kind of definite order.

Stellar light-change is of two fundamentally separate descriptions. The first may be termed “extrinsic,” because it obviously depends upon circumstances not inherent in the bodies it affects; the second, “intrinsic,” as resulting from constitutional peculiarities. They are readily distinguishable by the nature of their time-relations. Punctuality marks the former kind, a large measure of irregularity the latter. The difference becomes intelligible if we admit, as we reasonably may, that short-period or extrinsic variability belongs exclusively to compound objects, long-period variability to solitary masses. In no case—except where darkening by eclipse unmistakably occurs—can we define the kind of action to which the observed vicissitudes are due; but the power lately acquired of discerning between the “forced” periods prescribed by orbital relations and the “free” periods determined by fluctuating interior conditions, marks an important step in advance. The circumstance, although unaccountable, should not be lost sight of that eclipsing and non-eclipsing variables diverge somewhat emphatically in the character of their spectra. Those of Algol-stars belong to the first type, often to the helium division of it; those of Cepheid stars are mostly solar; and although enlarged experience may tend to invalidate these generalisations, it may, on the other hand, serve to reaffirm them. The spectrographic study of globular clusters—a task for the immediate future—should help greatly to widen the bases of knowledge regarding this curious relation; and the diligent collection of illustrative instances, by limiting the extent of its prevalence, may throw some light upon its cause.

The secret of short-period variability offers few points open to attack; but an attempt might be made to penetrate some of its outworks by comparing the systemic conditions of steadily lustrous close binaries with those of pairs fluctuating in magnitude. Stars of the same spectral class should, for the avoidance of complications, be chosen for confrontation. Polaris, for instance, θ Ursæ Majoris, λ Andromedæ, are solar orbs revolving swiftly round invisible companions. In what respects, it will be of interest to ascertain, do their orbits differ from those of the variables δ Cephei, η Aquilæ, and ζ Geminorum? Are they markedly less eccentric? Can they be inferred to be more spacious? No real discrimination may be possible, and if so, this line of search fails, and some other must be struck out. But it is clear that only tentative inquiries, pursued untiringly and successively, can avail to bring us nearer to the central truth we are in quest of.

The light-changes of stars are determinate in amount just in proportion as they are accurate in period. Irregularities of one kind are concomitant with irregularities of the other. They reach, in long-period variables, the limit of total disregard of traceable law. For one settled cause of variability a number of consilient causes would seem, in such objects, to be substituted. Should their co-operation break down—should one among them become temporarily ineffective—every vestige of cyclical recurrence may disappear. And it is important to note that stars with disturbed periods merge, by insensible degrees, into stars with no periods at all. Other stars show intermittent periodicity. They start cyclical variations only to drop them after a few recurrences, the alternations being often repeated at indefinitely prolonged intervals. A law of order is present; it has not been abrogated; but its workings are almost neutralised by adverse influences. Then again, stars are found subject to uncertain accesses of light-change. Fits of instability of the most marked kind may be followed by long epochs of constancy, as in the case of P Cygni, and, perhaps we may add, of η Carinæ; and many objects rejected from revised catalogues of variables as having failed to make good their title to inclusion are similarly stars reposing after more or less protracted crises of activity. The relations of stellar fluctuations to time are indeed unlimitedly various. They are, however, characterised by one feature which persists significantly, though far from immutably, notwithstanding the disguising effects, in Mira-variables, of accelerated, retarded, and duplicated phases. This is the more rapid increase than decrease of light. It is strongly pronounced in Cepheid and cluster variables; it asserts itself with modifications in variables of long period; in the sun it is never obliterated by the supervening irregularities of the spot-cycle. This wide range of agreement hints at some deep-seated principle of community, undefinable at present, and intangible to our mental grasp, yet lying at the root of the diverse phenomena of stellar light-change.

Their specific association with particular spectral types is a fact replete with meaning, no less than the exemption from variability, complete or partial, of whole classes of stars. Taking first the negative cases, we find the Wolf-Rayet family characterised by exceptional stability. Not one of its five-score members, so far discovered, shows the smallest symptom of luminous variation. Nor are their congeners, bright-line helium stars, ordinarily affected by it. Those that are so affected betray qualities in some way unusual. Among them are to be reckoned nearly all temporary stars, the singular short-period variable in Lyra, η Carinæ, and P Cygni. Nebulous stars are not as a rule variable. The Pleiades shine with approximate constancy; the star-groups at the hearts of the great Orion and Trifid nebulæ give no conspicuous sign of fluctuation;[798] and isolated stars with nebular appurtenances are, for the most part, steady light-givers. T Tauri, however, makes an exception; and disappearances, transient or lasting, of certain minute stellar denizens of planetary nebulæ have been, from time to time, asserted or surmised. White stars of either spectral variety are scarcely ever intrinsically variable. As spectroscopic doubles they are indeed subject to obscuration by eclipse; as telescopic doubles, to slight and capricious fluctuations in the magnitudes of one or both components, disconnected, apparently, from any of their orbital circumstances. Only three hydrogen or helium stars are known to be variable in any true sense; these are u Herculis, U Geminorum, and R^2 Cygni; and the rarity of this association of qualities makes it especially desirable that their spectral distinctions should be scrutinised with the utmost care. Very few second-type stars vary otherwise than extrinsically. Nor do they, that we yet know of, undergo eclipses. Yet many are binaries of the Cepheid kind, and thus become compulsorily variable. The solar quality of light must be regarded as highly favourable to steady radiation.

Just the reverse is true where banded absorption comes in. Stars of the third and fourth spectral types are not only pre-eminently variable, but they vary almost without exception irregularly, or in long periods. No spectroscopic binaries,[799] no Cepheid or Algol-stars have been found among them. In the character of their light-change, the two classes of stars with banded spectra are indistinguishable. But their spectroscopic study has brought into strong relief the different conditions of gaseous incandescence prevalent in each. In fourth-type stars it keeps aloof from fluctuations of brightness; the same vivid lines gleam in variables and non-variables; nor do they seem to kindle or fade in correspondence with the general light. Those that are visible in third-type stars, on the contrary, are an exclusive and unfailing mark of extensive variability, and attest, by a sympathetic course of development, their intimate connection with its processes. All this teaches us that the processes in question have their seat in limited regions, and are in a measure detached from the action going forward in other parts of the same stellar structures. Now variability of the kind prevalent in stars with banded spectra (as we know from certain of their spectral traits) originates in, or immediately above, the photosphere. Remote from the photosphere, accordingly, must be the location of the emitting gaseous strata in fourth-type stars, since the rays emanating from them shine undisturbed by the progress of the most conspicuous luminous vicissitudes. Hence the obvious deduction that these are conditioned by internal peculiarities. The brilliancy of stars depends, cœteris paribus, upon the rate of transport from within outward. Variability is an index to circulatory changes. In addition to this immediate cause, collateral influences doubtless come into play—forces which co-operate and combine, or mutually nullify one another, but so obscurely to us that there can be no profit, in the present state of knowledge, in prosecuting what could be no more than a conjectural inquiry.

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XXVII. IRREGULAR STAR CLUSTERS

Stars associated together into communities are probably subject to mutual influences of special kinds. With the relative movements produced in them by the stress or pull of gravity we are not here concerned; they form an extraordinarily interesting subject for future inquiry, but as yet no hint of their method is derivable from the scanty materials at hand. The physics of clusters, however, falls within our scope, and is a topic more immediately accessible. It is open to discussion chiefly in two ways—by studying the spectral peculiarities, and the luminous variability of the component objects.

It is not always easy to distinguish between a casual “sprinkle” and a true cluster. The Pleiades are the only family of stars which shows a wandering tendency; the rest are adscripti glebæ; they have no common drift by which they could be set apart from casual inmates of the same sky-region. There remains the argument from probabilities of distribution, with its indefinite variations of conclusiveness according to the circumstances of each particular case. Yet it usually suffices for conviction. None, at least, of the five hundred registered clusters present any real ambiguity of character, although many other groups doubtless subsist unrecognised because poor in numbers and loosely scattered.

Two varieties of stellar collections can be readily discriminated. One is characterised by a spherical form; the constituent bright points press inward towards a centre; they aggregate into “globular clusters.” Those termed “irregular” appear to be constructed on a different plan. Very slight traces of interior condensation are perceptible in them; they are made up of star streams, branches, and spirals, more or less closely intertwined and commingled. A glittering assemblage in Gemini (Messier 35) has an obviously radiated structure. Lord Rosse was struck with wonder at the arrangement into loops and arches of the stars in M 37, a similar object in Auriga. Still more definite and amazing are the “patterns, consisting of lines, wreaths, and curves of stars,” in Dr. Roberts’s photograph of a cluster in Cassiopeia (N.G.C. 7789), taken with an exposure of ninety minutes, 26th November 1892. The effect of a marshalled array is irresistible.

Interior vacancies seem correlative to a streaming conformation. They perhaps represent spots denuded of their bright inhabitants by the action of some unknown expulsive force. Irregular clusters, at any rate, are often remarkably perforated or furrowed. Some are rendered, by the development of “dark lanes,” essentially bifid or trifid. The well-known star-throng in Antinous (M 11) is broken up by partial clearings in a manner suggestive of eventual disruption. The breaches in the masonry (so to speak) are finely shown in a photograph taken by Dr. Roberts, 10th August 1896, reproduced, by his kind permission, in Plate XIX. M. Fenet, from an earlier Crowborough plate, mapped 395 components of this cluster, which includes altogether about 1200, and remarked that most were entitled, by the close attendance of satellite-stars, to be regarded as forming multiple systems.[800] Their collection into seven or eight separate allotments was evident to him, and could not, he thought, fail to become further accentuated with time. All, nevertheless, yield apparent allegiance to a ninth-magnitude star, which fully sextuples the brightness of any of its followers. A spectroscopic examination of this object would be desirable. The general quality of its light might be readily ascertained, and the detection of motion-displacements need not be despaired of. A catalogue of two hundred members of this brilliant assemblage, referred to their leader, was drawn up in 1870 by F. R. Helmert, and compared with measures executed by Lamont in 1836–39.[801] The agreement was complete within the limits of probable error; no discrepancies betrayed shiftings of relative position, although sure provision was made against their unnoticed occurrence in the future.

Helmert considered several of the stars observed by him to be slightly variable; but their changes made no show on the Arequipa plates of the cluster. Since, however, they were taken at an interval of a few days, only variables of short periods or rapid vicissitudes could have been disclosed, and none such, it is safe to say, are present. Yet there is some probability that a temporary member was added to the group upwards of sixty years ago. On the 12th of August 1839, Lamont entered No. 9 on his list as a “new star”; he had not perceived it before, and on the ensuing 9th of September it was gone. Helmert did not expressly look for it, but could only have missed seeing it through its extreme faintness.[802] Indeed, it was most likely, by that time, as hopelessly extinct as Nova Andromedæ now is. The stars of M 11, although devoid of nebulous attachments, are shown in one of Professor Barnard’s small-scale photographs to form a knot at the margin of one of the great cloudy formations in the Milky Way, their actual nuclear relation to which he regards as “hardly questionable.”[803] This may be; the supposition is plausible; yet it is a long way from being demonstrable. Meanwhile the practical inquiries to be made in connection with the cluster are these two: What is the spectrum of its leader star? and, Do any of its components vary in light?

The lovely double cluster in Perseus (N.G.C. 869, 884) resembles it in being non-nebulous, perhaps also in possessing galactic affinities. The twin groups are known respectively as h and χ Persei. Their connection is remote; dynamically they can scarcely be mutually dependent, but they are of closely analogous construction, and their components are similarly linked into festoons and spirals.[804] None of them can be perceived to drift, absolutely or relatively. This was put to the test in 1884, when 172 stars in “χ Persei” were photographically determined by O. Lohse for confrontation with the results of Vogel’s micrometrical measures of them fourteen years earlier.[805] Their seeming immobility will probably be maintained for many decades yet to come. Vogel’s special catalogue of the thirty brightest among them (all above the tenth magnitude) might, nevertheless, usefully be revised, from the photometric point of view, for the purpose of detecting possible alterations of brilliancy. The task would be the more hopeful that some of the objects in question have a note of colour. A “ruby” star was allotted a central position in χ Persei by Sir John Herschel;[806] the Parsonstown reflectors displayed rosy, yellow, and bluish tints in many of its sparkling associates; and Mr. Espin, much more recently, located in the outskirts of the cluster eight reddish stars with fluted spectra.[807] These may be expected to prove more or less variable, though not in the prompt and definite fashion prevalent in globular assemblages.

Professor Barnard recognises two varieties of irregular clusters—the purely stellar and the nebulously stellar.[808] They can be distinguished with certainty only by chemical means. Long photographic exposures are needed to test satisfactorily the condition of grouped stars. The characteristic nebulosity of many clusters may in fact be counted one of the most important discoveries made with the assistance of the camera. But at present we are dealing with the non-nebulous kind, such as the Hyades and Præsepe in Cancer.

Aldebaran, the great red “eye of the Bull,” is in, but not of the Hyades. The disconnection will be rendered obvious, and the relative drift more precisely definable, when the radial movements of the several stars can be spectroscopically fixed. This, indeed, is already feasible, were it not that the great telescopes of the world are otherwise occupied.

Forty-five stars in Præsepe have been located with rigid accuracy. Winnecke’s observations of them in 1858, Asaph Hall’s in 1870, above all, Schur’s Catalogue for 1875,[809] laboriously constructed from a triangulation with the Göttingen heliometer, ensure the detection of their future movements. They are extremely minute. In the course of thirty-two years they produced effects so small as to be barely determinable. The spectra of ninety members of this stellar family were photographed in or about the year 1896 at Harvard College.[810] Owing to their faintness—they ranged from 6·5 to 9·5 magnitude—the details of their classification remained uncertain; but the one main fact was brought out that the collection is of mixed quality. It does not seem to be expressly assorted in any way; no spectral pattern can be called predominant. Twenty-eight of the ninety associates were recorded as Sirian, sixty-one as solar or intermediate stars, and the third type was represented by a solitary specimen; that is to say, the percentage of first-type spectra in Præsepe is thirty-one, while it rises to sixty-five in the Pleiades, and sinks to fifteen in Coma Berenices. In this last asterism, however, the stars, although crowded, are not clustered.[811] Yet their crowding obtains significance through Professor Pickering’s notice of their almost exclusively solar character.

A spectrographic survey, carried out at Harvard College, of four southern clusters of the coarse-grained or irregular description, has lifted another corner of the veil from this department of astrophysics. One of these objects, still uncatalogued, is situated in the neighbourhood of η Carinæ; a second (N.G.C. 3523) is not far off; the third and fourth (N.G.C. 6405 and 6475 = M 6, M 7) are found in Scorpio. The plates exposed showed in the aggregate 705 spectra capable of characterisation, of which 576 were unmistakably of the first type.[812] The average proportion, then, of hydrogen stars in these four clusters is 82 per cent. Half a dozen helium stars were identified in the anonymous group, none in the rest. On the whole, spectral uniformity may be considered the ideal state towards which most clusters tend; it would perhaps, if accidental components could be eliminated, prove to be more nearly realised than it seems. In this connection it is of interest to note that M 37 in Auriga consists wholly of yellow stars, presumably belonging to the solar class. The sky, in Admiral Smyth’s phrase, appears in that spot as if strewn with gold dust. The companion cluster in Gemini is, on the other hand, resplendently white; and Dunlop registered at Paramatta a bluish globular cluster (N.G.C. 6723) likely to be packed with Sirian stars.

The jewel-cluster about κ Crucis is differently organised. Harmonies of contrast rather than of consonance may here be observed; but the collected brilliants display their various tints effectively only in the fields of large telescopes. From measures of 130 of them, Mr. Russell of Sydney derived in 1872 ostensible evidence of comparatively rapid interstitial movements during the thirty-five years elapsed since the date of Sir John Herschel’s corresponding work; but until a fresh set of determinations gives assurance that they are pursued systematically, little weight can be laid upon discrepancies otherwise explicable. His suspicions of variability in twenty-five components have not so far been verified. Several are bright enough to show distinctive spectra, the nature of which it would be particularly interesting to ascertain. This beautiful object lies near the northern border of the “Coal Sack.”

Irregular clusters obviously form systems of extreme intricacy. They cannot be pieces of mechanism set in action by some uniformly operating motive power. Their aspect is in most cases irreconcilable with the hypothesis of a dynamical equilibrium. Few, if any, betray by movement or conformation the influence of a preponderating centre of attraction. They rather suggest inconceivably complex aggregations of partial systems bound together loosely nor perhaps indissolubly. The investigation of their mutual relations will tax the resources of the old as well as of the new astronomy. Some of the problems to be confronted have just begun to take shape, others loom on a remote horizon. As a prelude to dealing with them, the separation might gradually be effected of the really physical from the merely optical components of clusters. The process will be greatly facilitated by the ready help of the camera; and the slow evolution of telescopic or tangential displacements can already in part be forestalled by the spectroscopic disclosure of radial velocities.

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XXVIII. NEBULOUS CLUSTERS—THE PLEIADES

“Tangled in a silver braid” of shining world-stuff, the Pleiades stand out as the typical nebulous cluster. They give signs of not being indefinitely remote. The assembled stars have a common drift, which is most likely a perspective effect of the sun’s advance in the opposite direction. If this be so, their light spends just 200 years in reaching the earth, the rate of our progress towards the constellation Lyra being taken at twelve miles a second. Alcyone then radiates at the very least 190 times more powerfully than our sun; in its place, Sirius would appear fainter than the fifth magnitude; it would be outshone, not only by the lucida of the group, but also by five of its companions—by Atlas, Merope, Electra, Maia, and Taygeta. Thus the glory of the Atlantids would be but slightly enhanced by the addition of the great Dog star to their number, and the scale of the system must be commensurate with the magnificent luminosity of its members.

A beginning has been made in the discrimination of the genuine Pleiades from their optical companions. The definite character of their proper motions has made actually feasible what in other similar collections is only remotely possible. The outcome of Elkin’s measures with the Yale heliometer in 1884–85[813] was to distinguish forty-five stars, including Alcyone, as inseparable travellers, while eight proved their independence by dropping out of the ranks. This group of forty-five members may be regarded as a nucleus round which additional stars will aggregate as their movements develop. How far it will extend, how many of the small stars swarming on long-exposed negatives it will eventually take in, remains conjectural. Its delimitation, however, should be practicable in the course of a decade or two; for the comparison of photographs taken about 1915 with those of 1885 and 1888 may be expected to bring numerously into view relative displacements consequent upon the abandonment, as it were in mid-ocean, of a multitude of pseudo-Pleiades exempt from the drift belonging to the true cluster. There is reason to think that this process of expulsion will have to be carried far. The self-selected assemblage will probably not be overcrowded. Exact numerical inquiry has led to many unexpected results, but to none more surprising than that of the thinning-out of faint stars within the area of the Pleiades. Professor Bailey counted nearly 4000 on a photograph including it taken with the Bruce twenty-four-inch lens in 1897; but their density, as a detailed examination made evident, fell off notably and systematically inside the precincts of the system. “It therefore appears,” Professor Pickering wrote,[814] “that the total number of stars in the region of the Pleiades is actually less than in adjacent portions of the sky of equal area, and it is much less than the corresponding number in many parts of the Milky Way.” Regarded, then, as a physical entity, the cluster includes only the brightest of the spangled points thrown together into the field. The spangles of the background would indeed presumably be still more numerous but for the absorbent effect of the nebulous masses attached to the brilliant stars in front of them. Their paucity, at least, must be somehow accounted for, and this explanation of it, suggested by Professor Pickering, seems admissible. M. Stratonoff,[815] too, was led, by a study of stellar distribution in their neighbourhood, to the conclusion that the physical associates of Alcyone are comparatively few; and the conclusion is the more interesting from the sure prospect of bringing its truth to the test.

The Pleiades might be described as not merely a nebulous cluster, but as a cluster of nebulæ, so numerous and so sharply characterised are the cloudy forms collected within its borders. All save one are photographic revelations. The exception is the “Merope nebula,” discovered by Tempel 19th October 1859. A mere “breath stain” on the sky, it is, to telescopic vision, a highly elusive object; yet it is always there, striated and definite, when looked for by chemical means, and the hypothesis of its variability has long ago been abandoned. The Maia nebula has something of the same striped aspect, but clings in a strongly curved whorl to the star which forms its nucleus. Mr. H. C. Wilson of the Goodsell Observatory described as follows a photograph of this object taken by him 30th January 1894.[816] “The region about Maia is especially interesting. A very bright horn-shaped patch of nebula runs out from the west edge of the star-image immediately northward, and extends to a distance of 3′ north of the star. The nebula here is full of irregularly parallel streaks similar to those about Merope, but making only a very small angle with the meridian. Some of them run to and beyond the bright stars north of Maia. A series of rather broad and diffuse patches extend from the middle of the group on a diagonal toward the north-west, reaching to a comparatively bright pair of stars in that direction.”

A second Merope nebula, totally unlike the first, was discovered by Professor Barnard with the Lick thirty-six-inch refractor 14th November 1890.[817] It is round, clearly terminated, and centrally condensed, 30″ in diameter, and presents the general effect of a distant comet. With the adjacent star it forms so close a combination as to indicate, almost of necessity, the slow progress of mutual revolution. Mr. Burnham, who measured the new nebula at Lick in the autumn of 1891, regarded it as “one of the most singular objects in the heavens,” and “unique with respect to its nearness to a bright naked-eye star.”[818] It is the brightest nebula in the Pleiades, and came out well on plates taken by Professor Keeler in 1898 with the Crossley reflector.[819]

Another cosmic species singularly exemplified in this cluster might be called “ribbon nebulæ.” They run in narrow, straight bands from star to star, in one case stringing together six or seven, “like beads on a rosary,” and they pursue with fair accuracy, along parallel lines, an east and west direction. What manner of communication they establish between the suns they connect it is impossible to divine. They may conceivably be mere survivals of a prior order of things, belonging rather to the past than to the present; but no structures more curious have been brought to our notice by the camera than these long, luminous highways built as if for the purpose of facilitating intercourse between the cities of space. An unfinished road starts from Electra towards Alcyone; it has been completed over only about one-third of the way. Or is it the wreck of a celestial causeway which formerly reached its destination, but has been gradually, for some ages past, falling out of use and repair? The question is a daring one; ultimately, however, the comparative study of analogous objects may supply hints for answering it, at least by a plausible surmise. The cluster is besides crowded, especially in the neighbourhood of Alcyone, with irregular or nondescript nebulæ, which choke the background as if with rolling fog. But in general the tendency is unmistakable to assume filamentous shapes, such as were shown with peculiar distinctness in two photographs taken by M. Stratonoff at Tashkent early in 1896 with multiple exposures of respectively ten and seventeen and a half hours.[820]

The history of the Pleiades nebulosities does not end here. Professor Barnard had long been aware of a dulling of the sky-ground over a vast adjacent area; and at last, in December 1893, he put these vague perceptions to the test by means of a ten hours’ exposure with the Willard lens.[821] “The resulting picture,” he wrote, “showed a number of singular curved and streaky nebulosities, apparently connected with the Pleiades and extending all about the group.” Some of them he was able to trace for several degrees on either side, especially towards the east. Yet doubts were expressed as to whether spurious photographic effects were not in question. The phenomenon disclosed was, indeed, so amazing that some degree of scepticism was excusable. Its reality, nevertheless, had to be admitted. Confirmatory photographs were produced by Dr. Max Wolf,[822] by Mr. H. C. Wilson,[823] and by Professor Bailey. A skilful drawing by Mr. E. Calvert, embodying the combined results, is reproduced in Fig. 46. But its limits are too narrow to include the whole of these far-spreading formations. There seems no end to them. The interior nebulosities are left undepicted for the sake of clearness. We are thus brought face to face with the “startling fact” (as Professor Barnard calls it) “that the Pleiades and their involved nebulosities are but the central condensation of an enormous nebula, intricate in details, and covering at least a hundred square degrees of the sky.”[824] The magnitude of this mixed system staggers belief and confounds the imagination. We contemplate it with imperfect apprehension of its scope and significance. For the present, the relations between its various parts appear scarcely open to investigation. The possibility even of speculating upon them will offer itself only when acquaintance begins to be made with the spectral characteristics of the Pleiades nebulæ. That they are all alike gaseous may safely be assumed; nevertheless, their dispersed light, when it becomes practicable to examine it, will perhaps offer diversities full of interest.

Among the stars of the cluster a single spectral type markedly predominates. It may be distinguished as “late Orion.” Hydrogen absorption is prominent; helium absorption also asserts itself, but with less emphasis than in the earlier stars of the same class.[825] This quality of light is common to all the principal stars; those that deviate from it are of inferior grades of brightness, and may eventually, through the effects of proper motion, be sifted out from an assemblage to which they do not properly belong. Hence, when the group comes to be organised on a definitive basis, the gross percentage of sixty-five helium stars may have to be raised very much higher. The first example of a mixed hydrogen series was met with in Alcyone. Professor Campbell was astonished to perceive in 1893 that its spectrum, otherwise marked only by absorptive action, included a glowing crimson C. Very remarkably, too, this solitary bright ray is coupled with a dark streak, situated, as usual in cases of duplication, on its more refrangible side. Which is the displaced line has still to be ascertained. Pleione, the only other member of the family showing signs of emission, is spectroscopically akin to γ Cassiopeiæ. Its bright rays bisect obscure bands.[826] The recession at the rate of eight or ten miles a second of the solar from the Atlantid system must occasion a perceptible shift towards the red of all the spectral lines of its members, to which constant element are superadded the varied, and perhaps varying effects of their individual motions. Spectrographic researches hold out, then, the best prospect of gaining, within a reasonable lapse of time, some insight into the working of this amazing piece of celestial mechanics. From a triangulation, executed with the Göttingen heliometer in 1889–91, Dr. Ambronn derived, as he thought, indications of a division of the cluster into several distinct parcels of mutually dependent masses;[827] but his measures, which included only sixteen stars, had too restricted a scope to be decisive of much.

No stars of assured variability, whether periodical or irregular, are found among the Pleiades. Yet light-changes, eluding definite recognition, are suspected to progress. Maia and Merope have both been held to fluctuate slowly; and Atlas, divided into a pair by Struve in 1827, is now single, with all powers, in the serenest skies. Only a twofold occultation, noted by Hartwig in 1876, seemed to intimate the obscure survival of the vanished companion. Possibly it may make itself felt spectroscopically, even should it never again be seen. The motion-displacements, accordingly, yielded by Atlas deserve attentive scrutiny, since from them may be obtained the key to one of the long outstanding enigmas of double-star astronomy. Another member of the group appears to have an authentically variable attendant. In Wolf’s map of 1874 an anonymous 7·2 magnitude star due south of Alcyone is marked as a wide double; it was single in the Paris photograph of 1886, but again double in that of 1888, when the satellite had risen to eighth-magnitude brightness. Yet M. Chevremont, reviewing the collection with a small refractor in November 1895, could find no trace of it.[828] This was the most precisely defined among several cases of presumable light-fluctuation met with in the course of his survey. Again, M.M. Müller and Kempf constructed at Potsdam in 1899 a photometric catalogue of ninety-six Pleiades,[829] forty-two of which, given in the Bonn Durchmusterung as of 9·5 magnitude, seemed to have diminished so considerably in brightness that their mean magnitude could not be placed higher than 10·7.[830] Very little real change, however, may here have been concerned, since the ordering of stars in the lower ranks of the Durchmusterung is known to have been a highly casual process. It can scarcely be doubted, indeed, that some components of the cluster are, in a measure, variable, but their variability is of a kind not easily certified; it follows no method; it obeys no time-prescription; its effects, perceived when least looked for, cannot be counted upon to recur. Hence the problems of light-change set by the Pleiades are of a peculiarly baffling nature.

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XXIX. NEBULOUS CLUSTERS—Continued.

Many nebulous clusters besides the Pleiades are known, but none in which the relations of stars and nebulæ are so highly specialised. In general, the stellar collection seems as if independently organised, and plunged as a whole into an ocean of cosmic fog, without any strong tendency on the part of its members to form individual nebulous attachments or connections. When, however, the structural details of such formations come more fully to our acquaintance, they may be found to contain evidence of a closer association between particular objects of the two species than can at present be vouched for. Such investigations can be conducted effectively only by photographic means and with carefully adapted instruments. For the purpose of bringing out the full extent of the nebulosity, small portrait-lenses have prerogatives, illustrated practically by Professor Barnard,[831] theoretically by Professor Wadsworth.[832] But pictures on a larger scale than those obtained with them are needed for the disclosure of many topographical minutiæ huddled together by the strong concentration due to their short focal length. For securing these, reflectors of powerful light-grasp are unsurpassable. Thus two lines of photographic inquiry should be made to converge upon nebulous clusters; one directed towards determining the limits of the involving nebulosity, the other towards ascertaining its constructive peculiarities.

To a superficial view the object we are now about to describe seems more like a cluster and nebula than a nebulous cluster. The nebula is “Messier 8”; it is just visible to the naked eye, and from the oval vacancies which interrupt its light, it has received the descriptive designation of the “Lagoon Nebula.”[833] The cluster—separately catalogued as N.G.C. 6530—immediately follows it on the same parallel, but not in complete detachment. The two formations unmistakably overlap, and, to discerning vision, inextricably intermingle. Professor Barnard’s negatives, taken with the Willard lens in 1892, showed the compound object to be “a singular mixture of stars and nebulosity. East and west its diameter is about 45′, and north and south some 42′. The southern side is sharply defined and serrated, with three distinct pointed projections. From its north-following corner a wisp of nebulosity extends nearly to a group of nebulous stars, and possibly with a longer exposure would be found to connect with them.”[834] A picture on so small a scale could teach little regarding internal structure; the “lagoons,” in fact, appear in it nearly silted up with diffused nebulosity. Many of the brighter stars are involved in the pervasive haze, and one in particular occupies too critical a position at the edge of “a very black hole” for the supposition of a mere chance arrangement to be permissible. It may be doubted, however, whether the 5·7 magnitude star, 9 Sagittarii, has more than an optical connection with the cluster, and the multitude of stellar points glittering in the background will almost certainly be separated from it by the slow discrimination of drifting movement. A preliminary step towards applying the test was taken by M. Comas Solà in 1898.[835] His photographic triangulation of the group fixes a starting-point for comparative inquiries which can yield tangible results only in the distant future. His plates showed the stars to be doubly implicated with nebulosity. One of the seventh magnitude acts as the focus to a cloudy mass, distinct, apparently, from a diffuse, elongated structure projected upon the central parts of the cluster, which it not impossibly encloses in annular folds.[836] The Lagoon Nebula gives a spectrum of bright lines. It will be interesting to learn whether they are displaced by motion, and whether, if so, corresponding spectral shifts in the clustered stars ratify the presumption of organic relationship between the two orders of formation.

The optical history of nebulous objects is often curious and instructive. As an open cluster, N.G.C. 2239, in Monoceros, was first observed by Sir John Herschel. An adjacent patch of nebulosity, seen by Swift in 1865,[837] again attracted Barnard’s attention in 1883. It proved to be a kind of knot on a great nebulous ring discerned by the latter in its entirety with the Lick twelve-inch refractor in 1889.[838] This encloses the star group like a ring-fence, and may be said to osculate with a second great filmy ellipse, of which only a section is perceptible. Then came the turn of the camera. Professor Barnard obtained a photograph of the complex arrangement 9th January 1894, which confirmed visual impressions while demonstrating their inadequacy. They were not misleading, but extremely partial. The photographed nebula is, in Professor Barnard’s words, “about one degree in diameter, and very irregular in brightness and outline.”[839] It involves the cluster with unequal condensations, which are especially heavy north of the bright stars. The nebulous knots, and the section of a large ellipse, fully depicted in the sketch of 1889, reappeared in the chemical picture. In this, the only effect of annularity left visible is that a vacant interior space seems reserved for the conspicuously grouped stars. Their chief being of the eighth magnitude, the determination of its spectral character should present no difficulty, and would be of particular value as a test of nebulous affinity.

The number of alternative titles by which the star catalogued by Flamsteed as “15 Monocerotis” is known expresses the curious variety of its claims to distinction. It is variable, multiple, and nebulous. In the first capacity it is designated “S Monocerotis.” Its fluctuations from 4·9 to 5·4 magnitude, in a period of 3^d 10^h 38^m, were noted by Winnecke in 1867. These elements are indeed still somewhat uncertain. They need verification and revision. The variable is, moreover, the leading member of a triple combination, enrolled by Struve as “Σ 950.” With a green companion at 2·8″, and a bluish one at 16·6″, it makes an exquisite telescopic object, but gives no sign of orbital motion. It occupies a dominating position in a collection of fifty or sixty stars, ranging from the eighth to the thirteenth magnitude, one of which is subject to fits and starts of extensive variation.[840] By virtue then of its chiefship of a cluster, 15 Monocerotis is registered among nebulæ as N.G.C. 2264. Nor solely on this account. Both Sir John Herschel and Lord Rosse suspected it to be nebulous, and Bruno Peter, who measured forty-five components of the group at Leipzig in 1879–82,[841] found their leader visibly wrapt in a hazy envelope. But the wide extent of its nebulous connections was entirely a photographic revelation. A picture of the region about 15 Monocerotis, taken by Professor Barnard on 1st February 1894,[842] showed it to be involved in a great nebula some three degrees in diameter. “It clusters densely,” he says, “about the groups of stars, and then spreads out in a weak, diffuse light, with rifts in it, and irregularly terminated along the edges of a vast vacancy in the Milky Way. The condensation, which is very strong, is not at 15 Monocerotis, but twelve minutes south-preceding that star, where it becomes a compact mass with numerous wisps and holes in it.” The absence of nebulous concentration about the individual stars struck him forcibly, especially by contrast with the different state of things in the Pleiades.[843] The nebulosity is free and general; no single member of the stellar assemblage appropriates a special share, or carves from it an appendage of its own. Yet it is difficult to doubt that the apparent association is real and physical.

The Wolf-Rayet spectrum is, broadly speaking, reversed in that of 15 Monocerotis. It includes both the hydrogen series and the upper “blue” band; metallic lines scarcely appear, but those of oxygen, nitrogen, and silicon are unlikely to be absent. About the peculiarities of the less refrangible section nothing is yet known. Here, perhaps, symptoms of emission will be found; indeed, a bright C might be looked for with success not only in 15 Monocerotis, but in some one or two of its principal associates.

An eighth-magnitude star in Auriga was noticed by Auwers about 1860 to be projected on a hazy disc (N.G.C. 2175). The object is placed centrally in a group of smaller stars, enveloped in nebulosity strongly manifested in Professor Barnard’s photographs of 1894.[844] The combination is quite similar to that just described in Monoceros. The spectrum of N.G.C. 2175 was examined by Professor Keeler with an indecisive upshot.[845]

Sir John Herschel observed at the Cape “a very remarkable object” in the Milky Way, where it crosses the tail of Scorpio. It showed to him under the guise of “a decided, tolerably defined, semi-nebulous mass, with abundance of very small stars forming altogether a telescopic Magellanic cloud. It fills about a field, and has branches and sinuses.”[846] This miniature Nubecula (N.G.C. 6437) invites photographic delineation. Long exposures may disclose in it constructive particulars of extreme interest.

Nebulous clusters are connected, by insensible gradations, with certain tracts of nebulosity in the Milky Way, which, since they are situated in, or in the line of sight with a stellar stratum, necessarily appear more or less densely star-strewn. The stars strewing them do not, however, collect into groups capable of individualisation; hence they cannot be termed “nebulous clusters.” These suggest some kind of organisation; they are more or less isolated and coherent entities, and are distinguishable as such from layers and beds of stars, however closely packed. Not that a clear line separates the two kinds of formation; the multiplicity of the heavens is too great for this to be possible; but it is well to maintain differences ideally, even though they be blurred, here and there in the concrete, beyond our perplexed powers of recognition. Nebulous clusters, on the other hand, shade off, as their members become fewer, into nebulous groups such as the Orion Trapezium, and reduce, “in the limit,” to simple nebulous stars. These will be the subject of a future chapter.

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XXX. GLOBULAR CLUSTERS

There is no possibility of failing to recognise in a globular cluster a true agglomeration—a structure teres atque rotundus. The systemic unity of such objects is as evident as that of a “globe of dew,” though it is by no means certain that they are not, like that “frail and fading sphere,” in course of more or less speedy evaporation. A gradual process of ejection is at least suggested by their streaming edges and filamentous appendages, formed of branching rows of stars, apparently on the move outward. One hundred and ten globular clusters were registered by Sir John Herschel in 1864 in his “General Catalogue” of nebulæ, and not many have since been identified. They are astonishing constructions. Their silvery radiance is a delight to the eye; the imagination is allured by their visionary beauty; reason is startled by the recondite nature of the problems they intimate. What, we cannot but ask ourselves, is the true nature of these mysterious “balls of stars”?[847] Are the luminous particles composing them suns in the proper sense? What are their mutual relations? How did they originate? In what are they to eventuate? Can mechanical stability be claimed for them, or must they be supposed to form temporary societies undermined by forces tending towards dissolution? On all these points definite information is still lacking; but there is no reason to despair of its future provision, since the inclusion of globular clusters within the scope of organised research is of quite recent date, and knowledge respecting them is accordingly in a nascent stage.

It may, however, safely be affirmed that their components are sun-like bodies—that they are spherical masses at an enormously high temperature, radiating into space by means of suitably adapted photospheric apparatus. Some are intensely actinic. They are much brighter chemically than visually. They hence presumably emit light mainly of the shorter wave-lengths, and are abnormally hot bodies. But the secret of their nature cannot be divined in our present ignorance of their spectroscopic peculiarities, and those of the individual star-points in clusters will long remain inaccessible. Their combined light, nevertheless, where it is concentrated into a “blaze” at the core, is capable of being effectually analysed with powerful instruments, and the determination of its quality is a sine quâ non for progress in this branch. Until this has been effected, there is no possibility of assigning to globular clusters their proper place in the celestial hierarchy.

The great southern agglomerations, ω Centauri and 47 Toucani, seem almost untouched by the wear and tear of time. They show few signs of dilapidation. No dusky rifts, no glades or clearings are perceptible in them; the subtraction of material (if it be going on) has made little progress; they are as yet well compacted to the centre. Nor is a flow of stars outward hinted at unless obscurely. They are cleaner at the edges than most objects of their class; tentacular appendages are wanting; their components are not visibly in marching order. This may mean that they have but newly arrived at their present state of being, and if so, their spectra should be of an early type. To the component stars, accordingly, a very low mean density may probably be attributed; their attractive power will prove small relatively to their light; the corresponding interstitial movements must be slow, and will be difficult of detection.

Just within the northern border of the Milky Way ω Centauri is visible to the naked eye as a hazy star of the fourth magnitude.[848] Telescopically it presents a grand aspect. Nothing more strikingly effective can be imagined than the transformation, by optical means, of a blurred light-spot into a glittering and multitudinous assemblage of separate suns. Nearly 6400 can be distinguished on sensitive plates, besides a crowd of others so small as to merge together into a grey mottling. No true nebulosity seems to be present. The photograph from which the counts were made by Professor and Mrs. Bailey is reproduced in Fig. 47. It was taken at Arequipa 19th May 1893 with an exposure of two hours in the thirteen-inch Boyden refractor. The area marked out for investigation covered 900 square minutes. It was “fairly well filled” with stars, and their statistical study defined the fact of their true central crowding.[849] They condense not merely in appearance, through the augmenting depth of spherical space in which they are distributed, but also as a consequence of actual compression inward. The number of stars per square minute was found, in fact, to increase in arithmetical progression with approach to the centre. Away from it the diminution ceased, a constant figure being reached along a line taken to be the boundary of the cluster; and this constant, since it must represent the areal population of the general sky, supplied a means of correcting the apparent results for that of the cluster. Deduction was accordingly made of 1616 unconnected stars, mere visual intruders from a limitless background; and there remained 5050 true components, collected within a circular space somewhat larger than that occupied by the full moon. They average about 12·5 magnitude.

The south polar cluster, 47 Toucani, is of equal loveliness with ω Centauri, although on a smaller scale. Its “computed diameter” is 22′[850]—that is to say, the extra stars which it projects upon the sphere die out completely at a distance of 11′ from its centre. They number about 2300, and are distributed in accordance with the same law noted as prevalent in the companion cluster. They are, however, even more densely aggregated; the realm is less spacious than ω Centauri proportionately to the throng of its inhabitants.

The great cluster in Hercules (M 13) presents a less uniform texture than its prototypes in the southern sky. Yet its constituent orbs follow virtually the same gradient of central compression.[851] Their distribution is affected besides by influences of an unimaginable kind. Three “dark lanes,” making an equiangular junction at a point south-east of the centre, were detected with the Rosse reflector in 1850.[852] An identical form of marking tends to recur in other parts of the cluster. On the Lick photographs of 1890–91, Professor Holden was able to trace no less than thirteen repetitions of it.[853] This insistence, he remarked in a local paper, made it evident “that a definite law was acting to produce this form, and that this law might be truly taken as representative for this cluster. In some way there are dark lanes produced and maintained among the hundreds of bright stars in this globular mass, and there are many such channels. How can we conceive of such a system? It is tolerably clear that either the dark lanes are absolutely empty of matter, or at least that they are empty of luminous matter.” Yet neither of these alternatives seems to be in accord with fact.

A plate exposed during ten minutes with the Crossley reflector by Professor Keeler showed all the brighter stars in M 13.[854] Two hours were, however, needed to bring the swarms of their faint associates into view. In all, more than 5400 stars, fairly within the precincts of the cluster, were counted on a negative taken 13th July 1899. A study of their distribution, made by Mr. Palmer, Professor Keeler’s assistant, elicited some noteworthy peculiarities.[855] The components separated, speaking broadly, into two distinct orders of brightness, those of intermediate magnitudes being comparatively scarce. Out of the total number of 5482 counted on the plate, 1016 were classed as bright, 4466 as faint, or below 13·5 magnitude. Now the mode of scattering of these two radial—they extend outward in curved rows; that of their minute companions is more nearly globular. Moreover, the characteristic dusky tracks are vacant only as regards the former class of objects. They are lightly strewn with the diffusive star powder found everywhere in the cluster. To its presence Mr. Palmer attributes the effects of nebulosity noticed in earlier photographs. It requires a very high resolving power to distinguish the stellar haze created by flocks and throngs of sixteenth-magnitude stars from genuine “fluid haze”; but in this case there seems little doubt that the feat was performed. Confirmatory evidence of the best kind was afforded by Professor Barnard’s direct observations with the forty-inch Yerkes refractor. They convinced him that globular clusters are non-nebulous formations.[856] Spectrographic impressions will probably before long add their testimony, at least in a negative sense.

With the thirteen-inch photographic refractor of the Potsdam Observatory, Dr. Scheiner obtained, 9th September 1891, the first plate of M 13 on which the stars were sufficiently defined for exact measurement.[857] He accordingly prepared a catalogue by which the places of 833 were fixed with the utmost accuracy; and these fundamental stars, henceforward kept under watch and ward, will perhaps one day disclose the plan of their movements, and thus enable future astronomers to attack, with some possibility of success, one of the most arduous problems in celestial dynamics. It confronts them, under a still more bewildering form, in a superb cluster in Scorpio (M 62 = N.G.C. 6266). Here a second focus of condensation is obvious; two star-globes are fused into one.[858] The contorted growth of a twin cherry may help us to realise, however imperfectly, the attendant indefinite complexity of the conflicting forces.

A compressed cluster in Serpens (M 5 = N.G.C. 5904), discovered by Kirch in 1702, presents telescopically the appearance of a softly radiant globe with divergent outliers. A photograph taken by Dr. Roberts 25th April 1892, indicated a nebulous interior; but no such effect came out on negatives exposed with Dr. Common’s five-foot reflector, nor was it perceptible visually to Professor Barnard. It was due, presumably, to an amalgam of faint stars forming a kind of matrix for those bright enough to be individualised. The discovery of their variability was begun by Mr. Packer in 1890.[859] He remarked the fluctuations of two components, and Dr. Common suspected many more to share the same character.[860] This premonition was followed up, though not until after five years, by Professor Bailey’s announcement[861] that many globular clusters—say one in five—are veritable nests of variables. Their abundance is such that as many as a hundred—in Professor Barnard’s words—“have been found in a space in the sky that would be covered by a pin’s-head held at the distance of distinct vision.”[862] Of 3000 components of ω Centauri examined within a radius of 22′, 128 were found to fluctuate to the extent of half a magnitude or more.[863] In one case a range of five magnitudes was observed; but that of most of the objects investigated was limited to one and a half, or two magnitudes. Very short periods are the rule; three are of less than seven hours; yet one is protracted to 475 days, and there will be a special interest in determining the nature of the light-change comprised in so long a cycle. A true “Mira variable” would seem an anomaly in a cluster made up of silvery white stars; since we are taught by experience to associate periods of many months with strong absorption and consequent redness of colour. But cluster-variables belong, for the most part, to a type apart, the character of which has been described in an earlier chapter. They have long minima, and brief maxima attained with extraordinary rapidity. The activity of their changes, when they set in, contrasts singularly with the completeness of their suspension during the intervals of rest. There is an entire absence of concert among the affected stars. Each is an independent, self-regulated phenomenon. No more curious spectacle is afforded by the heavens than that of a throng of seeming signal-lights waxing and waning every few hours under the sway, obviously, of some common law, yet with no trace of unanimity; some fading while their neighbours are on the rise, others stationary and semi-extinct, though only biding their time to enter upon a phase of renewed brilliancy; and none deviating by a hair’s-breadth from the course of change individually prescribed for it.

Eighty-five variable stars have up to the present been recognised in Messier 5, and they agree, for the most part, quite closely in a mode of fluctuation elucidated by Professor Bailey’s persevering inquiries.[864] The form of their typical light-curve is not different from that assignable to the components of ω Centauri; but they tend unmistakably to obey a common period of approximately twelve hours, and the oscillations of nearly all are between the fourteenth and the fifteenth magnitudes.[865] Yet they are not executed simultaneously; a congruity of epochs is not even distantly indicated. Professor Barnard took visual charge in 1898 of some half a dozen of these strange objects,[866] and his list included Packer’s original variables, Nos. 42 and 84 of the Harvard enumeration, stars exceptional in the great assemblage of which they form part, both as to the length of their periods and the manner of their change. This copies the pattern set by δ Cephei; it proceeds continuously, although not symmetrically, along a curve steep in its upward branch, sloping gradually downward, and interrupted by a “hump,” significant of an abortive second maximum.[867] Its time-measure is about twenty-six days. Professor Barnard was struck with a number of ink-black holes rending the brilliant surface of the star-globe in Serpens. They are closely adjacent to the dense central portion, and suggest tunnelling operations on the scale of those progressing in the great Hercules cluster.

A cluster in Canes Venatici (M 3 = N.G.C. 5272) is similarly perforated. Lord Rosse observed “several small dark holes” at its core, from which “rays run out on every side.”[868] A “bifurcated dark lane” was, moreover, perceived in the northern segment of the nuclear “blaze.” From which we can gather that the distribution of stars in M 3 is controlled by forces of the same nature as those ruling in M 13. Yet the two clusters are markedly differentiated as regards light-stability. That in Canes is already known to contain 132 variables; while in the Hercules group diligent inquiry has failed to certify the presence of more than two, its components actually shining much more steadily than the average of the stellar multitude outside its limits. Eighteen hundred stars were counted by M. Orbinsky from photographs of M 3 taken at Pulkowa in 1894,[869] and his measurement of their places will in the future supply a test of their relative mobility.

A starry sphere in Pegasus (M 15 = N.G.C. 7078) seemed nebulous in Dr. Roberts’s photographs;[870] but they were perhaps clouded by stellar dust, not by true cosmical fog. Of 900 members of this collection examined at Harvard College, 51 proved variable. The proportion in Ihle’s great cluster in Sagittarius (M 22 = N.G.C. 4424) is much smaller—16 to 1550; and only 10 among 600 stars tested for stability in M 2 gave responsive signs of fluctuation. This cluster, which is situated in Aquarius, might be the twin of that in Hercules plunged in a deeper depth of space.[871] The clusters ω Centauri and 47 Toucani, so much alike in other respects, deviate widely in the matter of variability. Periodical stars by the score occur, as we have seen, in the former stately assemblage; in the latter, only six have been registered, notwithstanding the most careful scrutiny. Whence the diversity? Professor Pickering[872] surmises that it depends upon the relation of a common plane of revolution to the line of sight. Each globular cluster would be, on this view, a system, the movements of which, whether axial or orbital, are conducted on the same level. And should this level happen to coincide with the visual ray, variability would result, either through the rotation of such components as possessed unequally luminous surfaces, or as a consequence of the eclipses of those provided with closely revolving satellites, Yet neither rationale of light-change can, without grave misgivings, be admitted. Suns with dusky hemispheres, or permanently spotted, may be treated as mathematical fictions. Nor is any evidence as yet forthcoming that genuine eclipse-stars ever find a habitat in clusters. Certainly none of the cluster-variables so far investigated can be accounted such.[873] That they are rapid binaries may be plausibly surmised, but they must be of the non-occulting sort. In eclipsing stars the maximum is essentially permanent; the minimum is accidental. In cluster-variables opposite conditions prevail. Habitually obscure, they brighten incidentally.

Professor Bailey’s discovery throws open a spacious field of research. Each variable cluster might well claim a sentinel appointed for the exclusive following of the complex, elusive, and rapid changes which ceaselessly develop within its compass. In December 1901, 509 components of star-globes were reckoned as periodical. Every one of these is perhaps a system apart; every one has its peculiarities, the inner meaning of which can only be drawn out by sustained attention. Powerful instruments are, moreover, required. The objects in question lie near the limit of practicable observation; work upon them taxes modern resources to the utmost. Nor can the photographic method alone be relied on. Long exposures are needed to show the stars at all, and they can naturally give no more than the “mean magnitude” during the intervals they cover. But when these intervals bear a large proportion to the entire period of change, such coarse-grained data cannot satisfactorily represent the manner of its progress. The resulting light-curve, as Professor Bailey says,[874] is always smoothed down; and it is smoothed to the limit of a straight line, in the ultimate case of the exposure equalling the period of a star’s variation. Hence the absolute necessity for supplementary visual determinations to fill out the peaks and corners rounded off by the camera. At the critical epoch, when the flash is being turned on, every minute counts. Estimates of brightness at the rate of ten or twelve an hour are not too numerous for the purpose of keeping guard over the swift alterations going forward. The hourly or two-hourly averages given on sensitive plates are wholly inadequate.

Two questions of fundamental interest present themselves in connection with the variability of clusters. Why, we must ask, are the stars in one globular assemblage luminously unstable, while in others, its strict analogues, they shine quite steadily? The contrast is not explained by any visible difference of constitution. Variability does not appear to come in at any particular stage of growth; it is not associated with a definite situation in the heavens; it does not characterise pierced and outworn globes preferentially to compact ones, or vice versâ; it can, in short, be correlated with no feature obvious to direct notice. It remains to be seen whether any spectroscopic peculiarity corresponds to it.

Again, the absence from, or extreme scarcity of Algol-stars in clusters occasions perplexity. We are led to believe that rapid variables are, in truth, binaries revolving in the light-period. But if so, the orbital planes of a certain proportion of them ought to pass through the earth, with the outcome of affording us the spectacle of so many occulting pairs. If these do not exist, we shall be forced to conclude that cluster-variables owe their punctuality to some other cause than the strict time-keeping of satellites.

One of the most signal services rendered by photography to astronomy has been in the facilities supplied by it for the measurement of star-clusters. The relative positions, especially of the components of compressed groups, can only with extreme difficulty be established by direct triangulations; while every negative taken of them fixes their configuration at a given epoch, and gives the means of determining its changes, should they occur. Thus the places of sixty-two stars in M 5, catalogued at Harvard College in 1897, were estimated by Professor Pickering to be of so high a degree of accuracy that annual displacements amounting to one-hundredth part of a second of arc can be detected by their comparison with results similarly obtained from plates taken a few years hence.[875] The foundations have then been laid for an extensive superstructure of knowledge, as regards both the physical and the dynamical condition of globular clusters; yet centuries may elapse before it becomes possible—in Kepler’s phrase—to “think over again,” with apprehensive minds, those wonderful “thoughts of God.”

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XXXI. WHITE NEBULÆ

“White nebulæ”—so called by Professor Young[876]—are those giving continuous spectra. They are in an immense majority. They are reckoned by thousands, or tens of thousands, gaseous nebulæ by the score. True, very little progress has been made with their actual spectroscopic examination, the faintness of their rays forming, in general, an insuperable obstacle to their analysis; but their shape and aspect supply indications, rarely misleading, as to the quality of their light. That of elliptical and spiral nebulæ is, to the best of our knowledge, always continuous; and with these may be classed the round, centrally-condensed objects which abound in every nebular region of the sky. Several other varieties of this great sidereal family might be indicated, but they are by comparison scantily represented, and have been but little investigated. The paragon of white nebulæ is the grand ellipse in Andromeda. No other is visible to the naked eye; it should be, judging by appearances, much the nearest to the earth of the whole tribe; its structure is splendidly definite, and profoundly significant; its spectrum shows peculiarities challenging inquiries which must be long-continued and arduous, but promise results of far-reaching importance. In January 1899 Dr. Scheiner,[877] employing a small spectrograph in combination with a mirror of nearly thirteen inches aperture, and only forty inches focus—an apparatus specially adapted for dealing to advantage with objects of extended surface—obtained in seven hours a legible spectrograph of the nebula. The indications gathered from it were of a most surprising kind. Dark rays were perceived to interrupt the continuous light, and they seemed to agree with the Fraunhofer lines in the solar spectrum. The Andromeda nebula was accordingly inferred to be a genuine cluster of solar stars; but this conclusion is very far from being securely established. No bright lines could be made out in the Potsdam photograph, but many have been seen at Tulse Hill.[878] On 13th November and 11th December 1897, when they were particularly distinct, approximate wave-lengths were assigned to six or seven, all of which fall near lines in the Wolf-Rayet stars. The reality of the coincidences cannot at present be pronounced upon; they are hinted at rather than asserted; but their verification would enforce an entire recasting of ideas as to the nature of white nebulæ.

A photograph of the Andromeda ellipse, taken by Dr. Roberts 10th October 1887, set the example, since extensively followed, of resolving into spirals, with the help of the camera, all sorts and conditions of nebulæ. It was, indeed, a memorable picture. The vast structure is shown in it and its successors[879] to be furrowed through and through by dark channels, or rather by a single continuous channel, winding in symmetrical convolutions in a left-handed direction from the compact nucleus outward to the dim, indefinite margin. Thus the nebula is not simply a concatenation of flat rings separated by vacant intervals; if it were, the problem of its construction would be less difficult; since the annular gaps might represent spaces cleared of their contents by exceptionally acute gravitational disturbance, while the ejection of matter along a spiral track belongs to a totally different order of phenomena, and implies the operation of laws scarcely yet brought within our ken.

The Andromeda nebula is presumably a round disc viewed obliquely. If so, the angle of its inclination is about 25°.[880] Remarkably enough, the nucleus does not share the elongation of the surrounding spires, as it should if it were no more than a flat condensation in their plane. Its outline is, on the contrary, circular,[881] and its true shape must be that of a globe. There is no probability that the innumerable stars strewing the formation have any physical connection with it. Two small nebulæ in its immediate neighbourhood, on the other hand, certainly belong to its system. The closer and brighter (M 32) was discovered by Le Gentil in 1749; the other, which is situated in a nearly opposite direction, by Caroline Herschel in 1783. Both can be seen with powerful telescopes to be included within the limits of the primary agglomeration.[882] Le Gentil’s nebula, indeed, appeared on a Meudon negative to lie as a condensed knot upon one of its external spires,[883] and the companion object doubtless owns a similar origin. The latter is an oval, apparently amorphous mass; its longer axis is inclined 60° to that of the great nebula. The two satellites may eventually yield signs of orbital revolution; or the whole disc perhaps rotates as one piece, and they along with it; we cannot attempt to decide which condition is the more likely to prevail. Perhaps neither to the exclusion of the other. It is conceivable that the more remote member of the system circulates independently, while the inner companion is borne onward with the general swirl.

Far inferior to this “Ajax” among the nebulæ, although eminent among the “other Argives,” is a large lenticular object in Cetus (N.G.C. 252), noticed by Caroline Herschel in 1783. Sir John Herschel[884] considered its “streaky and knotty” texture to denote resolvability into stars; but it came out instead as a fine spiral in a photograph taken by Dr. Roberts 25th December 1899, a reproduction of which is given, by his kind permission, in Plate XX. The whorls are evidently much foreshortened. They are studded, as Dr. Roberts remarks,[885] “with numerous condensations of a stellar character,” while six ordinary stars are probably seen in projection upon them. Measures of their positions relative to each other and to exterior stars might serve, he adds, for the detection of any movements, rotational or translational, by which the nebula may be affected. Its considerable south latitude brings it within the spectrographic domain of the Cape Observatory, and the McClean apparatus might be competent to obtain an impression of a spectrum sure to prove interesting, if only it can be made distinctly visible.

A nebulous “ray” in Ursa Major (M 82) was described by Lord Rosse as “a most extraordinary object, at least ten minutes of arc in length, and crossed by several dark bands.”[886] These run obliquely to the axis, and give the nebula—as Mr. Ingall said—“a twisted appearance, like a distaff of flax.[887] It appears to possess two centres of condensation, which must lend no slight complexity to its internal economy. Each is perhaps the starting-point of a separate arrangement of luminous coils, but no fair view can be got of them; they are foreshortened into mere broken lines.” The nebula, Dr. Roberts explains,[888] is presented to us “in section, and the upper and lower surfaces are very rugged.” The divisions between the rings hence took shape in a negative, to which he gave three and a half hours’ exposure, 31st March 1889, as “rifts and attenuated places” not obviously fitting together into a harmonious plan. The profile of a corrugated disc is not an easily intelligible object, and that is all that can be seen of M 82. Perspective has done its utmost to disguise its true aspect. Turned edgewise towards the earth, it betrays only by its indentations and rugosities the effects of the ploughing action to which for ages it has been subjected. Swift’s “hairline nebulæ” belong to the same category. They show as bare streaks of nebulosity, bulging a little where the nuclei protrude. Presumably they are flat, circular surfaces, the planes of which coincide with the line of sight.

The “ray” in Ursa Major is not solitary. It is placed at a distance of only 42′ from a larger structure (M 81), evidently of the same general character. The two were photographed together by Dr. Roberts in 1889, and cannot be wholly disconnected. The primary—if we may call it so—resembles the great Andromeda nebula, and, like it, was resolved into a fine spiral. The spectra of both objects were found by Sir William Huggins to be continuous; but the significant details disguised by apparent continuity have still to be revealed.

The essential formative law of white nebulæ is unmistakably that of spirality. This conviction, strongly upheld by the long series of the Crowborough pictures, was irrefutably established by Professor Keeler’s photographic survey with the Crossley reflector.[889] Owing to the strong light-collecting power of the instrument, the harvest of nebulæ garnered was so plentiful that the number within its reach over the whole heavens was estimated at no less than 120,000, and nearly all of these can be inferred, from the preliminary results obtained, to have a spiral shape. On the Lick plates, in fact, a small compact nebula, not disposed in luminous coils, stood out as a rarity. All spindle-nebulæ were resolved into spirals viewed aslant, but into spirals of various degrees of complexity. Some consist merely of two curved branches, shaped like the letter S, and diverging oppositely from a nuclear condensation. An object of the kind situated in Pegasus (N.G.C. 7479) is reproduced from Professor Keeler’s photograph in Plate XXI. Subjoined are the drawings by J. Herschel, d’Arrest, Lord Rosse, and Tempel, with which Keeler compared the autograph picture. They make an instructive study. Herschel saw the object as a narrow spindle “extended between two stars,” d’Arrest as a lozenge; Lord Rosse perceived, in addition, a mass of spiral convolutions surrounding a faint star, while Tempel caught the double effect of a round attached to an elongated patch of luminosity, but failed to discern their true connection. At last on the Lick plates the object disclosed itself under an intelligible aspect. “A glance at the photograph,” Professor Keeler wrote,[890] “shows that the nebula is a two-branched, left-handed spiral, with a nucleus or condensation near the point of inflection. The preceding branch is strong and single, but the following branch is split into two, which cross where their curvature is greatest, at some distance from the centre of the spiral, and unite again at their extremities. This appearance in the components of the following branch, and the fact that the ends of both branches curve around so as to approach the centre more closely than do the intermediate parts, are doubtless effects of projection, the plane of the spiral lying obliquely to the line of sight.”

Lord Rosse’s star occupies the centre of the space fenced round by the preceding branch (that to the left). “It would be of great interest,” the Lick astronomer continued, “to know whether this singular position of the star is accidental, or whether the star and the nebula are physically connected, and if so, in what way the star was left in its present position during the process of contraction. On the first of these questions an investigation of the spectrum, which will be made in due time” (the time, alas! never came), “may throw light. Assuming for the present that the star is physically connected with the nebula, it seems to me possible that the proximity of this star may account for the unsymmetrical appearance of the spiral, which may be due to an actual difference in the dimensions of the two branches, or to their lying in differently inclined planes.”

The first nebula in which a spiral conformation was recognised is still unsurpassed as a specimen of its class. We are enabled, by Mr. W. E. Wilson’s kindness, to reproduce in Plate XXII. his fine picture of this stupendous object. The coils are left-handed; they follow, as they issue from the nucleus, the line of movement taken by the hands of a watch. Our view of them is straight and square; they can be little, if at all, foreshortened. Yet they do not wind symmetrically round their origin. Their flow is broken and distorted, like the current of a river by jutting rocks. The spiral is fundamentally double. Two main streams leave the nucleus at diametrically opposite points, and preserve their separate individuality until they melt away into the outer darkness. Their course seems to be prescribed essentially by the combination of an ejective with a rotatory velocity; disturbances, however, manifestly supervene. The branches divide and reunite; they are cloven and bossy; they swerve widely from the circular track. This is especially remarkable in the case of the longest and brightest arm, which stretches irregularly outward to join a secondary exterior nucleus. This circumstance alone suffices to prove that the diffusion of matter in this formation has been outward. The perturbing mass was undeniably there before the luminous stream which it diverted began to flow; and its flow was quite plainly towards it from within. Other indications of centrifugal action are visible. Mr. Wilson’s photograph shows “cometary tails curved like a plume away from the central nucleus,” attached to some of the denser knots on the convolutions of the spiral;[891] and these effects of apparent repulsion are likewise clearly legible on Lick and Crowborough plates of the same object.

Indraughts or infalls from space are not here concerned, whereas, in Mr. T. C. Chamberlin’s words,[892] “the effects of explosive projection, combined with concurrent rotation, must obviously give rise to a spiral form.” Each such nebula (and there are tens of thousands of them) results, in his view, from the “approach without collision” of a roving star to a compact gaseous mass. Strained to the point of disruption by tidal influences, this embryo vortex would, at a given moment, project from both extremities of the ellipsoid into which it had become elongated, a stream of material curved into whorls through the continual slackening of its angular rate of rotation; and the double catastrophic outrush served to constitute a great system of shining spires, subsequently diversified by the supervening phenomena of minor outbreaks. This rationale has much to recommend it, and probably rests upon a substratum of truth; yet the events contemplated in it are on a small scale by comparison with the grandiose dimensions which we must ascribe to spiral nebulæ.

Lord Rosse described a nebula situated near the star 83 Ursæ Majoris (M 101 = N.G.C. 5457, 5458), as a large faintish spiral, with several arms and knots, at least 14′ across.[893] A four hours’ exposure at Lick brought into view a surprising wealth of intricate details. The groundwork of the structure agrees closely with that of the great spiral in the Hunting Dogs. It is composed of two main effusions, sweeping round from left to right. But they spread, and split, and ramify, drawn hither and thither by multiple attractions, while preserving in their complex interlacings, the whirling impress of their origin.

“Three-branched spirals” still survive here and there in catalogues. Such were supposed to be the delicate objects, M 99 in Virgo (N.G.C. 4254), and M 83 in the head of the Centaur. But the triplicate form ascribed to them was most likely of optical creation. There is no satisfactory evidence that it exists in nature. So far as we can judge, the spiral type originated, by fundamental necessity, through a double outflow, in contrary directions, from the parent mass. A mode of genesis is intimated which recalls, though distantly, the diametrically opposed eruptions not uncommonly witnessed on the sun. It may be added that no genuine spiral appears to be a simple watch-spring coil. This, to be sure, is, to some extent, a matter of definition. It depends upon what we agree to call a spiral nebula. Yet the difference will most likely prove to be radical between stars with curving trains, like Maia in the Pleiades, and those cosmic “whirlpools,” every trait of which testifies to the counterplay of multiple activities.

“Cometary nebulæ” are not very rare, and they present aspects of considerable variety. The nuclei are not always stellar, nor are the appendages attached to them in all cases inflected. A few have been photographed. Thus an object (N.G.C. 1999) 50′ south of ι Orionis was noticed by Lord Rosse as resembling “a comet coiled into a ring nebula,”[894] and appeared under the same form on a plate exposed by Dr. Common with his three-foot (now the Crossley) reflector in February 1883.[895] Its spectrum has not, that we are aware of, been examined. “Reaping-hook” shapes also occur. West of the Argo nebula, a falcated and forked tail, 10′ long, was observed by Sir John Herschel to issue from a granulated, perhaps a double nucleus[896] (N.G.C. 3199). The inner edge is sharp, but it fades gradually outward. Cometary, too, is N.G.C. 520. It has an indistinct nucleus and a bifid train.[897] A nebulous hyperbola with a star near the vertex (N.G.C. 2366) is met with in Camelopardalis;[898] and fan-shaped appendages to stellar condensations are a recognised variety of the species. A pair of these singular objects were photographed by Professor Barnard, 2nd February 1894,[899] in the immediate neighbourhood of the bright-line star γ Cassiopeiæ. His sketch, showing their positions with regard to it, is copied in Plate XXIII. Fig. 1. These do not seem to be casual. The opening out of the two fans straight away from the star suggests an express plan of orientation. Each nebula is about 15′ in diameter. They are “excessively faint and dilute,” and almost elude visual observation. In Professor Barnard’s opinion, they would never have been detected otherwise than by chemical means.[900] Yet they photograph easily enough; and if this actinic quality denotes, as seems probable, a gaseous constitution, mention of them should, properly speaking, be postponed to a later chapter. Their spectral classification, however, is likely, for some time to come, to remain matter of conjecture. Dr. Roberts obtained, with ninety minutes’ exposure on 25th October 1895, an excellent photograph of the twin fan nebulæ, in which traces of a luminous connection are apparent.[901]

“Rifted” nebulæ must be classed as a variety of the elliptical sort. They appear either as cloven discs—N.G.C. 5128 in Centaur is an example—or as parallel rays, such as a bifid streak in Leo (N.G.C. 3628). A probably analogous structure in Andromeda was photographed by Dr. Roberts in 1891.[902] It came out immensely elongated, and with just such a “chink in the middle” as had been seen by Sir John Herschel, whose opinion that the nebula was the foreshortened representative of a thin, flat ring of enormous dimensions thus received strong confirmation. Yet the added light-power of the Parsonstown mirror had extended Herschel’s “chink” into a channel, running from end to end of the formation. Or rather the retinal impression afforded by it had been misinterpreted in this sense; for that it was a misinterpretation the camera incontrovertibly asserts. The error may serve as an illustration of Professor Keeler’s remark that “the most obvious tendency of the draughtsman is to prolong a line or curve beyond the point at which it actually stops.”[903] A suspicion even arises that other telescopic presentments of rays split throughout their length are similarly misleading, and that they are, in fact, like the glimmering oval in Andromeda, rings thrown into perspective. Otherwise why should the dark rifts always coincide in direction with the major axes of such formations? If these are really circular discs, they might as well run across as along them; but they never do. We must then choose between two inferences. Either the nebulæ are in fact, and not merely by optical projection, elliptical, in which case a longitudinal line of cleavage would be intelligible, or they are luminous rings viewed very obliquely. It must indeed be admitted that the strong development of nuclear condensations in some rifted nebulæ appears almost to exclude the latter alternative. Dr. Roberts’s photographs, for instance, of N.G.C. 4565 in Coma Berenices, and of N.G.C. 4594 in Virgo,[904] exhibit an arrangement of parts insistently demanding a different explanation. What seems certain is that no single principle is valid all round. Modifications must be introduced to meet the exigencies of nature’s endless variety.

The great majority of white nebulæ might be called globular clusters in disguise. They present a round surface, condensed centrally by gradations testifying to their true spherical form. The only obvious distinction between them and “balls of stars” is that they are irresolvable by any telescopic powers that can be brought to bear upon them. And the suggestion lies close at hand that this quality depends wholly upon distance—that round nebulæ are neither more nor less than remote globular clusters. Yet it cannot be adopted without hesitation. The space-relations of the two classes of object are very different. Clusters frequent the Milky Way; white nebulæ avoid it. The discrepancy, it is true, may be capable of reconcilement, but by a somewhat elaborate artifice of speculation. Nor is there any immediate prospect of solving the difficulty by the aid of the spectroscope. We are unacquainted at present with any criterion for distinguishing continuous nebular light from that of compressed clusters. One may eventually be found, but its application must always be a matter of extreme delicacy.

Professor Max Wolf’s explorations of the heavens show them to be strewn with an incredible number of small faint nebulæ.[905] Directly visible only by elusive glimpses, they come out individually distinct and measurable on sensitive plates; and the Heidelberg observer has already laid his plans for the construction of a photographic catalogue of nebulæ, likely to be at least twenty times more voluminous than the most exhaustive visual enumeration. In certain regions he indeed found that only two per cent of the dim objects delineated on his plates had been previously recorded. The newly discovered crowd vary greatly in shape. Some are round and compact; many more are round and diffuse; there are spirals among them, and spindles, and draped or arched formations. A surprising number are marked “planetary,” and are hence, presumably, gaseous. The physical nature of the rest is inferable only when the definiteness of their shapes prescribe their arrangement in some established category.

Thus we are able to assert confidently that those disposed along coiling lines or projected into rays and ellipses, give continuous light; but if we attempt to go further, and obtain a clear conception as to how the light originated, embarrassments beset our path. Let us confront them fairly. The only white nebula of which the spectrum has been observed to any purpose is the great elliptical spiral in Andromeda. It almost certainly includes lines or bands of absorption; it is probably marked by traits of emission as well. Plausibility is thus lent to the opinion that the nebula is a genuine cluster of stars amalgamated by distance into a soft haze. The haze, however, shines very dimly; its lustre is almost evanescent comparatively to that of the sun. If, then, its component particles are true suns, they must be inordinately far apart. For the sake of giving some precision to our ideas on the subject, we will attempt to illustrate this numerically. If we assume the central parts of the nebula to possess ¹⁄₂₀₀th the intrinsic lustre of the full moon, or (what comes to the same) ¹⁄₁₂₄ millionth that of the sun, while consisting of scattered globes of solar brilliancy, it follows that this also is the proportion between the total bright area covered by their discs and the dark area of vacancy, the dimness of the nebula measuring the spread of the interspace. Hence the component stars, taking each to be half a million of miles in diameter, should be separated from its next neighbour by an interval of more than 5000 millions of miles, as seen projected upon a plane perpendicular to our line of vision. Their real distances, since they are presented to us slantwise, would of course be very much greater; but with them we are not just at present concerned. We must next try to form an estimate of how close together these sun-like bodies should appear to be in order to produce the observed effect of a smooth luminous surface. If the gaps amounted to one-tenth of a second, the nebula would certainly, with the powerful and perfect telescopes now in use, show symptoms of resolvability. Yet none appear. The fog does not even tend to condense into droplets, and the temporary star of 1885 stood out, to the last hour of its visibility, by contrasted light-quality from the soft surrounding glow. Allowing, then, that the linear intervals of 5000 millions of miles between the constituent bodies of the Andromeda nebula are represented by optical intervals of ¹⁄₂₀th of a second, we arrive at a parallax for that vast structure of less than ¹⁄₁₀₀₀″. In other words, its rays spend about 3300 years in travelling to the earth. At this distance, the stars we have supposed aggregated in it would appear of fifteenth magnitude. Now they should in the fainter outlying parts of the nebula be more sparsely distributed than near the centre, and as they thinned off they would inevitably appear in their proper guise as fifteenth-magnitude stars. But the texture of the glimmering haze remains the same in every stage of attenuation.

It may then be taken as certain that, if this object be of stellar constitution, it is made up of stars smaller and closer together than we have supposed; unless we are prepared to lengthen still further, and very materially, a light-journey already protracted to the verge of the incredible. It is not, however, easy to conceive that bodies much less than half a million miles in diameter can be truly sun-like. An outpouring of light and heat in the profuse measure exemplified by the sun, implies storage-accommodation on a colossal scale; and the spectrum of the Andromeda nebula, so far as it can be deciphered, seems to correspond to a high standard of temperature.

Undoubtedly the path “of least resistance” is to accept the stellar origin of nebular radiance. It is not entirely practicable, but every other is impassable. The solar corona presents no real analogy to white nebulæ, since it is kept incandescent by the potent agency of the sun, while their glow is self-sustaining. This it can only be—setting aside the vague possibility of electrical discharges—by the sacrifice of motion in some form. According to Sir Norman Lockyer’s well-known hypothesis, the collisions of swarming meteorites supply the evolved energy; but there is little or no evidence that the cause acts, or would be adequate if it did act. We, at least, have no experience of its operation. The only meteoric collisions we know of are with the earth, which spreads a wide net for the capture of flying cosmic particles.

As an alternative suggestion it may be worth considering whether the shining of nebulæ might proceed from a very slow loss of circulatory speed through the resistance of a gaseous medium. A pulverulent constitution, resembling that of Saturn’s rings, should then be attributed to them; they would consist of relatively small masses interfused with some highly subtle aerial remnant, the distinctive bright lines of which add complexity to the nebular spectrum. But the velocity of circulation in such structures should increase outward. Other things being equal, they should accordingly, if arrested motion were the source of their luminosity, gain brightness with increasing distance from the centre. The reverse is very markedly the case; but the attendant conditions are so intricate that the contradiction need not be fatal to the speculation. It cannot, however, be usefully discussed apart from a profound study of the dynamical condition of such a peculiar system as that just indicated; and this we must leave to more competent authorities.

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XXXII. DOUBLE NEBULÆ

“Double nebulæ,” Dr. See wrote in 1893,[906] “have been greatly neglected since the time of Sir John Herschel, but it is to be hoped that astronomers will again give adequate attention to these remarkable objects, which should be at once systematically studied and photographed. If accurate drawings or photographs of these objects were now made, it is not to be doubted that important changes could be observed fifty years hence.”

His special interest in them originated from a research into the “evolution of stellar systems.”[907] Sir John Herschel’s drawings of coupled nebulæ illustrated most aptly his theory of the origin by “fission” of double stars, some appearing actually modelled upon Poincaré’s “apioid”—the figure assumed by an ellipsoid when becoming unstable under the stress of increased axial rotation, and about to break up into unequal masses. Without entering into details regarding the process, it may be explained that disparity in the products of disruption indicates want of homogeneity in the parent body, so that the sooner the components separate, the greater the chance of their approximate equality. But the result of photographically investigating the pattern-objects was completely to alter the point of view from which they had to be regarded. “The actual nebulæ,” Professor Keeler stated,[908] “have almost no resemblance to the figures. They are, in fact, spirals sometimes of very beautiful and complex structure, and in any one of the nebulæ the secondary nucleus of Herschel’s figure is either a part of the spiral approaching the main nucleus in brightness, or it cannot be identified with any real part of the object.” There had been premonitions to this piece of “destructive criticism.” Many spirals are readily seen to be essentially duplex. Such is their exemplar in Canes Venatici, the second nucleus of which, separately catalogued by Sir John Herschel,[909] was brought into connection with the first only when the intervening whorl of nebulous matter disclosed itself at Parsonstown in April 1845. In many other cases, the brighter knots which tend to form on curving branches are seen isolated, for lack of light to bring the linking filaments into view, with the outcome of visually decomposing one formation into several. Thus telescopic improvements, which avail to analyse stars, have frequently a synthetic effect upon nebulæ. Even such adjacent objects as are presumably in mutual systemic relation, often show signs of being bound together by organic ties as well. Hence it is difficult to draw a line between single and double nebulæ. A pronounced “dumb-bell” form graduates insensibly into a pair of clearly individualised globes, barely united by a faint ligament. And their condition seems less alien to our ideas when we remember that the sun is nebulously connected with the earth by means of the zodiacal light.

Unification with increase of optical power was exemplified by Burnham’s observation, at the Lick Observatory in 1891,[910] of a nebula as single, though bi-nuclear, which Herschel had registered under two distinct headings (N.G.C. 7174, 7176). The condensations, which possibly offer to our view a double star in the making, are just 26″ apart, and belong to a nebular group in the Southern Fish. A more dubious object is situated in Aquarius (N.G.C. 7287). Detected by Müller at the M‘Cormick Observatory, it was described as an “excessively faint, slightly nebulous double star.” Burnham found the object to be indeed double at an interval of about 20″, yet not stellar, one component, at any rate, and perhaps both, appearing as small dim nebulæ. An authentic example of a double nebula was noted by Barnard in 1888, with the twelve-inch refractor of the Lick Observatory, in the neighbourhood of the wide double star 23 Orionis (Σ 696). The components are 36″ apart, faint and uncondensed. A tenth-magnitude star forms with them an equilateral triangle. Their measurement by Burnham in 1891[911] supplies a datum of first-class importance for the future determination of change in the system which they beyond question constitute.

Littrow described in 1835[912] a curious combination of three small nebulæ marking the angles of a triangle, the sides of which are formed by three nebulous bands, while a fine double star occupies the middle of the enclosure. The arrangement, met with near γ Pegasi, would make a promising subject for a photographic experiment. Close telescopic scrutiny, on the other hand, might advantageously be brought to bear upon a nebulous pair in Gemini (N.G.C. 2371, 2372). The distance from centre to centre of the components is only 32″, and they were seen at Parsonstown to be connected by “tails and filaments,” if not encircled by a filmy annulus.[913] An intermediate star, noted as “bright” 19th December 1848, was observed by Lassell in 1852,[914] and by d’Arrest in 1862, but has of late ceased to attract attention. Can it have lost light? D’Arrest seems to have had no difficulty in seeing it with an eleven-inch refractor, so the question might be readily answered. The preceding member of the pair was remarked by Dr. Dreyer in 1887 to be brighter and more condensed than its companion.[915] A similar nebular and stellar group was discovered by Dr. Common in 1880[916] in the constellation Crater. He regarded the nebulæ as planetaries; in the absence, however, of information concerning their spectra this cannot be held certain. The existence of the linking star—a feature of peculiar interest—has not, we believe, been verified, but need not be doubted.

Fine telescopic seeing avails to resolve, no less than to unify nebulæ. Some split up, like close double stars, under high powers. With a magnification of 250 Professor Swift perceived a nebula (N.G.C. 6679), earlier discovered by himself, to be a well-separated pair, and he obtained a similar result for one of Sir William Herschel’s. “It would,” he adds, “be a great satisfaction to be fully assured that they are binaries.”[917] We fear that the satisfaction is reserved for a future generation of astronomers. The Herschelian nebula in question was doubtless N.G.C. 3690 in Ursa Major, which had already in 1852 been divided with the Rosse reflector into two irregular masses at a distance of about 60″.[918] This is a coarse object compared with two delicate pairs discovered by Swift at Echo Mountain, California, in 1897. Each resembles a “double nebulous Uranus,” the conjoined discs being 5″ or 6″ apart.[919] They are numbered 6 and 27 on his eleventh catalogue; yet, although one seems the replica of the other, they are not near neighbours in the sky. Mixed pairs, stellar and nebulous, are less scarce than one might expect. Swift’s southern explorations yielded nearly a dozen specimens. Two are situated in Argo, near the small round nebula, N.G.C. 3267. Each proved resolvable, in the exquisite Californian air, into a star and nebula at a distance of 4″;[920] and the veteran observer’s concluding list of discoveries included five analogous couples, the widest having a span of 8″. They should at once be micrometrically measured; for until this is done they cannot be said to have started on their career in scientific history.

Double elliptical nebulæ are picked up now and again. They are not easily distinguishable from rifted nebulæ. Probably the true criterion is the duplicity of the nucleus. Rays stretched parallel to the main formation, but exhibiting no trace of independent condensation, can only be regarded as outlying portions of it; where there are two nuclei, there are, in esse or in posse, two distinct bodies. As such two lens-shaped objects in Pegasus (N.G.C. 7814), photographed by Dr. Roberts,[921] should probably rank. The “dark lane” shown by the Rosse reflector was perceived in the negative to bisect the globe-like nucleus, and to widen out on either side of it; so that each oval is complete in itself; neither looks like a fragment of the other. A true pair seems also to be constituted by the lenticular nebulæ (N.G.C. 3786, 3788) delineated by Spitaler at Vienna in 1893;[922] while the status of many more cannot be fixed until they have been photographed with a variety of instruments and exposures. Among those of uncertain nature should be reckoned a cloven ray in Leo (N.G.C. 3628), 15′ in extent,[923] and probably annular;[924] a similar object in Centaur (N.G.C. 5128), viewed with amazement by Sir John Herschel; a fissured ellipse in Leo Minor (N.G.C. 2964), thought to be “almost double” at Parsonstown; and a spindle in Draco (N.G.C. 5866), described by Professor Keeler as “divided lengthwise by a narrow, perfectly dark straight rift, on each side of which, near the north-preceding end, and involved in the nebulosity, is a minute star of about the sixteenth magnitude.”[925] A bifid beam in Coma Berenices (N.G.C. 4565), 14′ long, and with a protruding nucleus, appears to be essentially single. Sir John Herschel noticed that the segments of rifted nebulæ are sharp on their confronted sides, diffuse outwardly.[926] They recall the gaping shell of a bivalve; and this peculiarity, if photographically persistent, might serve as a secondary mark of unity. The unity, as already pointed out, may be that of a ring viewed edgewise; or, in some cases, a formation primitively one may have been sundered by disintegration, as a rock-ledge is cut by a mountain torrent. This is, of course, said merely by way of illustration. We are unable to conceive how disintegrating forces in a nebula really act. Nor should the possibility be forgotten that the occurrence of black chasms may indicate, not the removal of matter, but an abolition of light. These apparently breached objects perhaps subsist, after all, in substantial entireness.

Double nebular ellipses do not invariably lie parallel to one another. The Andromeda nebula, for instance, is, in a manner, coupled with N.G.C. 205, the longer axis of which, as has been said, makes an angle of 60° with that of its primary. A pair in Virgo (N.G.C. 4567, 4568) stand in yet more singular relations to each other. They coalesce at their following extremities, and diverge at an angle of about 45°.[927] They might be conceived of—were this mechanically possible—as revolving on a pivot. The combination is essentially reproduced by two spindle nebulæ near the hind foot of the Great Bear (N.G.C. 3786, 3788), which meet almost rectangularly. Dr. Spitaler’s drawing of them is copied in Fig. 48. A third pair, similarly composed, was noted by Swift, 23rd September 1897, at the Lowe Observatory.[928] A bright nebula in Sculptor (N.G.C. 55) was then too seen to have a dim companion. Both are elongated; they meet obliquely and overlap. Possibly indeed they form together a single curved nebula; yet the indications are more in favour of a genuine coupled arrangement.

Elliptical nebulæ are sometimes found less congruously associated with round, perhaps globular attendants. Thus an enormously long ray in Canes Venatici (N.G.C. 4631) is preceded by a tenth-magnitude star, and that again by a nebulous orb.[929] Of the ray, it was recorded at Parsonstown, 26th March 1848, that “masses of light appear through it in knots”;[930] and the drawing made there exhibits helical lines corresponding presumably to an extensive system of foreshortened flat spires. The star does not appear to have been seen; nevertheless it ought, unless greatly diminished during the score of years elapsed since Herschel’s observation of it, to have been conspicuous with the six-foot speculum. A spindle-nebula in Eridanus (N.G.C. 1532) has also a round companion;[931] and two are attached to an ellipse depicted by Spitaler in 1893 (N.G.C. 2781, 2785). Triple combinations of round nebulæ are fairly common. The varieties of multiple stars are recapitulated in them. Single primaries have closely double satellites, or single satellites wait upon compound primaries. Three nebulæ, which, from their central brightening, may roughly be described as spherical, were detected by Barnard in 1886 lying close together in the field of his six-inch refractor.[932] Yet they are faint objects even with the Lick thirty-six-inch. The intervals between them were determined by Burnham in 1891 to be respectively 94″ and 78″. A century hence there will be hope of eliciting evidence of incipient revolution by the renewal of these measures. Another of Barnard’s new nebulæ (N.G.C. 6302) was resolved by Swift into a triplet. It is plunged deep in the Milky Way in Scorpio, and has a gaseous spectrum.[933]

Compound nebulæ lead the way to groups and clusters of such objects. Swift counted twelve in a single field near Algol, and perceived at least twenty collected into a slightly larger space between κ and γ Herculis.[934] Barnard explored in 1890 a nest of eighteen small separate nebulæ in Ursa Major; and Max Wolf observed in March 1901 a real “Nebelhaufen” surrounding, though probably disconnected from the star 31 Comæ Berenices.[935] No less than 108 components, some elongated, some roundish and of various degrees of faintness, were found included in a circle 30′ in diameter; and a similar group, photographed in 96 minutes, has η Virginis for its centre.[936] White nebulæ, in fact, tend very markedly to gather into flocks; whether as a consequence of their mode of origin, or through the compulsion of their mutual attraction, remains an open question. And since the masses of these bodies are likely to be very small, and their real distances very great, circulatory movements only of the most leisurely kind can be ascribed to them. We are unable as yet to forecast, even remotely, the establishment on a settled footing of the dynamics of nebular systems.

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XXXIII. NEBULOUS STARS

“There is a vast difference,” Professor Swift wrote in 1897, “between a nebulous star and a star in a nebula.”[937] Vast indeed, since a star and nebula in reality billions of miles apart may be thrown by perspective into the same visual line, while a true nebulous star claims the ownership of its luminous appendage on the evidence of obviously adapted form. The nimbus is not more unmistakably fitted to the head of a saint in a picture than nebulous halos are, very often, to the stars they encircle. The relationship is patent. That the glow emanates from the star, and is no casual adjunct to it, the instinctive logic of the eye suffices at once to decide.

Sir William Herschel was the first to give express attention to “stars with burrs.” They struck him as remarkable, not only in themselves, but for what they implied. They served as the basis of a memorable train of reasoning.[938] Their atmospheres, he argued, being plainly “not of a starry nature,” must be composed of a “shining fluid,” the same which is seen to be diffused through space in milky tracts, or curdled into fantastic shapes of chaotic irregularity. He was, in a word, led by them to the capital discovery of nebulæ as a distinct sidereal order. On 13th November 1790, he came across the “singular phenomenon” which determined his abandonment of the view that the universe is constituted exclusively of stars variously aggregated. This was “a star of about the eighth magnitude, with a faint luminous atmosphere of a circular form, and of about 3′ in diameter. The star is perfectly in the centre,” he continued, “and the atmosphere is so diluted, faint, and equal throughout that there can be no surmise of its consisting of stars, nor can there be a doubt of the connection between the atmosphere and the star.” This exemplar object, situated in Taurus (N.G.C. 1514), he regarded as “decisive in every particular”; yet its nature is still to some extent dubious. It has even been classed of late as a planetary nebula, and certainly shares not a few characteristics of that family. The uncertainty of its status renders its study especially instructive.

A planetary nebula is definitely terminated; a nebulous star fades off into space. One shows a disc; the other is surrounded by an “atmosphere.” Moreover, a star, or stellar nucleus, is subordinate in the one formation, while it dominates the other. These distinctions, however, cannot always be unhesitatingly drawn, since the relative strength of the stellar and nebular elements varies widely in different objects. Hence the doubt as to the category in which Herschel’s typical specimen should be ranked. For the glow round it is uncommonly bright; d’Arrest found it to strike the eye with a four-and-a-half-inch refractor.[939] Nor is it equably diffused. The Parsonstown telescope showed it as spotted and patchy, and very curiously “ragged” at the edges;[940] and its aspect to Mr. Burnham in 1891 was essentially the same. A “broken and mottled” surface, about 126″ across, emerged in the field of the Lick thirty-six-inch.[941] He was inclined, though under reserve, to agree with Barnard in considering the object planetary. Its affinities might be settled by spectroscopic means; the attempt to do so could, at any rate, hardly fail to have an interesting result. So far, little has been ascertained about the spectra of true nebulous stars. Their halos, certainly in some cases, presumably in all, emit bright lines, and it might be expected that the same lines would show by absorption in the spectrum of the central star. The presumption has not, indeed, been fully verified; while the expectation founded on that presumption is disallowed by the facts scantily at our disposal. Stars with nebulous appendages are usually, if not invariably, distinguished by “early Orion” spectra—a combination already noted as significant in connection with the course of sidereal growth; but they show no special lines that could be attributed to light-stoppage by the immense bulk of rarefied incandescent matter interposed between our eyes and their shining photospheres. This is one of the many perplexities involving the luminous relations of nebular stuff, which, setting “Kirchhoff’s law” at defiance, exercises no absorption correlative to its emission.

One of the few nebulous stars bright enough for easy spectroscopic investigation is situated in Scutum Sobieski. Of 5·5 magnitude, it is enrolled in the Bonn Durchmusterung under the heading S.D. −10°4713. On a plate exposed by Professor Barnard with the Willard lens, 29th June 1892,[942] a large diffused nebulosity was seen to encircle it. The appendage must be visually very faint to have escaped notice so long. Its proper spectrum may then be nearly evanescent. The star it belongs to is No. 8198 of the Draper Catalogue, where it is set down, although doubtfully, as of the second type (Spectrum E). Now this is a point of crucial importance to theories of stellar development. The pronounced nebulous condition of a star near the solar stage would have a bearing on such inquiries that could not be ignored. Should it be established, current ideas will need revision. The spectral character of Barnard’s nebulosa in the Shield promises, indeed, to afford a test by which to try the validity of reasonings on sidereal evolution. The test ought, with the least possible delay, to be applied.[943]

A seventh-magnitude star in Eridanus was perceived by Swift in 1859 to be almost centrally placed in a shining corona.[944] It is perhaps identical with N.G.C. 5315. Its spectral classification should present no difficulty. A similar object, equally adapted for spectroscopic inquiry, was detected by the elder Herschel in Cepheus (N.G.C. 7023). The nebulosity is particularly strong north and south of the star. Irregularities of a more marked kind are apparent in other instances. A tenth-magnitude star in Monoceros was found by Barnard visually nebulous in 1888, photographically in 1894.[945] A “small dark space,” however, interrupts the encircling halo. A subsequent exposure with the same instrument disclosed as “closely nebulous” the 9·5 magnitude star, D.M. + 23° 1313.[946] And here again the illumination is unevenly distributed, the “fuzzy” border to the star-disc being denser south and east than elsewhere. This object lies almost midway between η Geminorum and χ^2 Orionis. One in most respects analogous (N.G.C. 2247), detected by Swift, 24th November 1883,[947] came out noticeably “blurred” on the same plate with Barnard’s nebulous star in Monoceros. Two further specimens of the class were photographed by Barnard in Sagittarius. One—D.M. −19° 4948—is fainter than the ninth magnitude. It has a narrow fringe of light.[948] The second—D.M. −19° 4953—is of 7·6 magnitude, and is encompassed by a far-spreading halo, 15′ in diameter,[949] conspicuous with the camera, although nearly invisible to the eye.

Nebulous stars are frequently compound—perhaps more frequently than stars clear of cosmic fog. Sir John Herschel recorded at the Cape a close pair (N.G.C. 5367) as involved in a bright glow two minutes of arc in extent; and a faint star with an aureola, discovered by Tempel in Cetus (N.G.C. 707), proved, when scrutinised by Burnham in 1891, to have a minute attendant at an interval of 10″.[950] A still more interesting detection concerned a nebulous triplet in Auriga (N.G.C. 1931). Discovered by Sir William, and described by Sir John Herschel as “one of the most curious objects in the heavens,” it consists of three stars, the brightest of 9·5 magnitude, forming an equilateral triangle with a side of about 8″, placed precisely at the centre of a small circular nebula. Mr. Burnham had repeatedly inspected it with minor instruments,[951] but it needed all the power of the Lick refractor to bring into view a fifteenth-magnitude satellite at a distance of little more than 2″ from one of the stars of the triangle. As a test for “seeing” facilities, the pair is of unsurpassed delicacy. Again, a wide double star occupies the middle point of a pretty large faint nebula in Monoceros (N.G.C. 2182). The attendant may be only optically such; the circumstances are on this point indecisive. The chief star, however, was found by Mr. Burnham to be double in a perfectly unequivocal sense.[952] A companion of nearly its own magnitude (8·6) is separated from it by a spatial gap of less than half a second, and the two must assuredly revolve round their common centre of gravity. Here, indeed, we are confronted by a profoundly embarrassing question. The couple are evidently plunged in nebulous matter; their movements must then, according to received ideas, be impeded, with the result of an eventual collapse of the system. We can see no escape from the dilemma except by adopting the startling hypothesis that the nebulous fluid does not constitute a resisting medium. The difficulty greatly enhances the interest of spectroscopically determining the velocities of bodies nebulously connected.

A 6·5 magnitude star in Cepheus (D.M. +57° 2309) appeared in a photograph taken by Barnard in 1893 “surrounded by a rather unsymmetrical dense nebulosity.”[953] A “hazy glow” could be seen with the Lick thirty-six-inch, which, in Burnham’s employment, had already revealed the star to be very unequally double at 4″.[954] A first-type spectrum is dubiously ascribed to it in the Draper Catalogue.

The nebulous triplet, ι Orionis, has been more completely observed than perhaps any of its congeners. It consists of a third and an eighth-magnitude star 11″ apart, with an eleventh-magnitude satellite at 49″, described by Admiral Smyth as “grape-red” in colour.[955] Sir John Herschel perceived the group to be “involved in a feeble nebula 3′ in diameter,”[956] and in the nebula (N.G.C. 1980) there was apparent with the Rosse reflector a central cavity containing the bright star-couple.[957] Possibly the effect was an illusion due to their effacing radiance; but this cannot be taken for granted, since “holes” in nebulæ are an attested phenomenon. And the early observations at Parsonstown approve themselves as singularly accurate through the confirmatory evidence of the best recent photographs. The spectrum of ι Orionis is of the helium type, and Dr. McClean identified in it three members of the Pickering series of hydrogen, besides many oxygen lines.[958] The surrounding glow emits the ordinary nebular rays, but they make no show, either directly or by reversal, in the dispersed stellar light. Yet before reaching outer space, that light has to traverse enormous volumes of incandescent or luminescent nebulium. The anomaly presented by the absence from the Fraunhofer spectrum of the solar coronal green line is here repeated in an emphasised form. A long nebulous streak, visible only on sensitive plates, links the hazy appendage of ι Orionis with the great formation in the Swordhandle.[959]

Far away in the northern part of the constellation, there is found in λ Orionis a combination very similar to that presented by the nebulous trio just considered. A yellowish and purple pair (Σ 738), of 3·7 and 5·6 magnitudes, at 4·2″, are immersed, with a comparatively remote twelfth-magnitude attendant at 29″, in a nebulous haze, photographed by Barnard in three hours, 17th September 1893.[960] The discovery was at once telescopically verified.

Five nebulous stars occur together in a narrow region of Sagittarius, and three of them are double. The two apparently single are Barnard’s stars, already mentioned. The pairs are N.G.C. 6589 and N.G.C. 6590, both first noticed by Swift,[961] and N.G.C. 6595, delineated seventy years ago by Sir John Herschel. The character of an object photographed by Dr. Roberts[962] near the spiral nebula M 81 in Ursa Major, needs to be more satisfactorily determined. Known to Herschel and d’Arrest as a condensed nebula (N.G.C. 3077), it appeared on the sensitive plate with a sharp, stellar nucleus in lieu of the woolly disc visually perceptible.[963] The spectroscope may perhaps help towards its rightful classification. Nebulous stars merge insensibly into stars with nebular appendages, such as ω Orionis and σ Scorpii. The former has a dimly luminous, curved spur running out from it, besides a larger mass hanging like a cloud above it to the north; the latter is winged with nebulosity, two pointed projections issuing from it in divergent directions.[964] The whole of these appurtenances were detected photographically by Professor Barnard. Both stars afford spectra marked by helium absorption. The relations of stars and nebulæ are manifold. Misty trains and tails of all sorts and sizes have stellar foci; they emanate from stars, or condense into stars; but nebulous stars are, properly speaking, what Herschel called “stars with burrs”; they give the usual sharply defined diffraction-discs, although a dim halo spreads more or less symmetrically round each. The nebular element in such a combination is entirely subordinate to the stellar; while in stars with appendages the disparity gradually becomes reversed.

“Rejected” nebulous stars are still worth attention in view of the possibility that they may be subject to genuine change. The case of 55 Andromedæ is particularly instructive, if only as illustrating the propagation of error. This is a 5·6 magnitude star, qualified as nebulosa by Flamsteed and Piazzi, and regarded by Sir John Herschel as a typical specimen of a hazy star. It figures as No. 428 in his General Catalogue of Nebulæ (1863), but was omitted by Dreyer from the revised edition of that work. Sir William Huggins, nevertheless, observed it in 1864 to be “a fine nebulous star with a strong atmosphere;”[965] and since he used a very perfect refractor, his confirmation of what Herschel had seen with a reflector had an independent value which might seem to exclude the hypothesis of association in optical illusion. Yet neither Lord Rosse in 1848 nor d’Arrest in 1856 had perceived any trace of nebulosity, and Schjellerup, during some years previous to 1866, always found the star sharp.[966] So again it appeared to Mr. Burnham in 1879–80,[967] and so it seems likely to remain. We can, however, scarcely persuade ourselves that several eminent observers conspired to blunder; and Schjellerup’s theory that Piazzi merely repeated Flamsteed’s note, which crept, he supposes, into the British Catalogue by a transference from the great Andromeda nebula, is rendered unacceptable by the circumstance that 55 Andromedæ follows the nebulous structure held to have been confused with it at an interval of considerably more than one hour of right ascension. Nor even if so extraordinary a mistake had been made, was Piazzi capable, one would think, of copying it unawares. His high astronomical reputation suffices in itself to clear him from the charge of such astounding carelessness. The spectrum of the star resembles that of the sun, a type never yet unequivocally associated with nebulous attachments. On the other hand, the normal quality of the light renders their optical creation more difficult of explanation. Thus the nebulous aspect of 55 Andromedæ must stand over as one of the unsolved problems of astronomical history.

A similar, but less convincing case is that of 8 Canum Venaticorum. On four separate occasions Sir John Herschel noticed this fourth-magnitude star to be surrounded by a “considerable atmosphere.” Yet since no one before or after him has vouched for its presence, he was presumably deceived. The spectrum of 8 Canum is of the solar class.

Finally, a 7·5 magnitude star in Cetus, discovered as nebulous by Stephan at Marseilles in 1880[968] (N.G.C. 988), appeared to Burnham and Barnard in 1891 devoid of any such peculiarity.[969] A photograph taken with suitable exposure would serve decisively to test its present condition. The criterion might indeed fail with stars so bright as 55 Andromedæ and 8 Canum Venaticorum; for their imprinted discs would become, through chemical irradiation in the time needed to bring out faint glows, sufficiently distended for their obliteration.

In connection with nebulous stars two lines of inquiry open out. First, the spectroscopic. The scanty evidence at our disposal is to the effect that the stellar rays of such objects are of the “Orion” kind; that they show the quality believed to characterise suns in a primitive stage, while their aureolas shine like gaseous nebulæ. But these generalisations rest on a very narrow basis of fact, and probably admit of interesting and significant exceptions. Indeed, each nebulous star should be treated as a separate spectroscopic problem, destined to afford in the course of its solution insight into many obscure secrets. A second branch of research relates to the structural peculiarities of stellar halos. Their luminosity is seldom, perhaps never, equably distributed. Its irregularity sometimes goes so far as to produce the effect of dark vacuities, photographically attested to be no mere visual deceptions. What their true nature and origin may be, is a subject for inquiries likely to be long and arduous. It is scarcely credible that they are what they appear to be, obscure tunnels, striking, in the direction of the earth, right through the heart of immense spheres of shining tenuous matter. The alternative view is preferable that the so-called “atmospheres” of stars are really effluences—that they consist essentially of spiral coils wound closely enough to merge ordinarily into an approximately uniform surface, while leaving in certain circumstances conspicuous gaps between their luminous folds. If this be so, nebulous stars fall into line with cometary nebulæ, the trains of which take a more or less completely annular shape; but their nearest allies are unquestionably to be found in the planetary family; and this brings us to the subject of our next chapter.

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