X. THE CHROMOSPHERIC SPECTRUM
The spectrum of the chromosphere is almost purely discontinuous. It is composed of detached bright lines. Some of these are always present, but the majority come and go. We will consider first the permanent radiations.
The chief of these are readily identified; they belong to hydrogen, helium, and calcium. Thirty hydrogen lines have been seen or photographed in dispersed chromospheric light, all members of the original, or “Huggins series,” which attained nearly to its theoretical limit on plates exposed by Mr. Evershed during the Indian eclipse of 1898. The fundamental C (Hα) is the brightest. To its intensity is due the crimson glow of chromosphere and prominences; and prominences viewed spectroscopically on this line appear larger than when imaged in any of the other qualities of hydrogen-light. This is generally explained by the influence of temperature, a higher degree of heat being needed to give complete development to forms of shorter wave-length.[194] A comparatively moderate chromospheric temperature would thus be indicated. But there may be other influences in question. Professor J. J. Thomson made the significant observation in 1895[195] that the red and green lines of hydrogen show marked differences of intensity at the two electrodes of vacuum-tubes, the red predominating on the positive, the green on the negative side of a partition. The hint must indeed be reserved for future use. We are not yet in a position to apply it profitably.
The helium-spectrum of the chromosphere also gives rise to some interesting considerations. For its various constituent series are not represented indiscriminately at the edge of the sun. These series, we may remind our readers, are six in number, and they are distributed with beautiful precision into two corresponding triple systems. They may be distinguished for convenience as “yellow” and “green,” D_{3} giving the tone to the former, several vivid green lines characterising the latter set. In the laboratory, as we have seen, they are inseparable; one set cannot, by any artifice so far devised, be procured apart from the companion set. But it is otherwise in the sun. Yellow helium is always present in the chromosphere and prominences; green helium only about one-fourth as often as it is looked for. The permanent chromospheric lines are four, namely, one far down in the red at λ 7065, the familiar D_{3}, a deep blue ray at λ 4472 (formerly known as “f”), and the ultra-violet “leader line” of the principal series at λ 3889. These have probably many associates of still shorter wave-lengths; but photographic data are too scanty as yet for purposes of discrimination between the constant and the occasional elements of the spectrum. They do not proceed, it must be borne clearly in mind, from two substances, but from one indivisible form of matter, differently conditioned. In what way, it is not easy to imagine. Laboratory experiments show that the green set of lines gains relatively in brightness with rarefaction, although the yellow set persists as well to the limit of practicable exhaustion. This indication, however, does not open a way out of the chromospheric difficulty. The gases near the sun are of inordinate subtlety. Helium ought there, if this alone were the determining quality, to be in the green state. Its most fundamental emanation, nevertheless, is D_{3}. Nor is its dominant position compromised in the highest prominences. On the contrary, the green rays nearly always proceed from lower lying, and therefore from denser strata than the yellow.[196] Supplementary influences are then active—temperature, mode of illumination, admixture of foreign materials. To this latter cause of spectral modification, helium, we know, is abnormally sensitive.[197] And it is quite possible that some of the series emitted by it may be more liable to suppression than others, in which case the emergence of the yellow without the green set would be an effect of damping, not of density. It must, nevertheless, be admitted that what little relevant experimental evidence there is, scarcely countenances this surmise. The complex spectra derived by Professors Liveing and Dewar[198] from the volatile residuum which survived the freezing-out of the main constituents of atmospheric air, included rays taken impartially from all the helium-series. No quantitative analysis of the contents of their tubes was, however, possible; and the fact has been otherwise learned that the helium-ingredient of a blend must be predominant to become spectroscopically conspicuous. Every volume of hydrogen present in the chromosphere (neglecting the effect of metallic vapours) should thus probably be diluted with two volumes of helium; constituting the solar appendage largely a helium envelope. This important piece of information was brought within reach only by terrestrial observation of the new element captured from clevite.
The significance of calcium in its chromospheric relations has only of late been fully recognised. And this for an obvious reason. It is represented by only one pair of lines—bright H and K—and these are so near the limit of visibility that they could be effectively studied only by the aid of photography. They were indeed registered as leading features of the chromospheric spectrum by Professor Young in 1872; but he was entirely incredulous as to their calcium origin, holding it impossible that a substance with a vapour density forty times that of hydrogen should mount to at least equal elevations above the sun’s surface. Sir Norman Lockyer,[199] on the other hand, maintained them to be characteristic of a subtle dissociation-product of calcium, alleging in support of his view the progressive enfeeblement of the “blue line” of calcium (λ 4227), concurrently with the enhancement of H and K, as the substance was more and more completely decomposed at each addition of intensity to the electric current transmitted through the vacuum tubes. The case for dissociation appeared strong; it has, nevertheless, broken down. Sir William and Lady Huggins in 1897 successfully reduced calcium to the “two-line” condition by attenuation alone.[200] Thus the enigmatical prominence-spectrum of calcium was at last artificially produced, and no escape was left from the identification with true metallic calcium of the form of matter encompassing the sun with violet radiance.
A formidable problem, however, remained. Calcium near the sun seems to possess a levitating faculty altogether inexplicable. It floats as high up as hydrogen, or even overtops it. H and K are the most diffusive of all the prominence-rays; they are derived from the summits of the tallest flames, and from every fibre of their texture. This anomalous agility in a comparatively heavy metal must, according to the late Professor Keeler,[201] be the index to some remarkable property unrecognised by ordinary chemical methods. Unless, indeed, something analogous to electrolytic action be in question. There is much to be said in favour of M. Deslandres’s opinion that the chromosphere is electrically luminous;[202] and if so, then “ions,” not molecules or atoms, are presumably its constituting particles. But ions are on a Lilliputian scale of magnitude, and they may be of nearly the same mass for all the chemical elements. Here, however, we trench upon a region of pure speculation. A region, nevertheless, that is likely ere long to be annexed, in part at least, to surveyed territory, since pioneers are actively engaged there. Much, in the interpretation of solar phenomena, depends upon the results of their work; for here, as in every department of astrophysics, the experimental decisions of terrestrial physics must be awaited, not anticipated.
The three ingredients of the chromosphere so far spoken of—hydrogen, helium, and calcium—are found as well in its eruptive outgrowths. Gaseous prominences of all sorts and sizes are thus triply compounded. But we have now to consider a form of matter permanently present in the chromosphere, though rarely projected to any considerable altitude above it. Its badge is a single green ray, which has a curious history. It was momentarily identified with an auroral line; it was long erroneously identified with the distinctive corona-line. It is apparently reversed in the sun—that is to say, a Fraunhofer line falls just in its place. This is Kirchhoff’s “1474” (known as 1474 K), which has proved, under close scrutiny, to be triple. It results from absorption by iron, by cobalt, and by an unrecognised substance. Now the chromospheric ray agrees in position with the iron line, which is one of secondary importance; yet it cannot at present be asserted confidently that it really emanates from glowing iron-vapour. If it did, it should be ordinarily associated with other iron-lines, and none have been ascertained to make part of the fundamental chromospheric spectrum. The vapour giving out “1474 K,” however, is never absent from the solar envelope,[203] although it perhaps subsides at times into its lower strata.[204] On the other hand, it occasionally rises in metallic prominences to a height of about fifteen thousand miles. Mr. Lord, at the Lick Observatory, observed the enigmatical line (λ 5316·8) as shining vividly at the base of a violently disturbed prominence on 4th August 1892,[205] and Mr. Evershed records similar experiences. Dr. Fényi caught a still rarer effect on 19th February 1892 in a prominence attending the great spot-group then visible.[206] The object was peculiar, though not unique, in showing complete forms built up of the various metallic and other substances injected into it from below. Fig. 14 reproduces an instructive drawing made on the spot. The bottom sketch was taken on the C-line when the eruption was at its height. It reached an elevation of 56,000 miles. The overarching of three filaments towards a point at some distance from the base is noteworthy. The second drawing in Fig. 14 is of a date twenty-four minutes later than the first. It depicts the flame in “parhelium” light. The image viewed was constructed on the red ray of “green” helium. This was much smaller than the simultaneous hydrogen-image, and that obtained on the “1474” line had shrunken still further. But it was measured at half-past ten, an hour after the drawing on “C” had been made, when the first vehemence of the outburst had subsided. The line 1474 K is singularly exempt from displacement effects through motion. It remains erect and undeviated in the midst of solar storms. Nor does it widen perceptibly with increase of pressure downward. Its invariable fineness contrasts remarkably with the wedged shape near the photosphere of C, H, and K. Unusual agitation is betokened by its emergence in prominences.
Eclipse-spectrographs do not include it, while they have afforded some other quite unexpected results. Of special consequence is Mr. Evershed’s detection of titanium as an unfailing chromospheric element.[207] Plates exposed by him in India, 22nd January 1898, proved to be crowded with ultra-violet lines belonging to this metal. Some among them had indeed been already recognised by Mr. Jewell in Professor Hale’s daylight photographs of prominence-spectra.[208] The height to which they extended indicates a diffusiveness for titanium-vapour equal to that of hydrogen and helium, although inferior to that of calcium. Its atomic weight on the hydrogen scale is 48; it is just as much lighter than iron as it is heavier than calcium. But comparative vapour-densities are, so to speak, impotent for the regulation of elemental distribution near the sun. Another surprise afforded by the Indian eclipse was the conspicuous presence in the chromosphere of scandium as well as of manganese and chromium; while Mr. Hartley[209] has identified as another of its constituents the rare metal gallium from two of its characteristic lines (λ 4172 and λ 4033) recorded during the eclipse of 1893.
On 29th September 1897, Professor Hale, using a grating spectroscope on the forty-inch Yerkes refractor, discerned the green carbon fluting bright at the edge of the sun.[210] Two years later, he found the corresponding yellow band, although the third in the blue remained imperceptible. He inferred the permanent existence near the sun of a shallow layer of carbon vapour.[211] Its rays do not come near the surface; they have to be dredged for. Hence the extreme delicacy of detective observations. The anomaly of this behaviour on the part of carbon is glaring. Its specific lightness ought to carry it to altitudes far beyond those attained by titanium and calcium. Yet it lies sunken almost out of sight, while they float manifestly aloft. Some other sorting-out influence besides that of gravity palpably comes into play in the chromosphere and prominences. The possession by the sun of a carbon-envelope, which, although thin visually, must really be at least 500, and may well be 1000 miles in depth, has an important bearing upon the vexed question of photospheric constitution, and tends to strengthen a barely indicated analogy between the sun and a peculiar class of red stars.
The chromospheric spectrum, when its adventitious elements are reckoned in, is highly complex. During a few weeks of 1872, Professor Young, by vigilant watching, determined 273 lines seen, in the clear air of the Rocky Mountains, to flash out intermittently, one by one, or in companies together. And this first systematic enumeration was subsequently greatly extended by its author, while crowds of ultra-violet lines have been added by the photographic investigations of Hale, Deslandres, Evershed, and others. For the most part the rays brightened in eruptions are reversals of Fraunhofer lines, picked out largely at haphazard, yet with an obvious preference, expressly noted by Sir Norman Lockyer, for such as are vivified when the higher excitement of the spark is substituted for the arc in laboratory experiments. This has been taken to imply that the chromospheric is essentially a high-temperature spectrum; but the facts may be differently interpreted.
Among the elements most apt to shine evanescently in metallic prominences are sodium, magnesium, “green” helium, barium, iron, vanadium, and strontium. Gigantic ragged forms, especially when they appear in high latitudes, are of relatively simple chemical composition; or it may be that their condition favours the visibility of only the most persistent radiations. Small compact prominences yield, at any rate, much richer harvests to gleaners of spectroscopic novelties. A specimen of this class was observed by Father Sidgreaves, 10th September 1891.[212] It presented the aspect of “four blow-pipe jets intensely bright at the bends,” was 16,000 miles high, and lasted only an hour. Twenty-six brilliant lines were counted in the visual part of its spectrum, the invisible part, in the absence of suitable appliances, remaining unrecorded. The shape of this flame was equally well defined in both the red rays of helium, and appeared nearly the same in sodium and magnesium. The spectral peculiarities of such objects, however, are most marked in the ultra-violet. A photograph taken at the Kenwood Observatory, 15th October 1892, registered, from an undistinguished prominence, no less than seventy-four bright lines between the wave-lengths 3970 and 3630,[213] most of them being of unknown origin. But since then it has become possible to identify a dozen and upwards with rays measured in the spectra of krypton and xenon by Professors Liveing and Dewar;[214] and the circumstance seems to associate those scarce atmospheric gases with helium as chromospheric constituents.
Occasionally, under circumstances not yet defined or understood, prominences emit continuous light. The sheeny white objects sometimes seen under cover of the moon’s shadow owe their peculiarity to this cause. The whole gamut of prismatic radiance is derived from them, with the addition of bright H and K and a suspicion of hydrogen lines. Thus they are essentially calcium-forms interpenetrated with glowing dust. Their light is probably original. If it were reflected, traces of Fraunhofer-absorption which seem to be missing should be perceptible. Distinctively “white” prominences are not common; none were observed during the totality of 28th May 1900. But “red” prominences differ considerably in colour-intensity, all the ruddy shades, from deep ruby to pale pink, being represented in them. Many perhaps consist of a crimson core veiled in almost colourless material. Certainly all are not equally well seen in and out of eclipse. From a comparison of drawings made during the totality of 1870 with his own simultaneous daylight observations, Tacchini inferred that the spectroscope disclosed only the cores of flame in such objects;[215] and the experience was repeated on the occasion of the Egyptian eclipse of 17th May 1882. The four prominences then measured were of a rosy tint, lightening towards the margins, which looked as if edged with a lustrous fillet. “These results,” Professor Hale writes, “may be accepted as establishing an important difference between the spectroscopic and eclipse-images of prominences.” Nevertheless the difference is not constantly present. Some chromospheric forms are identical, viewed prismatically at the edge of the sun, or directly in the dark beside the occulting moon. These are, of course, purely gaseous; the others presumably give a mixture of continuous and discontinuous light.
A very curious feature of the prominence-spectrum was ascertained by Mr. Evershed during the eclipse of 1898.[216] It becomes continuous in the extreme ultra-violet. The range of unbroken radiance begins abruptly just where the hydrogen series ends (near λ 3668), and extends to the limit of the plate’s sensitiveness. Not even a guess can be hazarded at the physical condition underlying this radiative vagary. A different cause must be ascribed to certain local outbursts of white light in eruptive prominences.[217] This symptom of disturbance has been interpreted by Professor Hale as follows. “Objects of the kind,” he says,[218] “are closely related to faculæ, and probably rise from them. It thus occasionally happens that a violent eruption carries some of the white-hot particles to a considerable distance above the photosphere. In such a case the prominence gives a continuous spectrum in addition to its bright lines.” The explanation may pass muster, since no better has been offered.
M. Deslandres succeeded in showing, early in 1892,[219] that the sun may, in a restricted sense, be designated a “bright line star”—that is to say, he elicited from the aggregate of its light evidence of gaseous emissions. Treating the sun as a star by admitting into his spectrographic apparatus the whole of its rays simultaneously, he obtained vivid reversals of the violet calcium lines. But this is only possible when the calcium flames crowning faculæ are widely and strongly developed. Ordinarily their emissions are drowned in the surging flood of continuous radiance. But facular maxima recur, coincidently with spot-maxima, once in about eleven years; so that the periodicity of the sun might conceivably be determined by this one feature at distances obliterative of all other signs of disturbance. Not that the sun viewed, say, from Sirius, could with our actual appliances be detected, even at culminating epochs of agitation, as a bright-line star. Some of his fellow-stars, however, may be in a greatly enhanced stage of his condition, and we may learn to follow their vicissitudes by spectrographic observations of the alternate glimmering and fading of fine rays projected upon the deep shadow of their calcium-absorption. Thus the means may be afforded of ascertaining the flow of change in remote and gigantic orbs; and we shall perhaps in the future be better acquainted with the cyclical peculiarities of Capella or Arcturus than with those of our own “particular star.”
XI. THE CORONA
The corona is exclusively an eclipse phenomenon. No sooner is totality established than it is there. It seems to have emerged from nothingness, to have arrived from nowhere. It starts into view with the abruptness, the inexplicableness, of an apparition. “The sun,” Professor Langley says, recording his impressions of the eclipse of 1869, “went out as suddenly as a blown-out gas-jet, and I became as suddenly aware that all around where it had been, there had been growing into vision a kind of ghostly radiance, composed of separate pearly beams, looking distinct each from each, as though the black circle where the sun once was bristled with pale streamers stretching far away from it in a sort of crown.”[220]
The corona presents various aspects, but it may always be described as composed of extended streamers springing from a much more intensely luminous ring, the so-called “inner corona.” There is no real separation; the entire appendage is evidently framed on the same constructive principle; yet the distinction is obvious visually, and convenient descriptively. “I do not know,” Mr. Francis Galton wrote of the corona visible 18th July 1860, “to what I can justly compare that magnificent meteor. It differed from other objects in the remarkable whiteness and purity of its light, and also in the definition of its shape as combined with a peculiar tenderness of outline.”[221] Both he and Winnecke noticed the curvature of some “long arms of light” protruding from the ring, while other rays took “a more or less tangential direction.” Mädler was struck with the determinateness of the formation. What he saw was no vague light-effusion, but a congeries of sharply terminated beamy sheaves. This is a radical characteristic. The solar corona is a texture of significant pattern. There is indeed much difficulty in laying bare the original design. A spherical agglomeration projected on a plane gives rise to intricate effects of perspective, from which the true relations in solid space of the objects originating them can be deduced only by careful and systematic interpretation on strict geometrical principles. The problem was attacked by Professor Schaeberle[222] with the help of an ingeniously contrived model, photographs of which showed divergent rods inserted over the surface of a globe as apparently intercrossing and interlacing in the flat picture imprinted on the plates. It is, however, well to remember that, while curved rays may be projected so as to seem straight, straight rays can never appear curved. Beams that show flexure are inflected. Nor can a semblance of double curvature be given by perspective to those bent simply in one direction; moreover, rays that are actually normal to the sun’s surface must, from all points of view, appear radial to the limb.[223]
Coronal structure is of immense variety. It is intrinsically of a radiated character, and the fact is of primary importance. There are no signs of concentric arrangement;[224] coronal materials do not form shells or envelopes, such as surround the heads of comets; they are, on the contrary, drawn out into fibres by forces acting upon them in minute detail. Comparisons to spun glass and to silken filaments indicate the delicate nature of the shining tissue spread round the obscured sun. There are indeed differences. The fibres sometimes, as in the corona of 22nd December 1870, look to have been “combed out,” but more or less of derangement is usually prevalent. Tangled hanks of thread are often suggested, or “masses of luminous hair in disorder.”[225] These contorted forms, although their complexity is doubtless augmented by the superposition of sundry groups of twisted rays presented at different angles, afford remarkable evidence of disturbance within the corona itself. Their photographic registration dates from the Sumatra eclipse of 18th May 1901. On plates there taken with the Lick forty-foot telescope, and by Mrs. Maunder at Mauritius, a tumultuous area, agitated as if by the effects of an explosion, was strikingly depicted. “A long thread-like prominence appeared,” it was stated, “to emanate from the same source.”[226]
Fleecy coronal tracts are at times intermixed with regions of striation. The corona of 1868 was perceived at Masulipatam to be “slightly mottled” near the sun, and the “curdled” aspect of the great nebula in Orion has often been recalled to telescopic observers. Sir Norman Lockyer, at Baikul, 12th December 1871, was struck with an “exquisite” and “strongly-developed structure.” “I at once,” he continued, “exclaimed ‘Like Orion!’ Thousands of interlacing filaments varying in intensity were visible; in fact, I saw an extension of the prominence-structure in cooler material.”[227] It may be remarked that nebular tufts, no less than prominence-jets, are resolvable into fibres under the best conditions of seeing. The aureola of 1871 was of such incomparable beauty that M. Janssen could scarcely rouse himself from its delighted contemplation to carry out his programme of work. Numerous coral red prominences were relieved against the “velvet white” of the corona, the exterior shape of which was rudely quadrilateral. Interiorly the streamers leaned together in pairs so as to imitate flower petals, the general effect resembling that of a gigantic lucid dahlia, with the black moon for its heart.
Again and again, in descriptions of successive coronas, the Orion-similitude recurs. In the “density, brightness, and species of its light,” that of 1st January 1889 strongly reminded a Nevada State observer of the nebula, and its slightly greenish tinge of colour completed the likeness.[228] Again, in examining the coronal photographs of 9th August 1896, Mr. W. H. Wesley detected an area “broken up by dark channels into flocculent-looking masses, giving to it somewhat of the curdled appearance of some parts of the nebula in Orion.”[229] The similarity is not merely superficial. Laborious photographic comparisons by Mr. Ranyard (assisted by Mr. Wesley) emphasised the organic analogy between the great nebula and the solar corona.[230] Synclinal forms (as the petal-shaped structures are called) emerge in both, and the branching effusions round the trapezium seem to mimic details legible in many eclipse-pictures.
A chain of “pearly cones” furrowed spirally, 200,000 to 300,000 miles in height, and rising above a long bank of red prominences, were perceived by Professor Cleveland Abbe in the corona of 1869. And in 1893 the sun appears to have been fringed in this manner nearly all round, the individual peaks being projected together into such close array as to be in many places undiscernable apart. “Systems of (approximately) concentric arches” were also distinguished by Professor Schaeberle in his large-scale photographs of the same eclipse.[231]
Inverted shapes are also, though less commonly, met with. Paraboloids, convex towards the limb, now and then replace arcs and cones based upon it. A curious instance was afforded by Schaeberle’s “coronal comet” of 16th April 1893.[232] This object seemed as if spitted upon a slender, solitary, nearly radial streamer, from which it had evidently developed. It was not the only specimen of its class. A well-known drawing by Liais of the corona of 7th September 1858 shows an immense double paraboloid lying behind and partly hidden by a “dahlia petal.” The vertex seemed to be just at the limb. A somewhat similar dusky arc was seen by Winnecke in the corona of 1860.[233] It looked, he said, like a tracing in sepia. Again, during the eclipse of 1868, a bright parabolic outline, “with its vertex towards the sun,” was noted by Weiss. Finally, Homer Lane observed at Des Moines, Iowa, 7th August 1869, two condensations of light which “might well be compared to small telescopic comets, with tails of some length, but without a head, and with no distinct indication of a head at one end rather than the other. They were not far from radial in direction relatively to the sun’s centre, and had their origin above the limb of the moon.”[234] He estimated the length of each at about 130″. These appearances are full of meaning. They plainly assert the subjection of coronal matter to a dual repulsion, such as acts upon the “crystal tresses” of comets. A local centre of condensation throws off a filmy envelope, the constituent particles of which, as they approach the sun, are swept backward into a train by a counter influence proceeding from him. The only genuine “eclipse comet” so far captured was that seen and photographed at Sohag, 17th May 1882. It was sharply characterised as such, the effects of swift motion being unmistakably impressed upon its curved plumage.
An eclipse, visible in the Western States of North America, 29th July 1878, disclosed a surprising spectacle. In lieu of the ordinary radiated corona there were seen “bristles” of light at the sun’s poles, enormous “wings” at each side of the equator. Professor Langley observed the phenomenon from the summit of Pike’s Peak in Colorado, at an elevation of 14,000 feet in a stainless sky. Thus favourably circumstanced, he was able to trace one wide beam to a distance of about five millions of miles from the sun, the other fully twice as far.[235] The direction in which they lay proved, when carefully measured, to agree closely with that of the zodiacal light, and “a faint central rib” emphasised the coincidence. “With the telescope,” he says, “the whole of the bright inner light close to the sun was found to be made up of filaments, more definite even than those seen in sun-spots,” and apparently exempt from the effects of spherical projection; they “fringed the sun’s edge in definite outline, as though it were really but a disc.”
At the time of this eclipse, the sun was in a state of exceptional tranquillity, and a search through the solar archives brought out the notable fact that a similar apparition had, eleven years previously, spots then too being nearly extinct, been described and depicted by Grosch of Santiago. He inferred from it the possession by the sun of “strong magnetic polarity.” And indeed the divergent light-fibres at the poles, in 1867 no less than in 1878, seemed to trace precisely the lines of force in a magnetic field. The concurrence of these phenomena with critical epochs in the sun’s activity started the idea, due, in the first instance, to Mr. Ranyard,[236] of varying coronal types. It was amply borne out by subsequent experience. From eclipse to eclipse, throughout the eleven-year cycle, the corona exhibits changes of form in marked conformity to spot-vicissitudes. In the accompanying plate, the original of which is by M. Hansky, the coronas of 1860, 1870, 1883, and 1893, all of pronounced maximum type, are represented in the first column; those of 1867, 1878, and 1889 in the third. The second and fourth show coronas of intermediate forms. The last figure in the third column has a prophetic character. It shows M. Hansky’s anticipation of the kind of halo due, on the theory of recurring types, in 1900. What was actually photographed is given for comparison in Plate VI., Fig. 1. The correspondence leaps to the eye. A definite law of variation indeed quite obviously regulates the shape of the effluence about the sun. At spot-maxima its component streamers issue from all latitudes indiscriminately; they pay little or no regard to heliographic co-ordinates. Then, as disturbance relaxes, they gradually draw away from the poles, and tend to form “synclinals” above the spot-zones, giving to the whole appendage the “form of a four-rayed star, the points of which are inclined 45° to the sun’s axis.”[237] In the polar regions abandoned by them, “magnetic” filaments meanwhile become noticeable, and what may be called the intermediate type is constituted. It is subject, however, to indefinite variations of detail. A good example is shown in Plate VII., Fig. 1, from photographs taken on the Amur, 9th August 1896, by a Russian party under the leadership of M. Bélopolsky.
One ray, it will be noticed, is greatly longer than the others, and the same peculiarity distinguished the corona of 1898. Only when the tide of solar agitation is dead out, is the finished type of minimum aureola realised. We have then a symmetrical arrangement of crested poles and equatorial extensions, with this one singular qualification to its symmetry, that the wings are not a pair. One is formed of convergent, the other of parallel, or even divergent rays. And they seem to be reversed east and west at alternate epochs. Both are radically double; they are formed by the closing down upon the equator, as spot-activity becomes exhausted, of the synclinal groups previously visible in middle latitudes. It is difficult to realise that these “wings” are merely the profile-shapes of a vast luminous disc completely encompassing the sun. Hence an extreme intricacy of structural details most baffling to efforts towards interpretation.
Coronal modifications are not so entirely isolated as might at first sight appear. Looked at more closely, they are perceived to correspond unmistakably with the cyclical changes in distribution of surface disturbances. This was insisted upon by M. Bélopolsky in 1897.[238] Spots descend into lower latitudes with the approach of each minimum. One after the other, the eleven-year waves of commotion attain their acme in medium zones, and die out near the equator. Coronal development pursues the same course. Its most intimate relations, however, are with chromospheric eruptions. Tacchini[239] was the first to notice that coronal outflows emanate from regions frequented by prominences, which at times of maximum spread all over the sun, but near minimum withdraw from the extensive polar tracts simultaneously denuded of far-spreading streamers. Particular agreement frequently accentuates this general correspondence. Thus the springing of a coronal arch has usually a prominence for its motive. Each pearly pavilion is erected over a red flame. Coincidences of the kind are of perpetual occurrence. Chromospheric jets seemed (and doubtless were) appropriated individually to the “striated cones” observed by Cleveland Abbe in 1869. In Schaeberle’s fine photographs of the eclipse visible in South America 16th April 1893, one-sixth of the sun’s circumference came out clear of prominences; and just over the same segment there is a gap in the elsewhere unbroken range of coronal arches.[240] In some cases arches are buttressed upon prominences; in others they are symmetrical as regards them; coronal streamers appear to be vaulted into domes, or bent together into ogives, through effects of eruptive action in the chromosphere. These relations were especially marked in the corona of 1896. The wide polar rifts were devoid of prominences, but a prominence lay at the root of each great streamer, and a prominence was enclosed by each synclinal structure. These M. Hansky inferred, from their interior darkness, to be hollow,[241] like the tails of certain comets; and he noticed curious effects of coronal transparency, a few beams showing traceably through the substance of those in front of them. The correlation of prominences with coronal forms was somewhat less conspicuous in 1898 than in 1896, and was barely perceptible in 1900. The Lick photographs of 1901, however, showed the envelopment of a prominence by a “series of coronal hoods,” besides other symptoms of community in disturbance between the chromosphere and the silvery aureola.
The more closely the spectrum of the corona is studied, the more interesting and enigmatical it becomes. It has a triple origin. Continuous reflected light is mixed up in it with continuous original light, and these again with bright-line emissions. The three elements are not easily separated and the proportions of them present vary from time to time. The gaseous spectrum is feeble, especially near spot minima. Its leading constituent is a green line at λ 5303, long confounded with the chromospheric λ 5317. Their disconnection first became apparent in Mr. Fowler’s eclipse photographs of 22nd January 1898, and was announced by Sir Norman Lockyer[242] as one result of their examination. The green line characterises the unknown substance designated “coronium,” the distribution of which round the sun can be investigated by its means. Photographs taken during totalities with the “prismatic camera” give separate images of the solar appendages in each quality of discontinuous light emitted by them, and the “green” coronal image proves to be approximately ring-shaped. The gas it is derived from seems to spread through the “inner corona” to an average height of from 160,000 to 200,000 miles, but not to extend into the sheaves and streamers beyond. There are indeed irregularities. The annulus of coronium is far from being uniform or homogeneous. It is wider, perhaps also more condensed, in some places than in others, and spectrograms taken with a slit by Professor Campbell at Jeur suggested, by the distortions of the characteristic ray impressed upon them, the progress of radial movements, such as might well be deemed inevitable in an aerial envelope obviously not in a state of equilibrium.[243]
Ten or a dozen coronal lines besides the green ray have been photographed. The wave-lengths of the most authentically recorded are as follows: 4232, 4086, 3987, 3801, 3643, 3456, 3388, and 3381. No success has hitherto attended efforts to arrange them in a series; nor is it by any means certain that all claim the same chemical origin. On the contrary, the fundamental green line, together with the most refrangible of those above enumerated, appears capable of segregation from the violet ray λ 3987 and the first three of its ultra-visible associates.[244] Two strange gases then, at any rate, are indicated as co-existing in the corona. And they are unmixed with any familiar substance. Evidence collected during recent eclipses testifies strongly to the absence of all the chromospheric materials. Not even the pervasive trio, hydrogen, helium, and calcium, extend into the vast solar halo. Some of their radiations, notably H and K, have seemed to be derived from it, but only through the effects of atmospheric scattering. They come as well from the black disc of the moon. Probably only some peculiar forms of matter, or forms of matter in a peculiar state, constitute the gaseous corona. So far coronium has not been recognised elsewhere.
The continuous light from the interior halo is mainly original. It emanates from incandescent solid or liquid particles. But their incandescence appears to be of an unusual kind. Bolometric experiments, carried out by Messrs. Abbot and Mendenhall of the Smithsonian Observatory during the eclipse of 1900,[245] showed the coronal beams to be almost wholly wanting in thermal power. Compared with them moonlight is a potent source of heat. They include, according to the results in question, next to no infra-red waves, and are surmised to be comparable in quality to the glow of phosphorescent or luminescent substances. Novel inquiries in the laboratory will be needed to ratify these significant conjectures; while it is to be hoped that the eclipse of 1904 will afford some positive data as to the distribution of energy in the coronal spectrum, which may serve as a basis both for photographic investigations and for theoretical conclusions.
Photospheric light must be reflected from the pulverulent materials of the corona, and a reflected ingredient is, accordingly, contained in its radiance. It is, however, small in quantity. Thus the dark Fraunhofer lines are barely distinguishable in it. They were detected by Janssen in 1871; fifteen were photographed by Pluvinel at Senegal in 1893.[246] On the same occasion, nevertheless, Deslandres could obtain no trace of them, and Campbell was equally unsuccessful in 1898.[247] The truth is that where the original emissions are strong they get drowned out. They show, and that with difficulty, only in the less luminous sections of the appendage. This was made perfectly obvious by Mr. Perrine’s discussion of the plates taken in Sumatra, upon which thirty-five Fraunhofer-lines impressed themselves, but only in regions remote from the limb. Polarisation-effects give accordant testimony.[248] They are slight but unmistakable, and plainly indicate the action of scattering particles right up to the limb. Some hints as to the manner of their distribution were obtained by Professor Turner in 1898, and again in 1900, through the ingenious device of photographing the corona across a plate of Iceland spar, and so obtaining two pictures in oppositely polarised light.[249]
Much remains to be learned about the nature of coronal radiance, and the opportunities for its investigation are restricted and unsatisfactory. Yet upon their outcome success in coronal portraiture largely depends. Eclipse-photography is an art in itself, and one beset by subtle difficulties. To ensure the best results, the plates employed should have a curve of sensitiveness as nearly as possible coincident with the energy-curve of the coronal radiations, and the form of the latter is entirely unknown. Many questions too arise regarding the quality of the plates, and the development proper to be given them, regarding the best kind of instrument for exposing them with, and, above all, the length of time that should be allotted to the process. And here the obstacle is encountered that no single exposure is suitable to the entire aureola. One long enough to bring out the streamers is too long for the delicate details of the brilliant interior. The choice has to be made between solarisation and incompleteness of representation. Two remedies have been tried. The first is by piecemeal delineation. From photographs of varied exposures, a picture showing the special features rendered by each is compounded by a skilled draughtsman. But it has no longer an autographic value; the forms embodied in it have been deliberately selected and unconsciously emphasised. “To a certain extent,” it has been well said,[250] “the same personality enters into the examination of a photograph that is known to exist in naked-eye observations of the corona.” M. Morin’s drawing of the corona of 1896 (see Plate VII. Fig. 1) is a fine example of what can be done by combining multiplied photographic impressions. The two best of those availed of were taken with an ordinary camera in one and two seconds respectively; the rest with a photoheliograph, getting exposures up to thirty seconds.[251]
The alternative method is purely automatic. It was invented and successfully applied by Professor C. Burckhalter of the Chabot Observatory, California, at the eclipse of 28th May 1900.[252] In his apparatus a system of revolving diaphragms is arranged so as to give exposures graduated to correspond with distance from the sun. The image being progressively covered at successive short intervals, time is allowed for the imprinting of faint coronal extensions, while the bright parts already effectively portrayed are shielded against further prejudicial action. The photograph “controlled” in this way was a striking record, and conveyed some curious intimations of perspective effects in the mutual overlaying of tufts and beams.
The outlying branches of the corona usually baffle the perception of the sensitive plate; for they merge into a dimly illuminated sky-ground, from which they are, unless by special precautions, chemically indistinguishable. The most conspicuous performance in this direction was by Mrs. Maunder in 1898.[253] With a lens of only 1½ inches in aperture, giving small but intensely bright images, exposures were experimentally made of inordinate length, proportionately to the shortness of the focus. The unprecedented result was achieved of photographing rays to a distance of nearly seven solar diameters from the limb. Mr. Wesley’s drawing from the original negatives is reproduced in Plate VII. Fig. 2. Four long narrow divergent rays are visible in it, each starting outward from a synclinal structure. Mr. and Mrs. Maunder hold it probable that such structures have always rod-like extensions, needing only protracted photographic exposures to bring them into view;[254] but this is more than doubtful.
A very perplexing appearance is that of dark markings in the corona. They are not mere interspaces between brilliant rays. Mr. Wesley, who is an expert in the scrutiny and interpretation of celestial photographs, vouches for their reality. Figs. 15 and 16 copy his diagrams of obscure streaks and veinings in the coronas of 1871 and 1896. In the first case, they cut right across the lustrous branches of the halo; in the second, they are in obvious connection with prominences. Indeed, black coronal and black chromospheric forms belong undeniably to the same order of effect, and cannot be separated causally. They took another shape in the corona of 1900. Mr. Wesley’s beautiful drawing from Mr. Maunder’s photographs (see Plate VIII.) shows rifts apparently darker than the general background of the sky, and hence of a positive character. Their substantial presence was confirmed[255] by negatives taken at Wadesborough, U.S.A., by Miss Gertrude Bacon; but the difficulty of accounting for them is at present insurmountable. That they are due to the interposition of opaque bodies can scarcely be admitted. The objections are prohibitive. Yet the phenomenon is none the less genuine for being incomprehensible. We must wait and compare.
The embarrassments attending coronal photography are enormously enhanced by the effulgence of daylight. Success here is more earnestly desired the less it can be hopefully anticipated; for the prospect is dim of realising Sir William Huggins’s scheme of 1882, or any modification of it. It is true that, during the partial phases of the last couple of eclipses, sensitive plates were impressed by the inner corona, but it only showed as a vague glow throwing into relief the small segment of the moon outside the sun some forty seconds before and after totality.[256] Still, even this scanty measure of success was welcome as an earnest of what the incalculable future might bring. Everything depends upon catching differential effects—upon obtaining plates capable of feeling the delicate gradation between daylight pure and simple and daylight plus corona. And this would be greatly facilitated by acquaintance with the law of intensity in the coronal spectrum. It ought to be stronger in the upper reaches than the ordinary solar spectrum, since the corona escapes the heavy toll of blue absorption exacted from the photosphere by the “smoke-veil”; the question is, can this presumable superiority be rendered predominant enough for the ends of portraiture? The use of coloured screens, letting through the more refrangible rays, and barring out those lower ones in which mere glare has the advantage, has proved ineffectual; and the “double-slit method,” so splendidly helpful in other departments of solar physics, has also been tried in vain.[257] In coronal photography, no bright line can serve the end in view because the gaseous spectrum belongs only to the inner corona, and the record aspired after is more especially of the outer corona, with its characteristic plumes, streamers, and aigrettes. Until the changes these undergo can be followed day by day, little will be satisfactorily known of their intimate relations with the different orders of solar phenomena, and still less of the underlying cause of variation.
It has yet to be determined whether the corona rotates with the sun. Opposite motion-displacements above the east and west limbs, of the green line or one of its companions, would settle the point; but they show, if at all, most evasively. M. Deslandres first attempted such measurements at Fundium in West Africa, 16th April 1893.[258] He chose, however, as the object of his attack the K-line of calcium, since ascertained to be non-coronal, so that his results were null and void. Yet they marked a starting-point, for they sufficed to introduce the research definitively into the eclipse-programme; nor will it be dropped out of it, we may hope, until a substantial increase of knowledge has been gained. Subsequent experiments, although legitimately conducted, have been indecisive. Mr. Newall, in those tried by him at Pulgaon, 22nd January 1898, went too far afield for their materials.[259] He directed his spectroscope to points 8′—corresponding to upwards of 200,000 miles—from each limb, whence no bright line could be obtained, since they lay outside the limits of the gaseous corona. Professor Campbell,[260] on the other hand, found in spectrograms taken east and west of the sun during the same eclipse, a difference in position of the green ray giving an ostensible radial velocity of 3·1 kilometers per second, suggesting rotation at half that speed; but he regarded its genuineness as open to grave doubt.
More hopeful than the method of simple displacements is perhaps the method of inclinations recommended by Deslandres.[261] A small, intensely luminous image of the corona being thrown upon the slit of a powerful spectroscope, the varied deviations of the bright lines derived from its different parts should tell something as to the mode of motion prevalent throughout the appendage. If it rotates like a solid body, all in one piece, the velocity increases outward, and the lines would lie aslant on the plate in corresponding directions. If, however, the regimen be analogous to that governing Saturn’s ring-system, in which every component particle revolves as an independent satellite, then the speed of coronal matter slackens with increase of distance from the sun, and the spectral rays emitted by it should be deflected the opposite way. The criterion, if it prove applicable, will be highly discriminative as regards rival hypotheses.
These may be classified as ejective, meteoric, and electro-magnetic. Professor Schaeberle analyses the phenomenon into “streams of matter ejected from the lower latitudes of the sun.”[262] To the materials of the longest rays he ascribes initial velocities up to 400 miles a second, and excursions outward to the remote neighbourhood of Jupiter or even Saturn; ogives and arches being due to eruptions of less violence; while the interplay of innumerable curving jets, foreshortened in all possible ways, explains the complex aspect of the sun’s lucent crown. Mr. and Mrs. Maunder, again, consider the prominences to “represent centres of strong eruptive action, and that in consequence of such action coronal matter is driven upward from the sun over a very wide area in domelike forms.”[263] A succession of arches results, “the outer being less definite and complete than the inner ones. Outside all we find the curves defining the boundaries of the synclinal group.”
There is much plausibility in these inferences. Prominences are visibly spouted or flung upward, and the lustrous filagree vaults often rising above them can scarcely have a totally dissimilar origin. But the whole secret is not thus laid bare. The rationale by eruptive action is no sooner sought to be made exclusive than it meets contradictory facts. It offers no admissible explanation of varying coronal types; it ignores the mysterious coronium envelope; it seems to be negatived by the bolometric observations of Abbot in 1900; for obviously the light of the supposed bombs, projectiles, or pulverulent ejected streams should contain the ordinary solar proportion of heat-rays, which are nevertheless markedly deficient.
The corona, according to Sir William Huggins, must consist of “incandescent fog.” And Professor Newcomb,[264] following a similar train of ideas, avers it to be made up of detached particles, wholly or imperfectly vaporised. They might be most sparsely distributed. Intense radiance would, he informs his readers, result from the occurrence of a single fragment of dust in every cubic mile of space about the sun. But how is the dust (if dust there be) supplied? Does it come from within or from without? Here the upholders of the meteoric theory join issue with the eruptionists. There must be rings and streams of meteors revolving quite close to the sun in orbits of all possible inclinations and considerably varied eccentricities. These furnish, we are told, the materials of the corona, which—as Dr. Scheiner has recently shown[265]—are raised to a temperature of incandescence by direct solar radiation. This view, nevertheless, like the eruption-hypothesis, is scarcely tenable in view of the non-thermal quality of coronal light.
There remains the electrical theory. Formally enounced by Sir William Huggins in 1885,[266] it remains unverified indeed, yet unrefuted. Coronal streamers are regarded by it as analogous to comets’ tails; they issue forth under the influence of a repulsive force emanating from the sun; they are illuminated by electrical discharges due probably to differences of potential at their bases and extremities.[267] The magnetic relations of the phenomenon, vividly indicated by the minimum forms exemplified at the two eclipses of 1889, were ably discussed by Professor Bigelow.[268] He analysed coronal structure by spherical harmonics, on the supposition of its dependence upon some mode of action similar to that of free electricity, “the rays being lines of force, and the coronal matter being discharged from the body of the sun, or arranged and controlled” by a power proceeding from it. He further showed that the power was of a repulsive nature and varied inversely as the square of the distance;[269] but he avoided speaking of it as “electrical” out of “deference to the doubt that free electricity can exist at such high temperatures as prevail on the sun’s surface,” content to have proved “that some force is present acting on the corona according to the laws of electric potential.”
The diagrammatic halo laid down on these principles was indeed the very “twin Dromio” of the corona of 1889; but they stood ill the test of prediction. The corona of 1893 failed to exhibit the special features anticipated for it by Professor Bigelow. Plainly the assertion that an illuminated magnetic field surrounds the sun, although scarcely deniable, does not comprise the whole truth. The same may be said of Hermann Ebert’s “electro-magnetic theory.”[270] He defines the corona as “the visible reaction of the finely-divided matter in the vicinity of the sun upon the dielectric polarisation proceeding from the different parts of the sun.” Luminosity is evoked by Hertzian oscillations propagated outward with the velocity of light, and its filamentous texture corresponds to differences in dielectric stress connected with the distribution of electricity on the sun’s surface.
The auroral aspect of the corona has often been commented upon. M. Ebert remarks[271] that the magnetic lines of force near the earth are not more definitely traced out by the play of polar lights than are those about the sun by the disposition of coronal rays. The truth of this, however, is patent only as regards aureolas of the minimum type. In those visible at maximum it is at any rate disguised. For, as the sun’s internal activity augments, beamy outflows predominate over tufted effluences, although both kinds of radiance may be simultaneously present. Professor Holden considered the sun to be hairy all over, and not merely at the poles. “There is no latitude,” he wrote, “at which we can say that here the polar rays end and a new species—equatorial rays—begins.”[272] This was also recognised by M. Hansky in his study of the corona of 1896;[273] but the superposed streamers and arches can with difficulty be included in any magnetic theory. On the other hand, Mr. Pupin of Columbia College obtained in 1892 striking imitations of the maximum type of corona by means of electrical discharges through partially exhausted bulbs.[274] Effects of polarity being, however, wholly absent, the reproduction failed to convey one fundamental characteristic of the real phenomenon. But the deficiency was supplied, three years later, by Ebert’s experiment of subjecting “coronoidal” tubes to the action of a powerful magnetic field. The organising effect upon the light-effluence was just what was needed, according to Bigelow’s contention, to give nature’s own imprimatur to his “magnetic theory of the solar corona.”[275]
A fact of high import in this connection is that the coronal bright lines are not reversed in the Fraunhofer spectrum. Coronium, and the gases associated with it, exercise no perceptible absorption upon the light transmitted through them. Now this kind of inertness, according to M. Cantor’s experiments,[276] is distinctive of substances glowing by electrical stimulation, so that we have here—as the late Professor Fitzgerald pointed out—a confirmation, absolutely sui generis, of the conjecture that coronal emissions are analogous to those of an illuminated vacuum-tube.
Yet none of the views propounded on the subject are completely satisfactory. They have points of contact with truth, but they do not closely embrace it. This, indeed, could hardly be expected at so comparatively early a stage of coronal research. For the questions involved are beyond measure baffling and intricate. We may re-enumerate them.
One that is fundamental relates to coronal heat. Its virtual absence, attested bolometrically at Wadesborough in 1900, must be confirmed during future eclipses before Deslandres’s contrary inference[277] as to the copious presence of long waves in coronal light can be finally dismissed.
The coronium-envelope offers a problem which stands almost apart from that of the stellate appendage surrounding it. And it is one that can scarcely yet be grappled with. No familiar substance enters into its composition. Its ingredients are altogether exotic. They do not diffuse into the chromosphere, while those of the chromosphere are as strictly excluded from the corona. The cause of this extraordinary circumstance will perhaps long remain obscure. Meanwhile, through the attempted apportionment of the coronal rays photographed in totalities between sundry hypothetical substances we are led to regard coronium as only one of a group of gases foreign to terrestrial experience.
The rotation of the corona can be measured only by the most refined methods; but there is little doubt that they will be successfully applied. Movements of other kinds may also be spectroscopically determined, since they are likely, in many cases, to take directions oblique to the limb, and therefore to have large components along the line of sight to the earth. Notwithstanding the lasting, and, in some respects, the growing importance of securing the best possible picture-photographs of the corona during totalities, novel revelations are scarcely to be expected from them. Their leading interest just now centres, first in the structural relationship of coronal arches with prominences, next, in the information they may afford about dark markings in the solar appendages. For the rest, we must look to daylight photography. When the great desideratum is attained of getting behind (as it were) the veil of atmospheric glare, we shall be able to trace the progress of coronal change, to follow the unbuilding and rebuilding of the typical aureolas, to witness, perhaps, sudden coronal developments in sympathy with chromospheric outbursts. From the vantage-ground thus gained, in short, the true function of the corona in the solar economy can be systematically investigated. During the few crowded moments of eclipse this is not possible.
XII. THE SUN’S ROTATION
The mode of the sun’s rotation is perhaps the most significant feature of his constitution. A thorough understanding of it would doubtless bring with it an explanation of many other outstanding difficulties. But it seems, unfortunately, a long way off. No more has been attained as yet than the representation of the observed facts by empirical formulæ. That is to say, a law of order has been discerned in them although their cause remains obscure.
No single period of rotation can be ascribed to the sun. Each element of the photosphere, probably each layer of the chromosphere, moves round the axis in a fashion of its own. The fundamental rate, if such there be, is so masked by local drifts as to be unrecognisable. The most hopeful road to its eventual detection seems to be by the gradual disentanglement of the solar influences affecting terrestrial magnetic phenomena. The immediate task in hand, however, consists in extending and giving precision to knowledge on the whole subject, in bringing varied methods to bear upon it, and in linking into some kind of sequence the circumstances ascertained.
Until spectroscopic and photographic means became fully available, the solar rotation could be determined only by timing the circuits of spots. And spots were very soon found to have “proper motions” precluding them from discharging the function of points of reference. Hence there could be no unanimity as to the sun’s rotation, the period arrived at by each observer depending upon his choice of spots. At length Carrington’s scrutiny during the years 1853 to 1861 showed the systematic nature of these baffling diversities. They proved to vary with heliocentric latitude, and the great solar swirl was brought into evidence. Once in about twenty-five days the visible surface of the sun sweeps round at the equator; but as the distance from it increases, the time lengthens progressively. The rate of retardation is given mathematically in the following expressions, which, being artificially adjusted to correspond with observation, might be multiplied and modified indefinitely:—
FORMULÆ OF SOLAR ROTATION.
X = 865′ − 165′ sin ⁷⁄₄ l (Carrington). X = 1011′ − 203′ sin (41° 13′+ l) (Spoerer). X = 862′ − 186′ sin^2 l (Faye). X = 858′ − 157′ sin^2 l (Tisserand), X signifying the daily angular motion, l the solar latitude.
Formulæ of the kind, moreover, are of restricted application. Based exclusively upon spot-measurements, they can scarcely be trusted outside of the spot-zones—that is, beyond 38° north and south of the equator. Carrington’s gives a period of 25^d 9^h 53^m where l = 14°, of 26^d 9^h 9^m for l = 30°; and he adopted as the standard period 25^d 9^h 0^m, conformed to in latitude 13½°. But what this average speed of transport actually represents, is the mean rate of motion of the multitude of sun-spots,[278] not the rotation of the body of the sun. Professor Bigelow argues from meteorological analogies that this prevails without disguise at the equator, while retrogressive currents of a “trade-wind” character lengthen the periods derived from observations in the spot-zones.[279] But such-like comparisons are plainly inadmissible. More plausible, although far from decisive, is his contention that the equatorial period must be the true one because it lends itself to the correlation of terrestrial phenomena—auroræ, magnetic storms, wind and weather changes—with solar outbreaks.
Solar rotational studies entered upon a new phase with the application to them of the spectroscope. The particular importance of the innovation lay in the change of venue which it involved. A new court, so to speak, was constituted, a fresh set of witnesses called. These were the Fraunhofer lines; and while requiring more delicate treatment, they seemed likely to prove more trustworthy than their predecessors. Spots do not float inertly on the photospheric tide. They are subject to individual hurryings and laggings that to some extent invalidate the record of their axial progress. Fraunhofer lines, on the contrary, are eminently steadfast. They fall short, it is true, of the absolute fixity formerly ascribed to them; yet after every deduction has been made, their reputation as natural constants remains substantially intact. It must, however, be borne in mind that what the Fraunhofer lines tell about the sun’s rotation is not strictly comparable with the information derived from spots. For they proceed from a different level, and may therefore obey a different law of revolution. They indicate the velocity, not of the sun itself, but of the absorbent strata in which they originate.
The eastern limb of the sun advances, the western limb recedes at the rate of 1·2 miles a second. The consequent line-displacements amount to just ¹⁄₁₅₀ the little gap between the D-lines of sodium. In 1871 their simple detection by Vogel was a feat of some moment.[280] Five years later, Young was enabled, by his early possession of a grating or diffraction spectroscope, to fix their range with approximate accuracy. Then Langley showed how, by their means, to distinguish at a glance lines of solar and telluric production. For in juxtaposed spectra from opposite ends of the equator, the solar lines, being affected by rotation, are manifestly “notched,” while atmospheric rays run straight on without a break. They stand self-announced as of domestic production.
The differential plan of measurement thus suggested obviates many forms of otherwise inevitable error. It was adopted by Dunér in his classic work on the sun’s rotation, presented to the Royal Society of Upsala 14th February 1891. He selected two iron lines in the red (λλ 6301·72, 6302·72) for comparison with a pair of adjacent oxygen lines from the “Alpha” band, known to be terrestrial, and therefore exempt from motion-shiftings.[281] They were then safely treated as fiducial; and the intervals between them and the solar lines differed, east and west of the equator, to an extent corresponding with rotation in a period of twenty-five and a half days. By the use of the utmost refinements, the observations were carried up to within fifteen degrees of either pole, where the period was found to be protracted to thirty-eight and a half days, the intervening zones showing intermediate velocities. Direct acquaintance with the sun’s axial movement was thus extended far beyond the regions of spot-occurrence; and only direct acquaintance is, in this matter, of any avail, since the formulæ devised to suit low latitudes break down nearer the poles, disclosing their unsoundness by a total want of agreement in the periods calculated from them.
“Carrington’s law” might have been true of spots only; it might have denoted some peculiarity in their mode of production, causing a systematic increase of backward drift, north and south of the equator. The Upsala measures, however, proved it to apply, irrespectively of spots, to the solar globe generally. They told, indeed, something more than this. The period deduced from them was longer than that given by spots. The difference amounted to about half a day, and it persisted in all latitudes—that is to say, the vapour of iron surmounting the photosphere gyrates more slowly than the spots in the photosphere. A variation of angular speed with elevation above the sun’s surface was for the first time indicated. Confirmatory evidence was soon forthcoming.
The photographic investigation of facular movements was attempted by Dr. Wilsing at Potsdam in 1888.[282] From 1012 measurements executed upon 108 plates, he obtained a constant angular velocity of 14°·27 per diem, equivalent to a period of 25·23 days, which is just that of spots situated ten degrees from the equator. But the conclusion that faculæ in all parts of the sun conform to it was certainly fallacious. M. Bélopolsky made this apparent in 1892,[283] and M. Stratonoff of Taschkent still more decisively in 1894–96.[284] His research included the determination of 2158 positions of 997 faculæ on 316 Pulkowa plates, and furnished the clearest evidence of their poleward retardation. The Stonyhurst drawings, discussed by Fathers Sidgreaves and Cortie,[285] yielded similar results. Concomitant increase of period and latitude is, in fact, a rule without exception on the sun.
Faculæ, however, have a mean rate of their own. In the same parallels they are transported more rapidly than either spots or absorbent vapours. Their period of rotation is the shortest attributable to any solar formations. The main facts of the case can be taken in by a glance at the following little table:—
PERIODS IN DAYS ┌────────────┬───────┬──────┬─────────┐ │Heliographic│Faculæ.│Spots.│Reversing│ │ Latitude. │ │ │ Layer. │ ├────────────┼───────┼──────┼─────────┤ │ 0°│ 24·66│ 25·09│ 25·46│ │ 15°│ 25·26│ 25·44│ 27·49│ │ 80°│ 25·48│ 25·81│ 31·83│ └────────────┴───────┴──────┴─────────┘
The equatorial facular period, it is worth noting, is nearly identical with that ascribed by Hornstein, on the ground of magnetic observations, to the mass of the sun.
The diversities exhibited above are as perplexing as they were unexpected. They do not even fall readily into any satisfactory order of progression. The slowest movement belongs to the reversing layer, or at least to the slice of it stopping out Dunér’s iron lines; for there is no certainty that the entire stratum rotates unanimously. It is, however, certain that it covers both spots and faculæ. The level of absorption is higher than the level of the photosphere with all its immediate appendages. This fact has been already insisted upon;[286] it may perhaps usefully be reasserted in the present connection. Fraunhofer-absorption is stamped in the prismatic rays of faculæ and sun-spots precisely as in those of the photosphere. They have then been demonstrably sifted through the same screen of incandescent vapours. We have yet to learn that they escape any minutest part of the absorptive effects produced in ordinary sunlight. They must accordingly be submerged beneath the whole series of strata occasioning them. At the same time, it has to be borne in mind that Dunér’s deductions rest upon a narrow basis. He measured only a single pair of lines, and we lack the specific assurance that those individual lines occur in the facular and spot spectra. Presumably they do; no reason is apparent why they should behave exceptionally; but a direct record of their actual presence would be satisfactory. They should also be looked for in the “flash” at the edge of the eclipsed sun. Their detection as brilliant lines would confirm and settle their status.
But here we encounter an anomaly. Since the reversing layer rotates more slowly, and lies higher than the photospheric formations, a law of retardation with altitude might be assumed. Its prevalence is, nevertheless, contradicted by the relations of spots and faculæ. Faculæ undeniably rise above spots, yet they wheel more rapidly; they give a shorter period. This seems to be established by Wolfer’s[287] confirmation of Stratonoff’s results. Thus while retardation outward is indicated by the reversing layer, acceleration outward is forcibly suggested by faculæ. And there is a general consensus of opinion that this rule applies generally, so that the comparative tardiness of the absorptive region stands over as an unexplained discrepancy. Even within the region itself, Mr. Lewis E. Jewell has found indications of diminishing angular velocity towards the photosphere, where also the equatorial quickening is small compared to its value higher up.[288] Moreover, Bélopolsky’s speculations as to the nature of the corona, which probably adumbrate, if they do not convey truths, require that it should rotate much more swiftly than the sun itself.[289] The forecast will doubtless be tested ere long by eclipse-spectrograms. Indeed, further evidence is much needed on a number of crucial points connected with the sun’s rotation.
Dunér confessed his inability to imagine a “Why” for the singular rotatory régime of which he had clearly expounded the “How.” “It constitutes,” he remarked, “one of the most difficult problems in astrophysics.” One theory after another has been proposed, and sunk out of sight, overweighted by manifest inconsistencies. A machine “going” like the sun the wit of man has not yet been able to devise. Nevertheless, a means of evading the difficulty has been found. The possibility has been perceived, and perhaps too readily admitted, of regarding it as a legacy from chaos to cosmos. The embarrassments surrounding the subject may be relegated to a far-distant age, and to a state of things for which the investigators of to-day disclaim responsibility. The anomaly, on this view, is a survival of nebulous conditions, which, so far from having a present sustaining cause, is, and has always been, subject to the destructive agency of friction. Why then, it may be asked, has it persisted throughout the æons of the sun’s growth? Should it not have been quite early abolished, if it be nothing more than a residual inequality, begun to be smoothed away in the dim foretime when the solar globe took shape? Mathematicians reply in the negative. Wilczynski of Berlin gave in 1896 an apparent demonstration that internal resistance cannot change the diurnal arc described by any point on the sun to the extent of two minutes in twenty-seven million years.[290] But Harzer showed it to be applicable only to cases non-existent in nature.[291] It would be valid under ideal circumstances; things being as they actually are, it falls to the ground.
Starting from less questionable premisses, however, Wilsing of Potsdam in 1891,[292] and Sampson of Durham in 1894,[293] reached practically the same conclusion. They agreed that millions of years must elapse before the sun comes to rotate “all of a piece,” like a solid body. There is, indeed, one flaw in their reasoning. Both limit convective circulation within the solar globe to a relatively thin shell of material. They assign to radiation an essentially superficial character. In the attempt to prove that the rotational currents flow without impulsion, they sacrifice the functional efficiency of our great light-giver. For assuredly its immense output of radiant energy can only be supplied from interior stores, rendered available by powerful and deeply rooted vertical currents. But the continued activity of such a system should speedily efface inequalities of surface drift, unless some countervailing force maintained them.
Dr. Wilsing asserts that, in the comparatively late “phase of celestial evolution represented by the fixed stars, radial currents are beginning to disappear.” It might more plausibly be argued, considering the intense brilliancy of such bodies, that radial currents are in them at a maximum of strength and volume. “We are therefore relieved,” the author continues, “from the difficulty of accounting for ‘Carrington’s law of rotation’ on mechanical and physical principles, since it appears as the result of earlier conditions of motion.”[294]
We leave a good deal to the coming time; why should we not shift an occasional perplexity back to the broad shoulders of antiquity? Much that is inconceivable in a sun might be possible in a spiral nebula. No such expedient, however, will answer the present purpose. The originating cause of the sun’s rotational anomalies, whatever it may have been, continues to act. In a globe so profoundly disturbed, they could not possibly have survived of themselves.
Professor Young finds the necessary driving power in falls of cooled materials, bringing with them to the photosphere the swifter motion appertaining to a wider circumference. The consequent accelerative impulse would be greatest at the equator, and would diminish to nothing at the poles, according, so far, with the observed facts. Spots, moreover, by onrushes at epochs of reconstruction or recrudescence, formally acknowledge the receipt of supplies from above. But Carrington’s “law” governs all the solar formations, and the rationale by continuous downward precipitations seems to be of very partial applicability. Faye tried to solve the problem on an inverse principle. Instead of descents from without, he postulated ascents from within, the equatorial retardation being less than that in high latitudes because the rising matter comes from nearer the surface. This, however, was a purely artificial arrangement, with no voucher for its reality. The explanation needed to be explained. Speculation here, as elsewhere, must await the progress of direct inquiry.
XIII. THE SOLAR CYCLE
Solar periodicity is a most complex phenomenon. The more it is studied, the less it seems to be understood. Its effects branch out into endless entangled and obscure fibres of fact, to trace all of which back to their root-source would be an almost superhuman task. At present they can only be dealt with in groups and tentatively. Their subtle, and often disguised relationships need much patience for unravelment. They need, above all, a free mind. Prepossessions are sure to compromise truth.
The sun is subject to a rhythmical tide of disturbance, ebbing and flowing in about eleven years. But the flow is irregular and spasmodic. Both the intensity of the crises and the intervals at which they recur vary largely and unaccountably. Probably the eleven-year cycle is involved in others. One, there is reason to believe, brings about alternate accentuations and partial effacements of change comprised within a term of some sixty-five years.[295] And minor pulsations—wavelets on the great rollers—are besides evident. Prediction, nevertheless, remains at fault. Spot-maxima are delayed or anticipated, they are languid or energetic, as the outcome of modes of action defying calculation. Not even the loose fetters of an arbitrary formula have ever been forged for them. The attempt would indeed be hopeless, since the laws governing them, besides being highly intricate in themselves, are plainly disturbed in their working. Circumstances intervene which we must call “accidental.” Could we describe them in detail the science of solar physics would lie before us as an open book.
The “error” of the spot-period may amount to nearly half its normal length. Thus sixteen years elapsed between the maximum of 1788 and the next certainly ensuing, and only 7·3 years separated the culminating points in 1829·9 and 1837·2. A characteristic feature of the representative curve is that it mounts more rapidly than it descends. Maxima succeed minima, on an average, after 4·5 years, while the corresponding minima are only reached after 6·1 years. Substantially, a disparity of this kind is probably always present, although now and again masked by the prominence of a secondary maximum. These peculiarities deserve the most careful attention, as sufficing in themselves to place the sun in the category of variable stars. His spot-curve might almost be said to be modelled on the light-curves of such objects; and the analogy is eminently instructive. We learn from it, for instance, that a spotted condition in the sun matches a phase of strong luminosity in the stars; and are hence led to infer that the sun radiates most powerfully when his disc is most maculated. The extreme difficulty of obtaining direct proof of this relation lends especial value to the side-wind of evidence thus brought to bear.
Now an increase of radiation involves a quickening of the sun’s internal circulation; and the process, when hurried, is likely to become tumultuous. So that a connection is easily traceable between heightened brilliancy and photospheric laceration. Spots, however, are only the most conspicuous symptoms of an agitated state. Faculæ and prominences follow suit. They are indeed too intimately bound up with spot-economy to do otherwise. The corona, by a less obvious necessity, sympathises, and the periodicity of all these formations has a double aspect. They fluctuate in mode of distribution as well as in vigour of development. Spots, faculæ, prominences, and corona, all vary similarly and simultaneously in heliographic latitude as the waves of disturbance rise and fall. The spot-zones are not stationary. They shift over the body of the sun according to a definite law enounced by Carrington in 1859, and confirmed by Spoerer in 1861.[296] At maxima they occupy mean positions in about fifteen degrees of north and south latitude; then, as the cycle advances, they close together, and the commotion finally dies out near the equator. Meanwhile, the start of a new series in high latitudes has anticipated the termination of the old. Feeble at the commencement, it gains strength as it departs more and more from its native regions. “Mobilitate viget, viresque acquirit eundo.” Fig. 17, copied from a diagram of Spoerer’s, illustrates the nature of this progression. The overlapping of the curves at minimum brings before us the remarkable circumstance that, as a consequence of successive disturbances breaking out before those antecedent to them have expired, the full duration of each is, not eleven, but twelve to fourteen years. Moreover, spot-production at minima, however inactive, has a twofold nidus; two widely separated zones in each hemisphere are appropriated to it. A further characteristic of the cyclical decline in latitude is that it proceeds in “waves.” “Every fourth or fifth rotation,” Mr. Maunder tells us, “there will be an effort to reach a higher level, a lift of one or two degrees, and then a gradual slipping back until a fresh effort brings another small lift, but a weaker one than the last. And so the cycle goes on; the decline is continual on the whole, but is broken and interrupted by these frequent little struggles to get back to a higher plane.”[297] The growth in spotted area accompanying the descent of the zones is similarly rhythmical. So closely connected, indeed, are these two modes of periodicity that irregularities in the cyclical progression frequently show under the double aspect of abnormal outbreaks of spots, and abnormal movements in latitude. Theories are accordingly valueless that fail to rationalise simultaneously both kinds of facts. Several, indeed, profess to do so, but by constrained expedients.
The zonal law applies, with qualifications, to faculæ and prominences. Eruptive prominences are strictly governed by it. They frequent the spot-belts, it may be said, exclusively. The quiescent kind, on the contrary, avoid them,[298] and have their main gathering-grounds within fifteen degrees of either pole.[299] They may even occur, near spot-maxima, right up to 90° of latitude. As activity decreases, however, they too move downward, and crowd more or less closely towards the equator, although maintaining at all times a wider range than spots. Faculæ show a divided allegiance. They attend on spots, and their principal maxima are hence displaced with the spot-zones, while their affinity with quiet prominences is evinced by the occurrence of secondary maxima in high latitudes. Not that their local arrangement is the same with that of prominences.[300] Discrepancies are frequently noted; and they are important as indications that the two varieties of outgrowth do not originate under identical conditions.
The succession of coronal types is in clear accord with the law of zones. Streamers and prominences march, on the whole, closely together. They unanimously quit the poles after each maximum; they linger in company over middle latitudes, where “synclinals” overarch red flames at epochs of medium activity; finally, they descend towards the equator, the white wings of the minimum corona meeting and spreading above the last members of each decadent eruptive series.
The spectral periodicity of the sun is less marked than might have been expected. It is, indeed, almost confined to spots. The ordinary spot-spectrum at minimum (to repeat what has been already stated) includes many broadened iron lines, replaced at maximum by vanadium and scandium absorption. The nature of the individual spot, however (as we have seen), not the stage of the cycle, is really the determining cause of this diversity, which recurs periodically, simply because minimum-formations are usually of the tranquil sort. The chromospheric spectrum has quite other relations. It does not vary fundamentally; but the metallic rays temporarily added to it become fewer as metallic injections fall off. Nor is the quality of coronal light subject to radical change. Only the relative strength of its constituents slightly fluctuates. In the “winged” type the gaseous emissions are feebler than in the “radiated” type; yet they are always present and always the same.
The virtual invariability of the Fraunhofer spectrum is more surprising, since the reversing strata are in immediate contact with the periodically agitated photosphere. They preserve, nevertheless, an almost inviolable tranquillity, and their composition remains unaltered from one cycle’s end to the next. A single recurrent modification is, however, just traceable. It is that produced by the emergence, at maximum, of the facular bright lines H and K. Pro tanto and pro tempore, the symptom constitutes the sun a “bright line star.” Analogous detections in stellar spectra would afford a possibility of determining the spot-periods of globes in the solar condition; but they are, for the present, scarcely to be hoped for. The observation is difficult in the sun; in a star, unless facilitated by extraordinary facular development, it would be impossible.
The throbbings of solar agitation affect his entire system. In how many ways, and by what hidden means, we can but vaguely surmise. Terrestrial meteorology, as a whole, is certainly embraced in the great cycle, although the details of its conformity baffle, by their intricacy, the most painstaking pursuit. Only in the magnetic department there is no room for doubt. A thoroughly satisfactory discussion of the subject was completed in 1898–99 by Mr. William Ellis,[301] who for long years controlled this branch of work at Greenwich. Comparing the observations of the diurnal range of magnetic declination and horizontal force made at the Royal Observatory during the years 1841 to 1896, with the sun-spot numbers for the same interval determined by Wolf of Zürich, he found between the two orders of phenomenon, not only a general parallelism, but a correspondence in irregularities of period and secondary variations of intensity. This is strikingly evident in Fig. 18, copied by kind permission from his paper read before the Royal Society, 10th March 1898.
The sympathetic relation extends to auroræ. They even obey a “law of zones” similar to that regulating the distribution of sun-spots. The earth is circled—presumably in each hemisphere—by an auroral belt, which advances into temperate latitudes at epochs of cosmic disturbance, but retires towards the pole as it quiets down. The reality of the connection was singularly affirmed by the simultaneous dearth of sun-spots and auroræ during the seventeenth century. A prolonged solar calm appears to have set in about 1643. Galileo and Scheiner had been at no loss for objects of study; but the diligence of their successors, although unrelaxed, went mostly unrequited. To Cassini, Flamsteed, Hooke, De la Hire, the occurrence of a spot was an event of rare interest, which rewarded perhaps a decade of fruitless watching. Yet, as Mr. Maunder says,[302] the cycle was “submerged” rather than actually abolished, “the crests of a sunken spot curve” being marked by the solitary spots perceived in 1660, 1671, 1684, 1695, and 1705. Definitively, the protracted minimum came to an end in 1716, and there was a normal maximum in 1718. Meantime auroræ too were in abeyance.[303] None were seen in England from 1575 to 1706, when a glimmer of polar lights heralded the magnificent display witnessed by Halley, 17th March 1716. That there was concomitant magnetic quiescence need not be doubted; but Gauss of Göttingen was still in the distant future, and nothing could be known on the subject.
Individual outbreaks on the sun are often unmistakably associated with commotions of the terrestrial magnetic system. These so-called “storms” are world-wide in their nature, abrupt in their origin, and bear witness to some sudden vital spasm attacking the globe as a whole, and at once. Auroras and earth-currents make part of these mysterious affections, which commonly reach their height when a large spot-group is nearly central on the disc—that is to say, when it is broadside on to the earth. Instances abound. On 17th November 1882, the photosphere was, to the naked eye, conspicuously rent. The coincident aurora and magnetic storm were said to “beggar description.”[304] A spindle-shaped beam, which darted that night across the sky, was indeed a unique phenomenon, and, on its farthing-candle scale, recalled the amazing solar flambeau of 1st September 1859. Nor can the contemporaneous twitchings of the Kew magnets on this latter occasion be regarded as accidental, any more than the sudden small disturbance of all three magnetic elements which accompanied an outburst of faculous light on 17th June 1891.[305] The magnetic turmoil raised by the transit of the enormous spot of February 1892 was exceptionally violent. Earth-currents seriously interfered with telephonic and telegraphic communication in all parts of our busy world;[306] the needles at Greenwich went completely off the prepared track of photographic registration;[307] and an auroral pageant completed the programme of response. Similar concurrences were observed in February 1894, September 1896, and March 1898, to mention a few out of a multitude of cases. Yet the sympathetic connection is not invariably manifest. A hole in the sun may evoke no earth-trouble. Mr. Maunder hence concludes that “though sun-spots are the particular solar phenomenon most easily observed, we must not therefore infer that their number and extent afford the truest indication of the changes in the solar activity which produce the perturbations we remark in our magnetic needles.”[308]
Not the spot itself, but the connascent agitation thrills the terrestrial organism. Quiet formations pass unheeded; crises of growth or reconstruction meet with instant rejoinders. Tacchini[309] accordingly holds that chromospheric, not photospheric phenomena are, in this respect, truly influential, and that it is to the fierce flame-rushes above spots that the magnetic nerve-system is sensitive. The view is favoured with some hesitancy by Professor Hale.[310] Professor Bigelow, as the result of much suggestive inquiry, affirms that “from the sun to the earth come two great supplies of energy, both types of radiation through the ether”—one propagated by plane waves, the other by vortical rotation—“and possessing very different properties, the one visible to the eye, the other visible to magnetic perceivers.” And he computes, from various indications, a value for the normal magnetism of the sun about one-fifth the maximum of steel, which may, at epochs of extraordinary disturbance, be augmented fifty-fold.[311]
Variations in the sun’s electrical state assuredly accompany his more obvious cyclical changes. “May not,” Dr. Schuster asks,[312] “the periodicity of sun-spots and the connection between two such dissimilar phenomena as spots on the sun and magnetic disturbances on the earth, be due to a periodically recurring increase in the electric conductivity of the parts of space surrounding the sun?”
The surmised alteration may be a consequence, it can hardly be the cause of solar periodicity. Its actual occurrence, however, is far from unlikely. Certain cometary phenomena lend it a qualified support. The grouping together near sun-spot maxima and minima respectively, of the bright and faint apparitions of Encke’s comet during a century (1786–1885), forced itself upon M. Berberich’s attention in 1888.[313] Correspondences of this unexpected form were even traceable with displaced epochs of activity, such as the retarded maximum of 1788, and the premature maximum of 1837. Some indications were besides gathered that comet-discoveries become more numerous as the tide of solar energy rises, many that would otherwise pass unseen being lifted into visibility by accesses of transmitted excitement. But this relation, M. Berberich admitted, might be more apparent than real.
One still more recondite and unaccountable has been lately adverted to by Mr. J. Halm.[314] He contends that the physical condition of the sun reacts perceptibly on the motion of the earth. Variations in its orbital elements, which have hitherto baffled attempts at explanation on gravitational principles, are comprised, he finds, within the “great” spot period of about sixty-five years. The obliquity of the ecliptic, for instance, instead of decreasing uniformly with the time, shows subordinate fluctuations synchronising with the long waves of solar activity. Its shorter waves, on the other hand, prove to be influential upon the variation of latitude. Mr. Halm considers that the deviations of the terrestrial pole conform unmistakably to the eleven-year cycle, with, however, a lag of about one and a half years in the corresponding epochs. “It may,” he writes, “be taken to be clearly established that the radius of the circle described by the pole of instantaneous rotation is greatest at times of sun-spot minima, and smallest at times of maximum displays of solar spots.” This “holds true,” he adds, “for the whole interval of about sixty years now covered by Dr. Chandler’s investigations.” His explanatory hypothesis is both simple and ingenious. It depends upon the large inclination of the earth’s magnetic axis to its axis of figure, combined with alterations, due to solar influence, in the total magnetism of our planet. Molecular strains along the magnetic axis would—it is plausibly assumed—occasion bodily distortions of the globe, whence should result displacements of the axis of figure relative to the axis of rotation. “The outcome of this hypothesis would” then “be that changes in the state of solar activity, since they produce a measurable effect on the terrestrial magnetic forces, should also be accompanied by corresponding changes in the motion of the earth’s axis.”
The machinery by which electro-magnetic impulses are propagated from the sun to the earth, completely evades scrutiny. Sundry conjectures on the subject have been hazarded, but none of them rest on any sure basis. What we know about modes of communication is chiefly negative. Thus, Hertzian vibrations are not transmitted to the earth with sunlight. They do not, at least, reach its surface. Wilsing and Scheiner tried for them in vain with an electric “bridge” and a galvanometer.[315] They might, indeed, as the experimenters noted, be atmospherically arrested. All that seems certain is that direct magnetic action is concerned in producing the observed perturbations of the terrestrial magnetic system, which are not explicable like ordinary meteorological phenomena as effects of thermal vicissitudes, or convective air currents.[316] They stand apart, and imply special conditions which cannot, without detriment to science, be ignored.
Little progress has been made towards ascertaining the cause of solar periodicity. We are only assured that it is not imposed from without, but arises from within; it resembles a “free,” rather than a “forced vibration.” This conclusion, it is true, tends to relegate the matter to obscurity; for the interior of the sun is a terra incognita, and seems likely to remain so. His cyclical changes may belong to his original constitution; they may date from nebular times, and be as inherent as the tone of a bell. Or they may simply characterise a stage of growth, and prove liable to modification and effacement. The study of variable stars will perhaps help to guide our ideas as to the probabilities of the case.
Its full bearings, meanwhile, can only be conjectured. The scope of the disturbance needs to be defined. There are still many open questions connected with it. Does the “smoke-veil” absorption vary with the abundance of spots? Are the temperature of the photosphere, and the depth of the chromosphere affected by it? Can periodical changes of pressure in the reversing layer be detected? These are among the problems of the immediate future. They are already within reach of attack.
XIV. THE SUN AS A WHOLE
Modern science contemplates in the sun a huge sphere 867,000 miles in diameter, bounded by a dazzling cloud-shell, and composed of materials 1·4 times heavier than water, yet heated so far above the “critical temperature” of any terrestrial substance, that they must be regarded as in the gaseous state. The imperious demands of radiative emission can be met only by rapid exchanges, implying the unceasing activity of a system of profound vertical currents; while rotational surface-drifts of a peculiar kind notify complexities of internal movement defying speculation or research. Incidental to them, doubtless, are the dark spots marring, at times, the brightness of the disc, and giving evidence, by their more or less copious occurrence, of that far-reaching periodicity which may be called the central fact in solar physics. Spots are garlanded with faculæ, which represent photospheric upheavals; and above them, to a height of some hundreds of miles, rise the metallic vapours producing, by their absorption, the Fraunhofer lines. The reversing strata seem to be continuous with the chromosphere, although the nearly perfect transparency of the latter establishes a noteworthy distinction between the two formations; but the corona is a thing by itself, sharply separated, physically and spectroscopically, from every other appendage of the sun. It has, of course, relations with them, just as our air has with the ocean it surmounts—relations, however, that do not even verge towards a confusion of identity.
The two fundamental problems connected with the nature of the sun are its rotation and its periodicity. They may be quite closely allied, and in regard to both, “counsels of despair” have begun to prevail. The spot-cycle, like “Carrington’s law,” is set down as a congenital peculiarity, and the mists of the past are invoked to cover the perplexities of the present. There seems little immediate prospect of their being removed. Early modes of investigating the subject have had no striking success, and fresh ones await development. So operations have come to a pause, yet by no means to a dead-lock. The difficulty of learning how the phenomena are occasioned should only stimulate diligence in unmasking and tabulating them. The ramifications of the period can be followed out, even if we cannot get at its roots, and there are signs that they will be found to take unlooked-for directions. Similarly as regards the sun’s rotation. Explanatory hypotheses avail little, but the sifting of facts avails much. The case has not yet been fully stated. Lacunæ need to be filled up, anomalies to be smoothed away, errors to be corrected. In the doing of all this, a clue to the labyrinth may present itself.
The nature of sun-spots must long be under discussion. Questions of extreme interest are involved, some of them being visibly answerable, since criteria are at hand to determine which way the truth lies. Discriminative, rather than numerous observations will serve the purpose. The relations of faculæ with prominences offer another promising topic of inquiry, as well as those of prominences with coronal streamers. But this last is an eclipse-problem, and so, unluckily, is at present the entire subject of the corona. No pains, however, will be spared in order to bring to bear upon it the full resources of daylight investigation. As to whether they will be rewarded or no, the balance of forecast swings pretty even. The unfolding of some condition, now hidden, may incline it either way.
Solar spectroscopy presents a variety of aspects. It can be studied from the chemical, thermal, electrical point of view; pressure, density, motion, possibly magnetic stress, are concerned in it. Nothing in this branch, however, is more instructive than the spectral diversities of the various solar formations, or even of different parts of the same formation. Thus each section, some few miles thick, of the reversing layer is probably distinguished by emissive modifications; and the Fraunhofer spectrum integrates the absorption of them all. The spot-spectrum is a more pronounced variety; the chromospheric spectrum is largely, the coronal spectrum, wholly peculiar. Now the task of interpreting these several scripts is not—could not be—easy; yet solar chemistry is, in the main, less unfamiliar than might have been expected. It includes only one element—coronium—that can be clearly distinguished as terrestrially unknown. For unidentified lines are not necessarily of exotic origin, as is proved by the rapid progress of their recognition in the Fraunhofer spectrum pari passu with advances in the photographic registry of metallic spectra. Nor is it probable that spot-cavities harbour strange forms of matter. The vaporisation in them of rare metals sufficiently explains what long appeared enigmatical in their absorptive action.
With the substitution of known for unknown substances in the sun, a leading argument for dissociation vanished. In this view, the supramundane species giving lines experimentally unrecorded were chemical fragments of our elements broken up by enormous heat. But if no supramundane species exist, the elements presumably remain intact. Absolute stability it would indeed be extremely rash to ascribe to them; yet it is certain that their individuality survives fierce ordeals. Evidence of other kinds tends towards the same conclusion. Mr. Jewell’s critical examination of the Fraunhofer rays indicated for the shadings attached to certain iron and calcium lines an origin quite close to the photosphere.[317] This goes far towards demonstrating the integrity of extremely complex molecules at the highest temperature prevailing near the sun. Outbursts of heat from the interior there may be, reaching a still more exalted pitch; but they must be transitory, since cooling by expansion should promptly and potently affect them. Further, the thermal relations of the terrestrial elements are distinctive and uniform. If these bodies are not really simple, they are at least compounds of a different order from those known to be such, and artificially producible. Finally, the extensive detection of spectral series invalidates the “one line, one element” principle which underlies most of the arguments for dissociation. Unity of origin is emphatically claimed, not only by each series, but by each connected set of series. Researches into the essential nature of matter have, however, entered upon a new phase through the aid of what we may call electrical analysis, in which “ions” play the part of atoms in chemical analysis; and their outcome may perhaps aid in the solution of many intricate solar problems.
There is a strong temptation to transfer to the sun the conditions prevailing on the earth, and to model solar upon terrestrial meteorology. This was the line taken by M. Egon von Oppolzer in 1893. He admitted, indeed, that it traversed dangerous ground, in view of the wide divergences of opinion as to the nature of atmospheric processes belonging to immediate and everyday experience. Still he judged it safe to apply the kinetic theory of gases and the dynamical theory of heat to the determination of the state of equilibrium in the sun’s aerial appendages. The results were scarcely encouraging. At elevations of one second of arc, or 450 miles, above the photosphere, the prevalent temperature was found to be 14,000° C. lower than at its surface, which should hence be extravagantly hot. And although this inconvenience was abated by forced assumptions, it could not, on the principles adopted, be removed. They involved, for example, a rise of temperature in downward currents to the extent of at least 5000° for each descent of 450 miles. Spots were regarded as “places of extreme alternations of temperature,”[318] where abnormally hot layers cover anticyclonic regions of increased pressure and reduced thermal excitement. They are “produced indirectly through a sinking down of masses upon the photosphere, and directly through extraordinary radiation, brought about by transparency of the overlying region.” To counterbalance descending movements in the spot-zones, a continuous uprising of heated matter was supposed to progress at the poles, constituted so far the analogues of our equatorial belt of calms. Two great permanent cyclones were thus centred on the axis of the sun. They had important functions assigned to them. Upon their regulative power was made to depend the working of the entire machine, and solar periodicity itself was referred to the alternate relaxation and enhancement of their activity. They were, nevertheless, a purely arbitrary creation. Not so much as a tortoise in mid-air was provided for the earth-bearing elephant to stand upon. The author claimed, it is true, only the merit of simplification; yet the contrast of conditions between the earth and the sun largely vitiated his reasonings. Atmospheric circulation on the earth is maintained by external heat; agitations on the sun by internal heat; they depend absolutely upon processes of cooling. Hence the impossibility of assimilation. Trade winds and cyclones lack in the sun the driving power by which they are kept going on the earth. They are characteristic of a planetary body—of a globe vitalised from without.
Eruptive hypotheses of the solar constitution have been proposed under various forms by a succession of writers—by Secchi, Faye, Lockyer, Schaeberle,[319] Young,[320] Sidgreaves.[321] Here, at any rate, we are in touch with reality. Volcanic forces are powerful in the sun, which might, in a sense, be described as organised upon a volcanic basis. The conditions of upheaval are everywhere at hand; only some casual relief of pressure, or access of heat is needed to provoke an actual explosion. Settling down on the photosphere in a cooled, though still gaseous state, the products of eruption would then give rise to spot-phenomena. Dark patches would mark the effects of their general and special absorption of light, while faculæ and flames attested the vehemence both of the original outbursts, and of those reactively started by their partial subsidence. Procedures of this kind on the sun appear inevitable; they at least count for something, if they do not explain everything.
A curiously subversive theory in solar physics was propounded in 1891 by Dr. August Schmidt.[322] Its interest, however, is largely academic. Yet it is no illusory speculation. It rests upon a sound foundation, and emphasises an undeniable truth. The mode of action brought into prominence by it necessarily plays a part in modifying the aspect of the heavenly bodies; the only question open is whether the part is conspicuous or insignificant.
The laws of atmospheric refraction were traced to their ultimate consequences by Kummer in 1860.[323] Some of them are remarkable.
The visual lifting of extra-terrestrial objects results, as is familiarly known, from the inflected character of the paths pursued by light-rays through our air. A beam reaching a spectator at sea-level from a horizontal direction is really bent upward with a curvature one-seventh that of the earth itself. But on a globe of seven times the earth’s radius—other things remaining the same—the refracted ray would possess identically the curvature of its surface, to which it would accordingly run parallel for ever, or until extinguished by absorption. It would never reach the eye of a spectator. “Circular refraction” would take effect upon it. A similar fate would befall a ray starting horizontally from the surface towards outer space, instead of attaining to which, it should follow an unending round within the air. On Jupiter, this critical stage must be considerably overpassed. From a Jovian atmosphere of the proportionate mass of the earth’s, light could only escape at angles of elevation exceeding 3° 22′.[324]
Applying his formulæ to the sun, Schmidt found that circular refraction would there be produced in a hydrogen atmosphere at a temperature of 10,000°C., and of one-ninth the standard density of air. The upshot was to “explain away” most of the solar appendages. The photosphere he showed to be an optical illusion, arising at the surface where circular refraction just comes into operation in an incandescent gaseous globe diffusing uniformly outward. The difficulty was thus overcome of accounting for the visibly sharp separation between the dense body of the sun and its tenuous surroundings; while a phantasmagoria of granules, spots, flames, and faculæ was easily evoked on the supposition of irregular refractions within the seeming disc.[325] To this dubious locality, the reversing layer, too, was transferred by Dr. Knopf of Jena, who hailed the speculation as the dawn of a new era in solar physics. Hence, opposite displacements of the Fraunhofer lines cannot be a rotational effect, and the singular accordance of what they should be on that supposition with the measures actually obtained, is the outcome of pure accident. But this is extravagant and unthinkable—a reductio ad absurdum of the optically composed sun. Plainly, the whole theory is in the air. Its reasonings, to be sure, are mathematically valid; but they refer, as Dr. Seeliger has pointed out,[326] to an ideally transparent globe, from which the absorptive effects, prominent in the sun, are absent. The solar spectrum, besides, fails to be accounted for by them “on the accepted principles of physics.”[327] Nor can practised observers of the solar surface readily be persuaded that they have watched, not realities, but mirage-effects. In dismissing the hypothesis as untenable, the important inference was, however, retained by Professor Frost “that refraction within and on the sun itself may modify in some considerable degree” observed phenomena.
An ingenious corollary has been added to Schmidt’s theory by M. Julius of Amsterdam.[328] It was suggested by certain anomalies in the dispersive action of sodium-vapour upon light, noticed by Becquerel, and confirmed by himself. They led him far. He was conducted by them to optical explanations of the “flash” spectrum, of the broadening of lines in sun-spot spectra, and of line-displacements, usually interpreted on Doppler’s principle. They are, however, given under reserve, and can be tested by comparing the lines affected in the sun with those in the vicinity of which special refraction is exerted. This is a laboratory task, which M. Julius himself is well qualified to discharge. Should the provisionallyassumed correspondences be found to subsist, the arguments for the phantasmic character of solar appurtenances will have gained imposing strength. But such a result cannot reasonably be anticipated. Refraction on the sun is likely, owing to the extreme rarity of the medium in which it should take place, to be of evanescent effect. Only in spot-cavities, it may perceptibly affect appearances. The entire matter, nevertheless, deserves to be thoroughly sifted, and cannot in future be forgotten or ignored.
Here again a fresh prospect is opening into view, and many others invite exploration by difficult and devious routes. For solar science does not become less arduous as it advances. One of the surest marks of progress in any branch is, indeed, the development of new and unforeseen problems. And the sun, as we have learned by degrees to recognise, is a body organised in too complex a method for easy apprehension. We perceive that its energies are specially directed; the purpose of the machine is obvious, and it is admirably fulfilled. In part we can see how, but much remains mysterious. Right to the core of the mystery we may never penetrate; nature is virtually invincible by man; but in urging our way forward, we shall gain continually larger and more lucid views.
I. PROGRESS OF SIDEREAL PHYSICS
Sidereal physics includes stellar and nebular physics; the two branches cannot be separated. They have interlacing offshoots, and progress during the last hundred years has tended more and more to unite them in one main stem. Objects closely akin are dealt with in both, and they are dealt with by methods substantially the same. Stars and nebulæ are not only related as fellow-members of the grand galactic system, but they coexist in numerous sky regions, and often in such close connection that it is difficult to define them apart. A nebula with a stellar nucleus can scarcely be distinguished from a nebulous star, and nebulæ altogether devoid of star-like condensations are perhaps non-existent. Chemical affinity ratifies visual conjunction. Spectroscopic classification proceeds from stars to nebulæ with hardly a break. The establishment of an exceedingly low standard of density for certain varieties of stars forges an additional link between the two sidereal orders. For it brings to our acquaintance bodies in a transition-stage from nebular diffuseness to solar condensation—bodies attracting feebly while radiating powerfully. Variability in light is another quality, the common possession of which by stars and nebulæ has recently been placed beyond doubt; and strong evidence is forthcoming that binary systems occasionally consist of a stellar and a nebular member, united by origin and inseparable to all time.
The methods of solar and sidereal physics do not differ fundamentally; all alike depend in the main upon light-analysis and chemical delineation, and resort to direct telescopic observation only as a subsidiary expedient. The aims of sidereal science are, however, profoundly modified by the remoteness of the objects it is concerned with. Many kinds of inquiry, successfully prosecuted in regard to the sun, are impracticable for application to suns deprived by distance of sensible dimensions. Surface-phenomena are in them wholly out of reach, no less than the paraphernalia that lend their glory to total eclipses. No star sends us more than a single pencil of light, collected indiscriminately from a wide hemispherical area to the obliteration of its local peculiarities. Spots, flames, faculæ (if such there be) mix their rays inextricably together; for stars have no parts. But while their distance narrows in some directions the scope of possible inquiries concerning them, their multitude immensely widens it in others. They are not all similar, and their differences supply grounds for a classification, the import of which deepens with every advance in physical knowledge. Delicate spectral traits are found to be indexes to conditions of temperature, density, magnetic strain, or electrical excitation, in part imitable in the laboratory, in part transcending, and hence contributing to enlarge terrestrial experience. Classification, moreover, is dominated by the idea of development. Comparative sidereal study leads inevitably to far-reaching speculations on cosmical growth.
The sun is solitary; he exercises a “sole dominion.” But it is not so with all his compeers. Nor are the mutual relations of those linked together expressed solely in terms of motion. They do not fall within the exclusive competence of the mathematical astronomer. They involve constitutional modifications of profound import. Coupled stars, clustered stars, stars immersed in or attached to nebulæ, are probably subject to influences, the nature and modes of action of which remain largely obscure. Their investigation has, however, been tentatively set on foot, and may give results of peculiar interest. Thus sidereal science extends and supplements solar science. It assigns to the sun its status in the universe; it provides objects with which it can be compared or contrasted. The two modes of knowledge mutually act and react; what one acquires the other assimilates. The progress of each is, by this comparative action, quickened and assured.
The population of the heavens is so dense that general conclusions as to its characteristics can be attained only by statistical methods. And to employ these effectively, the command of vast masses of information is required. Data must accordingly be secured wholesale, and the necessity has been met by the creation of a world-wide international organisation. But its mills grind slowly, and individual enterprise will not be stayed. Sir David Gill has already completed, in the Cape Durchmusterung, a preliminary work designed purely in the interests of geometrical astronomy, but fraught with importance to cosmical physics. Professor Kapteyn of Leiden, who undertook the examination and measurement of the plates, detected relationships between the kind of spectrum given by the stars imprinted upon them and the mode of their scattering, which promise to affect more and more profoundly all future conceptions of the universe. Hints of their prevalence had, it is true, been already gathered. Father Secchi noticed long ago that spectral types are not indifferently distributed over the sky, a conspicuous example of local preference being afforded by the constellation Orion, which might be described as a colossal group of helium stars. The work of Pickering, McClean, and others has also brought out the facts that the Milky Way is a distinctive spectroscopic region, and that the stellar tribes in general show aggregative tendencies not to be mistaken. Astrophysical considerations then enter into discussions of celestial structure; they are not wholly alien to questions as to how the heavens move. The monumental “Draper Catalogue” of stellar spectra served as the foundation of most of these extensive researches; and the entire bulk of photographic and spectrographic data collected at Harvard College with astonishing persistence and skill during the last score of years, has incalculably promoted the larger interests of sidereal astronomy.
Physical and descriptive catalogues of nebulæ are still a desideratum. The materials for their compilation are, indeed, lacking. Comparatively few such objects have been examined with the requisite care. They are not easily dealt with. Those of a gaseous nature are strongly characterised by rays high up in the ultra-violet, where absorption by glass is formidably effective. Their due photographic registration is then feasible only by means of reflecting telescopes combined with prismatic apparatus of crystal and rock-salt, or some other materials transparent to the shortest wave-lengths. But since these special arrangements are rarely made, nebular spectrography makes slow progress. Its conditions, in the case of “white” nebulæ, are still more embarrassing. Nor are they much alleviated by substituting direct vision for the camera. All nebulæ are intrinsically faint, and most give continuous spectra. A scant supply of light makes, however, a much better show, as can readily be imagined, when concentrated in a few bright lines, than when dispersed uninterruptedly along the colour-scale. To the eye, the resulting variegated streak is dim and featureless; the sensitive plate takes cognisance of it only under the compulsion of prolonged exposures, and then imperfectly. The task of overcoming these obstacles is arduous, yet far from hopeless. It has been taken in hand, and on the success attending its prosecution the future of nebular physics essentially depends.
The discovery of helium as a terrestrial element marked a fresh point of departure in the chemistry of the heavenly bodies. Its leading chromospheric ray, D_{3}, had already been noted as an emission-line in the Orion nebula by Dr. Copeland, as an absorption-line in Rigel by Professor Keeler; but this was only preliminary to what was to follow when recognition-marks were multiplied by complete experimental acquaintance with the associates of the yellow beam. One of these, a line in the blue (λ 4472), is much more conspicuous in stellar spectra than D_{3}, and it was established by Vogel in 1895, with other members of the conjoined series, as distinctive of a large class of “helium stars.”[329] Some insight was thus gained into the extraordinary profusion with which this strange gas, so sparingly occluded by the earth, is dispensed to the suns in space. In order to show spectroscopically, it must, as already stated, be voluminously present in a glowing atmosphere. Its rays scarcely endure competition. Our sun, for example, although surrounded by huge volumes of helium, is not a “helium star.” Pretty sure indications, on the other hand, can be gathered that helium is one of the principal components of all gaseous nebulæ; and it blazes where stellar incandescence is strong—in “new” stars, in bright-line stars, and in some “long-period” variables. The disclosure of its great cosmical rôle is among the most important consequences of the modern alliance between astronomy and terrestrial physics.
The identification of oxygen and nitrogen as stellar constituents by Mr. McClean and Sir William and Lady Huggins respectively, and the detection of the “Pickering series” of hydrogen in certain stars, are advances of scarcely less moment. These unusual kinds of absorption emerge, as a rule, in stars showing helium as well, and generally assumed to be at an early stage of growth. They apparently tend to supersede metallic action, which becomes imperceptible when the inchoate stage is approached. Thus the nebular spectrum includes no lines of known metals, and they are likewise apparently missing from “Wolf-Rayet” stars. Acquaintance with these remarkable objects has profoundly altered the views of stellar physicists. It has introduced them to a borderland where the prevalent conditions defy forecast. Who, for instance, could have anticipated Campbell’s observations of mixed bright and dark spectral series, derived from the same element, in the same object? Nothing could well be more perplexing; but perplexities are often the raw material of discoveries.
The application of photography to the examination of “blaze stars” began with the apparition of Nova Aurigæ in 1892. Opportunities for its continuance have since been frequently afforded, which would probably have slipped by unused but for the automatic watch kept on the changes of the heavens at Harvard College and its southern dependency. Two generalisations have thus been authorised. One is that the spectra of Novæ are mainly composed of bright and dark lines in pairs, emanating from the same substances, but pushed asunder as if by the effect of swift opposite motions. The second is that fading Novæ put on a nebular light-vesture. Spectroscopically, they simulate minute “planetaries.” These surprising facts are seeds of future knowledge; they need time to germinate and yield fruit.
Stellar variability no longer remains outside the pale of successful research. The mystery surrounding it has not, to be sure, been dissipated; but some of its attendant circumstances have become manifest. Eclipsing stars are now fully open to investigation; and although not physically variable, they have indisputable connections with stars that are. Fluctuations executed quite punctually in periods of a few days or hours indicate a compound nature in the objects undergoing them, even if they cannot be explained as occultation-phases. Short-period variables, in short, are non-eclipsing spectroscopic binaries. Yet spectroscopic binaries, indistinguishable from them as to their orbital conditions, shine with a perfectly steady lustre. And these are the majority. It remains to be discovered what are the special attributes of the differentiated classes. Nor is variability found only in conjoined objects. Stars apparently single are subject to extreme, although more or less irregular vicissitudes. The only clue to the nature of these vicissitudes yet found is the fact that increase of light is ordinarily attended by the spectroscopic flashing out of hydrogen-rays. Moreover, the resemblance of stellar light-curves to sun-spot tracings gives a strong hint that the solar analogy should be made a starting-point for inquiries into “long-period” variability.
The branch of stellar astronomy concerned with double and multiple systems gains extension and importance year by year. Their evolution under the influence of tidal friction has been studied by Dr. See. Sir William Huggins’s device of a slit with reflective jaws having facilitated spectroscopic observations of close stars, the analysis of their light has at last entered upon a stadium of progress; apart from which perennial obscurity must have hung over sidereal chromatics, and enveloped theories of sidereal growth. Spectroscopic binaries, meanwhile, are multiplying on our hands, and their varieties offer a brilliant field for investigation. Their periods range from less than one day up to two years; some revolve in subordination to larger combinations; their orbits are variously inclined, and of various degrees of ellipticity; and one of the circulating bodies is, more often than not, sensibly devoid of light. The function and place in creation of “dark stars” have thus come into the foreground of inquiry. They occur also in telescopic systems, where their disturbing power produces a visible swaying in the movements of their bright companions; but less commonly, it would seem, than as members of spectroscopic couples. These are, so to speak, just out of the shell; hence dark stars can hardly be effete suns, although suns presumably lapse with age into obscurity. The distinction is a delicate one to draw, but should not be lost sight of.
The scope of sidereal research is limitless—limitless because its objects are indefinitely numerous. The difficulty is to lay hold of them. Inferences based on partial surveys are felt to be unsatisfactory. The star-depths beyond continually invite farther and farther advances. Astronomical curiosity is only temporarily appeased by learning, for instance, the radial movements of a few score of stars; they are wanted by the hundred, and when at hand in hundreds, they will be in demand by the thousand. Exhaustive inquiries, while remaining unattainable, must, by the nature of the case, be perpetually aimed at. And what is primarily needed for them is a plentiful supply of light. Ambition in telescope-building is then justifiably insatiate, nor has it, so far, overreached itself. Each addition to instrumental capacity has, on the contrary, notably widened the horizon of feasible research. The erection of the great Lick refractor made it possible to obtain legible spectrographs of the Wolf-Rayet stars, and set the nebulæ in motion by enabling Professor Keeler to determine their radial velocities. With the Crossley reflector, mounted in the same superb situation, nebular photography made a fresh start, and the law of spirality, as a structural principle, was confirmed and generalised. The Yerkes telescope has given access to several closed fields. By its means, variable stars are followed through their semi-extinct phases. Professor Barnard has performed the unique task of verifying visually the rapid light-changes of the minute components of globular clusters; and Professor Hale has examined spectrographically a large number of “carbon” stars, too faint as well as too red for satisfactory treatment under average circumstances with the camera. The completion, at the same observatory, of a five-foot reflector will shortly afford even better opportunities for prosecuting this work. It may be added that a special function in nebular investigations is reserved for instruments of abnormally short focal length, such as the Meudon reflector, in which the rays collected by a mirror thirty-nine inches across form an image at only thrice that distance from it.
Centuries, however, of invention and contrivance must elapse before the minuta plebs of the sky can be individualised by the peculiarities of their spectra. Inducements to keep up this strain of toil are not wanting. “A star’s a star for a’ that,” even though it lie beyond reach of human questioning. To bring it virtually nearer, is the object of perpetual efforts. And they cannot fail to be rewarded. Novelties in the heavens are likely to prove endless and surprising. The enticements they offer to progress are irresistible. No “anchor dropt at eve or morn” can stay the ideal voyage. “When a man hath done, then shall he begin” his scrutiny into the works of the Most High.
II. THE CLASSIFICATION OF STELLAR SPECTRA
Various methods of classification may be applied to the stars. The most obvious and the most antique is that of relative brightness. From of old the stars have been collected into ranks by “magnitude.” Or the amount of their “proper motions” may be taken as the principle of distinction. Within certain limits this is practicable, and for certain purposes it is useful. But apparent lustre and projected movement alike depend in part upon distance; they include an extraneous element; and astrophysical science considers the heavenly bodies in themselves, without regard to their spatial relations. Hence an absolute quality must be made the basis of their arrangement, and it is found in the kind of light emanating from them. This system is of far more than conventional value. It affords the only clue within reach to the intricacies of stellar constitution. Schemes of spectral classification may be amended and altered; but in one form or another they are indispensable to progress.
They naturally tend to become more complex as facts multiply, and finer shades of difference are rendered manifest; yet Father Secchi’s four “types” continue fundamental. It is well, then, to keep their characteristics steadily in mind. The first is marked by strong hydrogen absorption. It consists of radiantly white stars. The second by innumerable fine metallic rulings; the sun is an example. The third type includes red stars with banded spectra like Antares, the bands being sharply terminated towards the violet, diffuse towards the red. The fourth is composed of deeply-tinted, mostly faint objects, showing wide bands facing redward, due to carbon absorption. These four groups form irremovable landmarks; but beside and between them many subordinate divisions have been set up. Vogel, Huggins, McClean have all modified, while broadly adopting Secchi’s “law of order.” They have, moreover, regarded it, not as a mere empirical formula, but as prescribed by the necessary conditions of development. Modes of classifying the stars have come to be equivalent to theories of their evolution. With this aspect of the matter, however, we are not just now concerned. The aim of the present chapter is to establish convenient distinctions without regard to their essential meanings. The unravelment of these will be attempted later on.
Miss Maury’s arrangement of the stars[330] is designed for a life-history as well. It is the most elaborate yet put forward. Based on the examination of some 4800 spectrographs taken at Harvard College, it embraces 681 objects, disposed in a progressive series of twenty-two groups, most of which are further comprised within three collateral divisions, established to meet the visible necessity for a secondary characterisation. The work is a monument of industry and skill, and will long hold a place of standard authority; but the fine gradations it emphasises, although worth putting on record, are scarcely suitable for committing to memory. Our object here is to present large outlines, leaving minute shades of difference to be dealt with as occasion may arise. There is danger of stellar classification degenerating into a maze of provisional distinctions. The best remedy is to fix attention on the summit-ranges of the landscape; when they are clearly imprinted on the mind, mastery of detail can be safely and readily acquired.
A framework of eight compartments accommodates practically all the stars. The separation is easy and natural, and the “notes” of the various classes present themselves unmistakably. The constituents of the four first show absorption spectra only; those of the four last are marked by emission as well as by absorption. No hypothesis of growth or affinity is implied by the order of succession given to the bright-line objects; only the interests of clearness have been consulted in its choice. We will now briefly describe these stellar families.
Class i.—Helium Stars.—In the spectra of these brilliantly white stars, absorption by hydrogen and helium predominates. The complete “Huggins series” is stamped upon them, from the fundamental C to its “head” in the ultra-violet;[331] and at least twenty-six of the strongest helium lines, culled impartially from all the six series, show conspicuously besides. In a few helium stars, suspected of nebular relationships, the “Pickering series” of hydrogen is represented, while others betray the action of oxygen, nitrogen, and silicon. Metallic lines are faint and scarce; those identified belong to sodium, iron, calcium, and magnesium. Especially remarkable is the comparative prominence of the magnesium line λ 4481, to the exclusion of the triplet b, which takes the lead in the solar spectrum. The substitution, according to many authorities, indicates enormous heat. Scheiner’s criterion for high temperature is precisely the development of λ 4481, and Professor Keeler remarked that the effacement of b marked a stage of heat beyond the possibility of artificial production.[332]
In these stars there is almost no general absorption; their photospheres are unveiled. Moreover, they seemingly possess reversing layers of very simple composition, to which circumstance their display of helium may, with much probability, be attributed. Originally included in Secchi’s first type, they were separated from it by Vogel in 1895, on the identification of their distinctive lines with those of terrestrial helium; and their importance in the sidereal scheme was accentuated by McClean’s spectrographic researches in the southern hemisphere. The lucid orbs of Orion and the swarming Pleiades are leading members of the class, which are also thickly disseminated in the Southern Cross, the Centaur, and the Greater Dog. Miss Maury’s first six groups are subdivisions of helium stars, arranged in the assumed order of their development from a nebulous condition.
We are indebted to Sir William and Lady Huggins for permission to reproduce their admirable spectrograph of Rigel, the premier helium star (Plate IX. Fig. III.), and to Sir David Gill for that of ε Canis Majoris (Plate X. Fig. 1), taken by his assistant, Mr. Lunt, who detected many lines of silicon and oxygen, in addition to strong helium-absorption, in the spectrum of this star.
Class ii.—Hydrogen Stars.—These stars are distinguished by intense hydrogen-absorption of the ordinary kind, no Pickering lines being present. Helium-influence on their light is null, or barely perceptible. The “H” and “K” of calcium are thin but distinct. Feeble iron lines can be numerously discerned. General absorption is slight; the ultra-violet end of the spectrum lies open, imparting to hydrogen stars a bluish-white colour. Vega is a perfect example; its spectrum, photographed by Sir William and Lady Huggins, is shown in Plate IX. Fig. I. The black band to the right is the fifth line of hydrogen (Hε). It masks the calcium “H”; but “K” appears well to the left.
Hydrogen—sometimes called “Sirian” stars—abound in the heavens. They form the main part of Secchi’s first type, and are distributed by Miss Maury into five groups, numbered vii. to xi.
Class iii.—Solar Stars.—The Fraunhofer spectrum sets a pattern copied, with slight variations, by the members of this class. Its leading feature is the powerful development of “H” and “K.” Other metallic lines are innumerable, but mostly sharp and thin. Four hydrogen lines are normally present, ultra-violet members of the series showing decisively only in stars like Procyon and Canopus, which may be regarded as intermediate between the Sirian and the solar classes. A yellow tinge corresponds in the latter to a veiling of the blue end of the spectrum, similar to that perceptible in the sun. Solar stars, then, resemble him, not only in the composition of their reversing layers, but in the possession of “smoky” envelopes. A spectrograph of Arcturus by Sir William and Lady Huggins is shown in Plate IX. Fig. II., and one by Sir David Gill of α_{2} Centauri, the brighter member of the southern binary, in Plate X. Fig. 2. The precision of its correspondence with the solar spectrum may be seen by a glance at the comparison strips in this latter figure. Stars of the solar class constitute Secchi’s second type, and are included in Miss Maury’s groups xii. to xvi.
Class iv.—Stars with Fluted Spectra.—Two kinds of absorption are distinguishable in them. A linear system, somewhat reinforced from the Fraunhofer model, has superposed upon it a set of dusky flutings, about ten in number, of undetermined chemical origin. They suggest action by oxides, the formation of which in stellar atmospheres would seem to imply a considerable reduction of temperature. None of the bands occur in the more refrangible part of the spectrum, so that the photographic differences between solar and “fluted” stars are easily overlooked. Nor is there an abrupt transition from one class to the other. From Capella the line of connection passes unbroken through Arcturus and Aldebaran to β Andromedæ and α Orionis (Betelgeux), absorption settling down more heavily on the blue rays, and metallic lines gaining strength at the expense of the truncated hydrogen series, until the fluted type is definitively formed. It is equivalent to Secchi’s Type iii. Within its compass a progression of objects with deepening bands, such as Miss Maury has arranged in her Groups xvii., xviii., and xix., can readily be followed out. It may be said to terminate with α Herculis, a star of the third magnitude, displaying magnificent prismatic chiaroscuro. A drawing of its spectrum by Mr. Espin is copied in Plate XI. Fig. 1.
Fundamentally, the same series of bands recurs in all the individuals of this class. They vary from star to star both in relative and in absolute intensity, but their identity remains unmistakable. The presence of certain determinate atmospheric ingredients fixes the type, and no others can replace them. The stars belonging to it are in diverse degrees red or orange, their blue emissions being largely arrested in the precincts of their photospheres. They must hence be intrinsically brilliant far beyond the proportion of their visual, and, still more, of their photographic magnitudes. Their apparent lustre, that is to say, is small compared with the masses that may reasonably be assigned to them. Their light is markedly unstable, and many are subject to periodical variations of exceedingly wide range. These give bright-line spectra of a very definite character, which it seems advisable to set in a class apart.
Stars with fluted spectra (conveniently designated as “Antarian,”[333] from their exemplar, the lucida of the Scorpion), although rare compared with Sirian and solar stars, are found plentifully in every part of the sky. No comprehensive catalogue of them exists; the number of those already known might, however, be roughly estimated at a couple of thousand. They must be vastly remote. None have sensible parallaxes, and very few show appreciable proper motions.
Class v.—Carbon Stars.—These have also banded spectra, but of a totally different stamp. Three shadings, particularly conspicuous in them, testify to strong absorption by carbon vapour. Vogel found them to reverse exactly the spectrum of an alcohol flame.[334] Scheiner conjecturally identifies the absorbent material with acetylene.[335] Others hold it to be pure carbon. There is much uncertainty on the point. The carbon-bands are of different degrees of obscurity in the various members of the class, nor do they in all preserve the same relative strength.[336] Subordinate bands, too, of unknown origin diversify these spectra more or less strikingly. But all are designed on the same pattern; it is only the mode of printing off that varies. They include as well many dark lines, notably Fraunhofer’s “D” and “E,” representing absorption by sodium and iron. A characteristic dusky streak at λ 576 awaits chemical interpretation.
Carbon stars glow like rubies in the sky; they are, for the most part, fiery red objects. To the eye they make a poor show. The brightest—19 Piscium—is of 5·5 magnitude; and only three in the northern and four in the southern hemisphere, out of about 250 recorded, exceed the sixth. This is not surprising when we consider that but a small percentage of their rays can escape stoppage by enfolding vapours. They would seem, besides, to be plunged in greater depths of space than Sirian or solar stars; so that they ought perhaps, allowance being made for these disadvantages, to take rank as potent light-givers. The coloured “zones,” or luminous intervals in their spectra, are, at any rate, curiously vivid and sparkling. Most are to some extent, some are to a large extent, variable. Their photographic examination—rendered arduous by the quality of their light—afforded Professor Hale in 1898 the interesting discovery, imperfectly anticipated by Secchi, of several unfamiliar bright lines superposed upon their dark shadings.[337] Hydrogen lies low in carbon stars. It exerts no visible absorption, and displays no traceable emission. Their relationships with other stellar families are obscure; connecting spectral links are not altogether wanting, but they are of secondary importance. The class formed by them is coextensive with Secchi’s Type iv. and with Miss Maury’s Group xxi. Four specimens are given in Plate XII., from spectrographs taken by Professor Hale and Mr. Ellerman with a train of three prisms fitted to the eye-end of the Yerkes forty-inch refractor. The wealth of detail shown is so great as almost to obliterate the general columnar effect.
Class vi.—Stars with Fluted Spectra showing Bright Hydrogen Lines.—Mira is the typical star of this class. All its members (save one doubtful case) are pronounced variables. They fluctuate in colour too, but show in general a decided orange or ruddy hue. The flutings are very marked, and they tend to deepen and widen as the stars lose light. Essentially the same as in Class iv., they overlie a similar metallic line spectrum. Vivid hydrogen rays come into view with the approach of each maximum, and fade after it has passed. They seem, however, to persist much longer in some stars than in others. The series is, indeed, at all times incomplete. Its first term—the crimson C—is often missing; the second, F, is by no means invariably present; the stress of brilliancy is, in certain stars, laid upon the third, in most upon the fourth line, the fifth being always concealed by the dense, distended H of calcium. The succession of bright lines is resumed in the ultra-violet, and continued to the limit of the spectrum, which is curtailed by strong general absorption. A spectrographic impression of Mira by Father Sidgreaves is shown in Plate XIII. (1). It extends from orange to indigo, but stops short of H and K. There is no assured trace of green hydrogen, while the two blue beams are lustrous. A corresponding print of the spectrum of α Herculis appears on the same plate below that of Mira. The yellow ray of helium shines in some members of Class vi., and several dark lines in the spectrum of Mira coincide approximately with more refrangible lines of the same substance. Through the presence of emissive symptoms in fluted spectra, more than a hundred new variables have been photographically discovered by Mrs. Fleming and her staff at Harvard College. She subdivides them into eleven families, marked by the varying relative brightness of the hydrogen lines,[338] while Miss Maury includes them all in her twentieth group. Secchi’s third type likewise claimed them; its limits were indeed defined before their singularities had been noticed.
Class vii.—Helium Stars with Bright Lines.—These objects give the characteristic dark-line “Orion” spectrum, variously emblazoned with rays of hydrogen, helium, and a few other substances. In some the bright and dark lines are ranged side by side, in others they are superposed, the system of reversal being tripled by the addition of dark threads drawn across the emission rays. The historical variable “P Cygni” exemplifies the former variety, γ Cassiopeiæ the latter. The fundamental C is perhaps always the brightest line in these spectra, and there is a uniform decrease in the lustre of the hydrogen-series as it progresses upward.[339] In many cases its lower members show by emission, the rest by absorption. The bright spectrum may indeed be reduced to a solitary C. The same rule applies to helium. The circumstance, however, that the lowest terms of each series are those vivified, becomes evident only when the lines present are sorted out in their due sequential order. It is unapparent on a collective view of them.
About fifty bright-line helium stars are known, and fresh specimens are yearly swept up in the course of space-sounding operations at Harvard College. A fuller acquaintance was gained with thirty-two among the number by Professor Campbell’s scrutiny of their spectra with the great Lick refractor in 1895. Much hesitation prevails as to their proper place in systems of stellar classification. Most usually they are treated as a subdivision of the dark-line helium class, but Miss Maury and Miss Cannon leave them outside the framework of their respective schemes, appending valuable discussions of individual peculiarities.[340] There is much to be said for this mode of procedure. We are enabled, by the kindness of Father Sidgreaves, to reproduce in Plate XIV. a spectrograph of γ Cassiopeiæ taken at Stonyhurst, 7th March 1898. Among the bright lines imprinted on it are several due to magnesium,[341] namely, the b-triplet prominent in the sun, and the blue ray at λ 4481, specialised by Scheiner as marking a high grade of heat.
Class viii.—Wolf-Rayet Stars.—Acquaintance with these objects began in 1867 with the discovery, by MM. Wolf and Rayet of the Paris Observatory, of three small stars in Cygnus, giving a spectrum composed mainly of blue and yellow effluences. Then on 24th December 1871, Respighi[342] observed the brilliant prismatic radiance of γ Argûs ( = γ Velorum), which proved to be of the same quality, although vastly superior in quantity. No other star of the kind exceeds the sixth magnitude, and over one hundred of them have been already recognised. Their distribution is remarkable; all are situated in or quite close to the Milky Way, except a considerable group located in the Magellanic Clouds;[343] and the Magellanic Clouds obviously reproduce many of the conditions of the Milky Way.
The leading spectroscopic distinction of the Wolf-Rayet class is the display of the Pickering series of hydrogen. Five of its constituent lines have been recognised, but they are not all equally bright. The upper ones may even appear dark. Emission-bands in the blue, on the other hand, never fail to be visible. Two at least are simultaneously or alternatively present. The more refrangible at λ 4688 is the azure beam identified by Rydberg with the leader-line of the otherwise unknown principal series of hydrogen. Its associate at λ 4652 may possibly owe its origin to nitrogen, but this remains to be proved. Helium lines show, both bright and dark, in these stars; in the same spectrum D_{3} occasionally gleams golden beside its dusky fellow, the noted “Orion” absorption ray at λ 4472. Similarly, in the Huggins hydrogen series, a vivid C may have for its companions an almost neutral F (Hβ), and obscure Ηγ and Ηδ.
The Wolf-Rayet spectrum is then triple. A band of continuous light, fairly strong in the ultra-violet, forms its basis. Absorption lines and bands are superposed, a few of them due to hydrogen and helium, but for the most part unclaimed by any terrestrial substance. To these are added hydrogen, helium, and anonymous bright rays in varying degrees of profusion. No metallic lines, bright or dark, have been recognised. Stars of this description are white or yellowish. They are rarely or never variable. Pickering combined them in 1891 with planetary nebulæ into a “Fifth Type of Spectra”;[344] yet, certain nebular affinities notwithstanding,[345] they lie well away on the stellar side of the dividing line between the two sidereal realms.
The eight stellar divisions just enumerated comprehend as nearly as possible all the stars spectroscopically examined up to the present. The few left outstanding are, in general, difficult objects, which have been casually or defectively observed. When better known, they will probably avow affinities not at first sight apparent. Our classification may then fairly claim to be exhaustive; it certainly rests upon broad and unmistakable distinctions. And it is no small achievement to have obtained a bird’s-eye view of the celestial “maze of error.” Order is not knowledge; vere scire est per causas scire; but it is an indispensable preliminary to its attainment.
III. HELIUM STARS
Helium stars are often palpably connected with nebulæ. The entire Orion region, where they brilliantly congregate, is pervaded with cosmic fog; cosmic fog enwraps the Pleiades; and individual instances of the same association abound, and are likely to multiply as exploration proceeds. It is, however, visibly closer in some stars of the class than in others; and these nebulous gradations appear to correspond with spectral gradations of a very interesting kind, accurately represented in the progressive order of Miss Maury’s groups. The earliest are strongly impressed, not only with helium and ordinary hydrogen lines, but with the Pickering series as well, noted by Mr. McClean to characterise a primitive stellar condition.[346] Satisfactory evidence of oxygen absorption in them was adduced by him in 1897, and lines of nitrogen and silicon are recognisable besides. Among metals only calcium and magnesium make a feeble effect,[347] the one with a just discernible K, the second with the “high-temperature” line in the indigo (λ 4481). No sharp rays are found in these spectra. Their shadings take the form of hazy streaks.
The chief of a triple group in Monoceros is a specimen peculiarly worthy of consideration. It has the uncommon property, for a helium star, of being variable in a short period, whence its catalogue title of S Monocerotis; and it dominates a small cluster (N.G.C. 2264), measured by Bruno Peter in 1880.[348] An involving nebula, evasive of telescopic vision, came out fully in Professor Barnard’s photographs of 1894,[349] and was described by him as “a very wonderful object, irregular in outline but quite well defined, with numerous black gaps running into it, and conforming in general with the peculiarities of the Milky Way in that region.”[350] A picture on a larger scale was taken by Dr. Roberts a year later.[351] That S Monocerotis will prove to be a spectroscopic binary revolving in three days and ten hours, may be judged probable from the analogy of its fellows in variability.
One of its contemporaries is the third magnitude star, ι Orionis (N.G.C. 1980). Sir John Herschel perceived it to be wrapt in a large feeble nebulosity, and a divergent streak, linking it to the great “trapezium” nebula, disclosed itself photographically to Professor W. H. Pickering.[352] It is widely triple, but sensibly stationary. Although its spectrum is in the main a copy of that of S Monocerotis, important distinctions may present themselves to scrutinising inquirers. Both the Wolf-Rayet blue bands show by absorption in these “early Orion” stars, the upper one being especially pronounced.
The three stars forming the Belt of the Giant are slightly more “advanced.” Helium has gained strength in them relatively to the Pickering series; the reversal of Rydberg’s azure band verges towards effacement, and in the middle star, ε Orionis, the spectral lines are fairly well defined. Oxygen, nitrogen, and silicon contribute each its quota of absorption. A great stream of nebulous matter sweeps through the Belt, and its two lower gems, ζ and ε Orionis, claim besides shining appurtenances of their own.[353] An analogous object, σ Scorpii, came out on Professor Barnard’s photographs with a couple of nebulous “prongs” attached to it,[354] and also as a focus of marked condensation in the great nebulous field near Antares. The spectrum is perfectly similar to that of ε Orionis.
Among southern helium stars one deserves special mention if only for its association with a notable discovery. This is Mr. McClean’s “oxygen star,” β Crucis. His identification in 1897[355] of numerous lines in its spectrum as due to the absorptive action of our vital gas was fully confirmed two years later by Sir David Gill,[356] who employed for the purpose the splendid apparatus bestowed by Mr. McClean upon the Cape observatory. Silicon is also present, and the usual reversals of the indigo line of magnesium and of the violet K of calcium appear distinctly. Nitrogen, however, is not evident, nor the Pickering series of hydrogen. The spectra of β and ε Canis Majoris, and of β Centauri are of the same stamp. One of the brilliants of the Southern Cross, β Crucis lies immersed in the Milky Way, at an unmeasured, perhaps an immeasurable distance from the earth. We have thus no means of estimating its actual radiance, which must, however, greatly exceed that of the sun. A secular proper motion of fourteen seconds is ascribed to it, and Sir David Gill finds it to be receding from the sun at the rate of about eleven miles a second.
Bellatrix, in the shoulder of Orion, is a typical helium star, the chief representative of Miss Maury’s fourth group. No Pickering lines have been found in its spectrum, but Professor Keeler noticed the comparative prominence of a subordinate nebular ray at λ 4390.[357] Nitrogen and oxygen were identified in it by Sir William and Lady Huggins,[358] and silicon by Sir Norman Lockyer[359] and Mr. Lunt.[360] The effacement of iron, remarkable in nearly all helium stars, subsists also in Bellatrix, and is explained by Sir Norman Lockyer as an effect of transcendental temperature. It has a spectroscopic alter ego in the lucida of the Southern Cross.
In the spectrum of Rigel some iron lines faintly emerge, and the sodium D appeared conspicuously on Professor Campbell’s isochromatic plates. The Huggins series of hydrogen is magnificently displayed from its first term to the limit[361] (see Plate IX. Fig. 3); helium lines are strong and numerous; those of nitrogen, oxygen, and silicon come out in photographs, and they were found by Sir William and Lady Huggins to be associated with certain distinctive rays of titanium.[362] The lines in this spectrum contrast markedly by their sharpness with those of other stars chemically similar, such as Regulus. Rigel belongs to Miss Maury’s sixth group; it approximates to the stage where helium sinks out of sight, and yields the sole predominance to hydrogen.
In order to form some idea of its prodigious light-power we must remember that it has no sensible parallax (Gill), and is all but stationary in the heavens. This implies that it is so far off as to make almost no perspective response to the sun’s centennial advance through space. In other words, a base line some thirty-three thousand millions of miles in length (allowing for foreshortening) shrinks to little more than a point as seen from Rigel. Assuming the reality of the minute proper motion of 1·5″ a century deduced from its catalogued places, and that it is a parallactic effect of our system’s progress towards an “apex” on the borders of Hercules and Lyra, at the rate of twelve miles a second, we must ascribe to the star a distance of at least 367 light years, corresponding to an annual parallax of ¹⁄₁₁₂ of a second. It follows that Rigel gives about 8000 times more light than the brighter component of α Centauri, an orb considered by Sir David Gill to be the exact match in every respect of our sun. But the sun is dimmed to about one-third of its native lustre by effects of absorption which are virtually absent from the star. Hence a total light emission 8000 times greater would represent a radiating surface only 2667 times more expansive than the solar photosphere. Rigel, moreover, is certainly not massive in the proportion of its luminosity. Stars of the helium variety are composed of highly rarefied materials. This has come to be known through the study of eclipsing stars. Taking, then, the density of Rigel to be about that of Algol, or one-fourth that of the sun, we find it even so to be of no less than 34,000 times the solar mass, while gravity at its surface is of just thirteen-fold power. Nevertheless its spectrum indicates extreme tenuity in its gaseous surroundings. Calcium, for instance, in the reversing layer of Rigel emits violet rays only. There is no trace of the blue line (λ 4227). The vapour exists there in much the same state as in the solar chromosphere and prominences—that is to say, in a state of the utmost attenuation, which implies the counterbalancing of gravity by a strong antagonistic influence, presumably of an electrical nature. This merely extends an inference already derived from solar phenomena.
The relationships of Deneb (α Cygni) have been variously assigned. There is, however, no longer any doubt of its affinity to Rigel.[363] The lines in its spectrum are numerous and clearly defined. Sir Norman Lockyer measured 307 on photographs of the section above F,[364] and there are hundreds besides. Those of helium are of subordinate importance; they are being replaced in the supposed evolutionary progression by metallic lines. Magnesium absorption is deeply graven in the ultramarine (λ 4481), and begins to appear through the green triplet (b). Iron lines of the kind “enhanced” in the spark are fairly abundant; gallium shows at least one strong line,[365] and titanium lines are prominent. Among non-metallic substances, besides helium and hydrogen, only silicon is unquestionably present. It comes, however, well to the front. Mr. Lunt regards α Cygni, Rigel, and Sirius as some of “the best examples of silicon stars” yet known.[366] Nevertheless, spectrographs of them fail to show the three silicon lines most conspicuous in the β Crucis group, while Lockyer’s enhanced lines imprint themselves with some emphasis.[367] These celestial modifications of the silicon spectrum afford “valuable data,” in Mr. Lunt’s opinion, “for the elucidation of the problem of relative stellar temperatures.” Their interpretation on current principles would lead to the conclusion that α Cygni, Rigel, and Sirius are hotter than the “earlier” suns typified by the “oxygen star” in the Cross. But there is no real certainty as to what causes the difference in kind between the luminosity of the spark and arc. Temperature may not be the sole, or even the chief, agent in its production.
The width and density of K in α Cygni are noted by Sir William and Lady Huggins as anomalous, calcium absorption usually remaining feeble when that of helium is visible. The curious thinness of the hydrogen lines may result in part from their projection upon a photospheric background of exceptional brilliancy.[368] The star, at any rate, is one of those which “stand apart through a distinctive individuality,”[369] and it invites, as such, special attention.
Regulus—an intermediate specimen—and β Centauri are perhaps the only helium stars at determined distances from the earth. For the latter Sir David Gill found a parallax of 0·046″, equivalent to a light-journey of seventy-one years, so that it is by no means a near neighbour. In its place the sun would be just perceptible to the naked eye; it would appear of sixth magnitude, while the star is only a couple of grades below the first rank (its photometric magnitude is 1·2). Its emissions, in fact, surpass the solar radiance rather more than 150 times. We may then allow that they proceed from a photospheric expanse fifty times ampler than the sun’s, which must encompass a globe 342 times more voluminous. Assuming further for β Centauri (as for Rigel) a density one-fourth the solar, we obtain the result that it is of 85 times the solar mass. Owing, however, to the comparative remoteness of its surface from its centre, gravity has there less than twice the power which it exercises on the sun. Putting it otherwise, the acceleration of a falling body on β Centauri is about 750 feet a second. The value of this “constant” is probably of essential importance in determining the character of stellar spectra; hence attempts at its estimation, despite the uncertainties that hamper them, are worth making.
The Pleiades are tolerably mature helium stars; Alcyone was selected by Miss Maury as the type of her fifth group. Some of its associates show hazy, others sharp lines. Algol approaches still closer to the boundary of the Sirian class, its “Orion lines” being quite secondary to the hydrogen set. Regulus is of nearly the same standing, but its spectral markings are dim and diffuse; those of inferior intensity thus make no appreciable impression, and the star’s rays are all but exempt from absorptive encroachments.
They indeed tell but slightly throughout the entire class of helium stars, and this seems to indicate the absence of any strong contrast in temperature between their photospheres and the encompassing incandescent vapours. That these are exceedingly tenuous is rendered almost certain (as already pointed out) by the non-reversal of the blue line of calcium. As to their relative temperatures, no dogmatic assertion is possible. The effects of transcendental heat evade inquiry. We cannot without hesitation assume that known rules apply under unknown conditions. The “temperature” of the electric spark is a purely conventional expression; to what state of matter it actually corresponds, can barely be surmised. Yet it is important to remember that the progression of helium stars—if the testimony of their silicon lines be credible—is towards this state from a lower degree of molecular excitement; while their “high-temperature” magnesium ray, present at the start, gains prominence by accordant gradations.
The inverse relationship between helium and metallic absorption is extremely significant. They seem to be almost incompatible; one tends to effacement with the incoming of the other. Yet it must be borne in mind that both subsist together in sun-spots, where a condition of things temporarily arises enabling helium to exert its proper stoppage upon light. Spot-spectra thus approximate, so far, to stellar spectra of the “Orion” stamp. Here, no doubt, we hold the clue to some profound physical analogy, the investigation of which may help to dissolve part of the mystery shrouding the “process of the suns.” Moreover, oxygen, nitrogen, and “cosmic” hydrogen (if we may so call the modified gas giving the Pickering lines) are even more sensitive than helium to the adverse influence of metals. This statement is scarcely impugned by the fact that a trace of oxygen-absorption survives in the sun.
The closest connections of early helium stars are with members of the Wolf-Rayet family. But for the dusky lines of magnesium and calcium apparent in them, their spectra might indeed be said to reverse the Wolf-Rayet radiations. Later on, when nebular symptoms disappear, when helium fades, and faint iron lines crowd in, they slide imperceptibly into the Sirian stage of existence. No halt is cried. The frontier is crossed without advertence.
Helium stars are not equably scattered over the sphere. Their condensation towards the plane of the Milky Way, first noticed by Pickering,[370] was strongly emphasised by McClean’s southern survey. “In the contiguous constellations of Musca, Crux, Centaurus, and Scorpio,” he tells us,[371] “there are twenty-seven helium stars out of a total of thirty-six” brighter than 3·5 magnitude, and the proportion in Perseus, Taurus, and Orion is fifteen out of nineteen. It would be desirable to ascertain whether objects of inferior lustre are similarly swayed by this galactic attraction. But up to the present nothing is certainly known as to the prevalence of the type among faint stars. Below the sixth magnitude, its distinctive marks are hardly recognisable. The conjecture, however, is plausible that Milky Way aggregations are composed mainly of helium suns, large and small. But since their great intrinsic brilliancy renders them visible at distances completely quenching the rays of solar stars of the same size, they should preponderate in the Milky Way for this reason alone, apart from any real numerical superiority. So that the question of their distribution, like most others in stellar physics, has complex bearings. Helium stars are plunged, without any known exception, in abysmal depths of space. None have been found within a radius measured by about seventy years of light-travel. They frequent a sidereal region different from ours, where nebulæ linger and stars with blazing chromospheres have their habitat.
IV. HYDROGEN STARS
Helium and hydrogen stars cannot be quite definitely set apart. The two classes commingle. A transition specimen of uncommon interest is found in η Leonis, which combines some of the peculiarities of α Cygni with a powerful development of the hydrogen series.[372] Another is presented by θ Aquilæ, placed by Mr. McClean beside such clearly characterised helium stars as Algol and Pleione, but by Miss Maury in the group with Sirius and Vega. Even Vega, although a perfectly normal member of the hydrogen class, preserves a vestige of helium absorption in the typical “Orion line,” λ 4472; and its frequent companion, λ 4026, emerges to view in stars like ζ Aquilæ, in which hydrogen approaches a maximum of strength. Both these lines are in the laboratory immediately associated with D_{3}, as members of the first subordinate series of the “yellow” helium set. None of the special Wolf-Rayet lines occur in hydrogen stars; the Pickering series is unrepresented; the “blue bands” have no dark counterparts; oxygen and nitrogen lines seem to have died out. Between the broad black bars of the Huggins series, however, crowds of ghost-like metallic rays are discernible. More than one hundred and thirty of these nascent markings were counted by Miss Maury in the photographed spectrum of Sirius, and her search, owing to the limited range of the plates, could only be partial.
Sirius is perhaps a slightly “older” star than Vega. Helium has entirely disappeared from its spectrum, and more familiar elements take its place—sodium, iron, magnesium, calcium, silicon, with titanium, vanadium, barium, and perhaps chromium and nickel.[373] A remarkable group of lines high up in the ultra-violet is of unsurmised origin. Photographed by Sir William and Lady Huggins 4th April 1890,[374] their approximate wave-lengths are λλ 3338, 3311, 3278, 3254, 3226, 3199, while the head of the hydrogen series stands at λ 3646. They are accordingly more refrangible than any possible hydrogen line; nor could impressions of them be obtained with apparatus including glass prisms or lenses, for which reason they are to be found only on the Tulse Hill spectrographs. The interpretation of these recondite characters offers an alluring problem. Although absent from the light of Vega, they will doubtless be recognised, when duly sought, in other Sirian stars; but exposures of the requisite kind are laborious, and seldom undertaken. Yet just such special investigations are likely to be the most fruitful.
Sirius is the best-known luminary of its class. This for two reasons. First, because of its vicinity. Light travels from the star to the earth in rather less than nine years. Next, because of our fairly complete acquaintance with the nature of its binary revolutions. They have now been closely observed during forty years, and their period is fifty-two. Hence the mass of the system has been determined, and it has, moreover, been apportioned with satisfactory exactness between the members. Their disparity in gravitative power proves to be small compared with their enormous inequality in lustre. The companion is a mere point of light outshone 36,000 times by its radiant primary, which is, nevertheless, more massive only in the proportion of 2·36 to 1·1. This quasi-obscurity of the Sirian satellite is very curious; but our present concern is with the majestic orb, in the blaze of which it is almost lost to view. From Dr. See’s orbital elements, combined with Sir David Gill’s parallax, a mass is deduced for it just two and a half times that of the sun. If, then, it were a body of the same average density and surface luminosity it should give nearly twice (1·84 times) as much light. In actual fact Sirius is of at least twenty-one times the solar brilliancy. This estimate is arrived at by taking α_{2}, the brighter component of α Centauri, as an intermediary. Sir David Gill has shown that its spectrum is a replica of the solar spectrum; its revolutions prove it to be of equal mass with the sun, and it is hence assumed with the highest probability to emit sensibly the same amount of light. It may accordingly, in comparisons of stellar brightness, be substituted for the sun, the uncertainty attending the direct confrontation of enormously unequal light-sources being thus avoided. Now the distances from the earth, and the photometric magnitudes both of α_{2} Centauri and of Sirius, are well known, so that it is easy to calculate how one star would appear in the place of the other. Sirius is twice as far off as the southern binary; transferred to that remoteness α_{2} Centauri would then show one-quarter its present brilliancy; it would be of 1·9 magnitude, just matching the chief star in the Plough. It would accordingly be 3·3 magnitudes fainter than Sirius, which is as much as to say that it gives only ¹⁄₂₁ part of its light. And the sun, similarly located, would be of the same faintness.
Thus Sirius, while two and a half times more massive, is twenty-one times more luminous than the sun. Or, putting it otherwise, the solar ratio of light to englobed matter is exceeded more than eleven-fold. Three causes may concur to produce this effect. One of them we know to be present. It is quite certain that the Sirian beams are almost undimmed by self-absorption, whereas those of the sun are reduced probably to one-third their original intensity. A second contributory cause to the brilliancy of this star may be found in its great bulk. It is likely to be much less condensed than the sun, consequently to possess a much larger extent of photosphere relatively to mass. A luminous area multiplied four times would explain the outstanding disparity of brightness, but would involve a reduction of mean density to one-eighth the solar standard, or about one-sixth that of water. There remains the third factor of absolute areal brilliancy; but its value presumably depends upon temperature, and comparative stellar temperatures must for the present be left an open question. We are only able to conclude that if Sirius and the sun be on a par as to intrinsic shining power, then the star is probably about eight times more tenuous.
Vega is so much more remote than Sirius that it may safely be stated to quadruple its emissions. Its mass, however, remains undetermined, since it sways no detected companion with a measurable force. That it is small compared with its light can hardly be doubted. Indeed, throughout the hydrogen class, this rule prevails to all appearance universally.
The temperature of the stars, as already remarked, is one on which dogmatic assertions are best avoided. All authorities agree nevertheless that the conditions governing light-production in such orbs as Sirius and Vega approximate in many important respects to those present in a disruptive electric discharge. One important item of evidence to this effect is the prominence in spectra of this class of metallic lines weak in the arc, but strong in the spark. “The general result,” Sir Norman Lockyer says,[375] “of the investigation of the enhanced iron lines in stellar spectra confirms the view that the absorbing regions of the hottest stars exist at a higher temperature than is attainable in laboratory experiments.” Concurrent testimony was derived from variations of relative intensity in the magnesium and calcium lines shown by particular stars. But no allowance was made for modifications resulting from differences of pressure, which the Tulse Hill researches had proved to be highly influential. Hence the absence in Sirian stars of the “blue” line of calcium (λ 4227) tells nothing by itself as to their temperature. The special value of Dr. Scheiner’s magnesium-test is that the opposite behaviour of the two lines considered (λ 4481 and λ 4352) excludes the density-factor. For increase of heat may occasion the weakening of individual lines concurrently with the strengthening of others; but changes of pressure must always act in the same direction—though not necessarily to the same extent—on every element of the spectrum affected by them.[376] Professor Keeler[377] suggested that by means of the magnesium triplet, b, inferences as to temperature in stars might be extended to grades beyond the possibility of artificial production. This group is conspicuous alike in the flame, arc, and spark; it cannot be experimentally abolished, yet it fails (as we have seen) to appear in Rigel, and emerges very feebly in Sirius and Vega. Now it belongs to a subordinate series due to a special molecular arrangement, which could not easily persist in an extreme stage of heat. And a break-up of the arrangement would be marked by the effacement of the triplet in the green. “If this reasoning is correct,” Professor Keeler wrote, “the aspect of the b-lines in stellar spectra gives us an extension of the method proposed by Scheiner, and it shows that the temperature of certain stars exceeds that of the most powerful electric spark.”
The long range of powerful hydrogen lines in Sirian spectra, on the other hand, cannot be regarded as a sure symptom of excessive heat. It seems rather to indicate an approach to homogeneity in the originating stratum. The abridgment and enfeeblement of the series, and the development of metallic absorption, follow the same course, which is certainly not prescribed by thermal change. This inverse relation has not so far been satisfactorily accounted for. The most plausible hypothesis regarding it is that of Sir William and Lady Huggins, who connect it with the inevitable gain of effective gravity in condensing globes.
Nor can the intensity of the higher spectral sections be taken as an unequivocal sign that hydrogen stars are hotter than the sun. For it may be caused not by the intrinsic emissive superiority of their photospheres, but by their unveiled condition. We know that the sun’s more refrangible rays would, through the removal of his absorbing atmosphere, acquire strength enough to turn the balance of colour from yellowish to bluish, and it is amply possible that its spectrum, displayed to equal advantage with that of Vega, might rival or outdo its actinic compass.
Fomalhaut (α Piscis Austrini) is a fine example of an advanced hydrogen star. Metallic lines are considerably more developed in it than in Vega or Sirius; the McClean spectrograms show a profoundly grooved K-line, and its blue associate (λ 4227) is faintly reversed. The “spark ray” of magnesium (λ 4481) is prominent. Miss Maury prints a table of wave-lengths[378] measured from the Harvard spectrograms of this star which deserves particular attention as a record, perhaps, of a transition epoch in stellar growth. Lines of iron, titanium, and silicon are readily identifiable in it, but most of the entries have no obvious meaning. Fomalhaut is of 1·3 magnitude, and Sir David Gill has determined for it a parallax of 0·13″ showing it to be almost six times more remote than α Centauri. Its real brightness is hence easily found to be fourteen and a half times that of the sun. Its mass and density, however, remain entirely unknown.
As illustrating the physical differentiation of bodies to all appearance chemically similar, two stars may be singled out. These are Castor and γ Ursæ Majoris, the third of the Plough. Both are included in Miss Maury’s Group viii., and both show deep and broad furrows of hydrogen. The spectra, in fact, bear the same inscription, only printed from dissimilar types. In γ Ursæ, the spectrum of Castor is viewed, as it were, out of focus. The lines distinct in the one are hazy and diffuse in the other; none probably are really missing, though a good many are effaced by expansion. This peculiarity is met with in a considerable number of helium and hydrogen stars forming Miss Maury’s “division b.” In “division c,” on the contrary, of which α Cygni is the best exemplar, the lines are notably sharp and narrow, while in “division a” they are of normal appearance, some thin, others fringed or winged. The cause of these variations is obscure. It would naturally be connected with differences of pressure in the stellar reversing layers; and this again must depend in great measure upon the locus of absorption, which probably varies, not only from star to star, but for each separate substance in the same star. So that the conditions to be regarded, even from this point of view alone, are highly complex.
V. SOLAR STARS
The transition from hydrogen to solar stars is effected as gradually as the transition from helium to hydrogen stars. Metallic absorption comes more and more to the front in successive objects, while the Huggins series retires into the background. There are no definite stopping-places; the course of change flows on continuously. At a certain stage of progress, however, the characters distinctive respectively of the condition that has been, and of the condition that is about to be, appear evenly balanced. The hydrogen lines, although reduced to about one-quarter their Sirian intensity,[379] still muster strong even in the ultra-violet, the metallic spectrum being at the same time pronounced and crowded. This medium state can be studied to advantage in Procyon, the lesser Dog star. So perfect is the blend of types shown by it, that Professor Pickering found it difficult to decide whether the spectrum was actually intermediate, or combined the Sirian and the solar light of two separate, but closely conjoined stars.[380] Either alternative is possible, but the former is the more probable. Nevertheless, the presence of the full complement of Huggins lines, together with a K-band of ten-fold the intensity possessed by it in Castor, must be regarded as somewhat anomalous.
Procyon is one of our nearer neighbours in space. Dr. Elkin has measured for it a parallactic shift of 0·325″, corresponding to a light-journey of ten years. And the revolutions of a faint companion complete the data requisite for finding the mass of the system. It comes out 2·7 times that of the sun, and we cannot be far wrong in assigning to the brilliant component twice the solar quantum of matter. It gives nearly quadruple the solar light; yet the disparity between light and mass is notably reduced from the Sirian standard. Absorption has increased as condensation has progressed. The rays of Procyon are perceptibly tinted with yellow.
A similar spectrum is shown by the splendid Canopus. The extreme remoteness of this orb, which is second only to Sirius in apparent lustre, compels us to attribute to it a prodigious real light-power. It has no measurable parallax, and no sensible proper motion. Only a minimum estimate then of its magnitude is practicable. Sir David Gill attached to the zero representing its parallax a “probable error” of 0·011″. Hence the measures executed do not exclude a parallax of this amount, although they are just as consistent with an equal negative value. Canopus then may be no further off, but cannot be nearer than a light-journey of 296 years. Admitting, for the sake of illustration, that it is in fact at this distance, which is thirty times that of Procyon, we obtain the astonishing result that it gives no less than 3600 times its radiance. And since the spectra of the two stars agree nearly line for line, this figure must represent approximately the ratio of their photospheric areas, that of their cubical contents being 216,000 to one. In other words, 216,000 bodies of Procyon’s size would go to make up one such globe as the star of prehistoric Egypt. Yet Procyon, as we have seen, is a sun constructed on a larger scale than our own. The existence of a luminary so vast as Canopus, although bewildering to imagination, need not appear incredible when we consider the immense scope of creation, and the boundless resources variously displayed throughout the ethereal spaces populous with stars.
Another interesting specimen of the Procyon variety is γ Cygni. Visually of 2·3, it is only of 3·2 photographic magnitude. This implies blue absorption to an extent unusual in the presence of the ultra-violet hydrogen series. It is accompanied by a disproportionately strong K, well brought out in Mr. McClean’s spectrograms.[381] The star resembles α Cygni in the definite character of its lines, although their chemical meanings are very different. They have, however, as yet been most imperfectly deciphered.[382] The spectroscopic relations of γ Cygni derive added importance from its apparent connection with a far-spreading galactic nebulosity photographed by Wolf and Barnard. But the star may be merely seen in projection upon it. The peculiarities of its light recur with less accentuation in that of Polaris.
A very close approach is made to the solar spectrum by χ Orionis; virtual identity is reached by Capella, η Boötis, and α_{2} Centauri. It is scarcely compromised in Arcturus, or any of its numerous associates in Group xv.; the same lines subsist, only drawn somewhat more heavily, and there is an added shade of ultra-violet absorption. The steadiness with which the solar type is maintained, all but unmodified throughout a large collection of objects, is very remarkable. Of the 681 bright stars investigated by Miss Maury, 19 are Capellans, 111 Arcturians; the latter are barely distinguishable one from the other, the former only by the finest grades of difference.[383] This seems to indicate a particularly stable phase of stellar existence. Our sun’s constitution, we can infer, is adjusted to a high degree of permanence; he is moving along a nearly level tract of his evolutionary journey, and will decline with extreme slowness from his actual state.
Solar stars are to be found of all sizes, their variety in this respect forming an instructive commentary upon their spectral similarity. Consider Arcturus. Dr. Elkin, from a long series of skilfully planned observations, assigned to it in 1897 a parallax so small (0·024″) that its light cannot reach us in less than 136 years. And since at this abysmal remoteness it outshines the sun’s twin, α_{2} Centauri, by one-third of a magnitude, the actual excess of its brightness must be at the very least thirteen hundred-fold. In view, then, of its spectral identity, Arcturus may confidently be asserted to possess a photosphere 1300 times more extensive than the sun’s. The globe it encompasses is, accordingly, about 47,500 times more voluminous, and in the same proportion (assuming equal mean densities) more massive. It follows that gravity exercises over the surroundings of Arcturus thirty-six times its solar power. Yet its spectrum bears no trace of sensibly augmented pressure. We are confronted everywhere in sidereal physics with this seeming inconsistency between the nominal force of gravity and its effective action.
Pollux (β Geminorum) conforms strictly to the spectral pattern of Arcturus. It is, however, a full magnitude fainter and at only half its distance; it must accordingly be a much smaller body. Its superficial area is, in fact, one-tenth that of Arcturus. Nevertheless it contains fifteen times more matter than the sun, and gravity at the surface of Pollux has more than eleven-fold its solar power. Planets revolving round this star would have, at the same distances, periods about one-quarter the length of those belonging to the earth and its sister worlds, our year, for instance, being reduced to ninety days, so that a whole summer would be consumed in a brief holiday excursion.
But there are small as well as large solar stars. An insignificant object in the Great Bear, catalogued as “Groombridge 1618,” and noted for its rapid proper motion, is, according to Sir Robert Ball’s measures, comparatively near the earth, its light reaching us in 10·2 years. The sun, however, in that position would be four and a half magnitudes brighter, for it radiates fifty times more powerfully. The spectrum of 1618 Groombridge is of the Arcturian sub-class, so that the proportion of its mass may, under reserve, be taken to follow the proportion of its light. About 350 such stars, then, should be put into the scale to balance one sun, and gravity at its surface has one-seventh its value at the photospheric level. Another minor sun is “Bradley 3077” in Cassiopeia, although the inferiority is here slighter, since Bradley’s star emits perhaps ten times more copiously than Groombridge’s. Further examples of the kind will certainly come to be known when some progress has been made with the investigation of faint spectra. But this is most baffling work, subject to the illusions that everywhere haunt the limits of distinct visibility.
Enough has been said to make it clear that the Fraunhofer spectrum is exactly copied in orbs of most various dimensions. This points, in Dr. Scheiner’s opinion,[384] to the closest agreement, not only in the percentage of the chemical elements entering into their composition, but also in conditions of temperature and pressure. How such uniformity can be combined with widely different gravitational constants, is extremely hard to understand. The Tulse Hill experiments, already referred to, showed the predominant influence of pressure in altering spectral characters. Since, then, they are the same in Arcturus and Groombridge 1618, there is practical certainty that the calcium envelopes (for instance) of both stars do not differ appreciably in tenuity. Yet the compulsive force acting upon one is 252 times more powerful than that exerted on the other. The persuasion that it is somehow neutralised is irresistible. We might even venture tentatively to define solar stars as bodies in which the ratio is the same between gravity and electrical repulsion. In the course of time, doubtless, it will change; one or the other force will gain relatively to the other, and the spectral type will vary to correspond. Presumably the augmentation of strength will be on the attractive side; but cosmical electricity is still an unexplored region.
The symptoms of approach towards the fluted description of spectrum set in gradually, and are of two kinds. General absorption of the more refrangible rays spreads and deepens, and specific absorption becomes intensified in certain dusky lines. Conspicuous among these is the “blue” line of calcium (λ 4227), the stress laid upon which unquestionably signifies increased density in the absorbing vapour. This is just what might be expected to accompany the progress of cooling and contraction, through which domestic gravity gains advantage, as acting in a steadily narrowing sphere. The symptoms described are visible in α Hydræ and β Cancri; they are particularly well marked in Aldebaran. The last is a glaringly red star, its blue emissions being mostly arrested by its own atmosphere. Incipient flutings, too, are traceable. It is the “type star” of Miss Maury’s sixteenth group, which includes twenty-three objects scarcely to be discriminated as regards the quality of their light. “From the greatly-increased width in them,” she writes[385] of the line at λ 4227, “it would appear to be complex, and to include lines weak or absent in the stars of the solar type.”
Aldebaran has a parallax of one-tenth of a second, and is of standard first magnitude. Its real brightness is then certainly twenty-eight times greater than that of the sun; and since it has suffered much more heavily from absorptive encroachments, the emitting surface must proportionately exceed the spread of the photosphere. Even apart from any allowance for increased density, the Taurus luminary may be considered by a quite moderate estimate to be of 200 times the solar mass. On the other hand, the primary member of the famous pair, 61 Cygni, which as to spectrum is a faint duplicate of Aldebaran, ranks very low in the hierarchy of suns, emitting, in fact, only ¹⁄₃₆₄ the light of that great red orb. Here, again, the lesson is enforced that the widest variety of size and mass may consist with spectral identity. Aldebaran is encompassed by gaseous strata apparently no denser and no hotter than the absorbing layers in 61 Cygni. This circumstance is evidently of vital moment in stellar natural history.
VI. STARS WITH FLUTED SPECTRA
The blue light of these stars is powerfully absorbed by an intensification of the screening effect observed in the sun. They are accordingly rufous, or red. Their spectra are profoundly scored besides with metallic rays, generally agreeing in position with, although differing in relative intensity from, the Fraunhofer lines. Thus the calcium line in the blue has gained still further upon the great pair in the violet than in transition stars of the solar type. Finally, banded absorption has come in. A complete system of ten or eleven flutings, sharp towards the violet, graduated insensibly towards the red, shadows nearly the entire visible spectrum. It is printed in stereotype. The bands are variously impressed, but similarly located, in all members of the class. This gives strong assurance of an identical origin. We do not yet know how they are produced, or by what substances, but there can be no doubt that their explanation in one star will apply to all.
With high dispersion the bands can be resolved into fine lines set very close together.[386] The fluted effect is due to the crowding of these lines towards a limiting wave-length prescribed, beyond question, by a rhythmical law. For that each band represents, as it were, a condensed series there need be no hesitation in admitting. Indeed, by mere reduction in scale the hydrogen procession in a white star assumes the aspect of a genuine fluting. A promising start has even been made in the research of laws regulating the distribution of lines in bands.[387] Each of the stellar stripes is then, so far, a separate entity, while all may be linked into harmony by subordination to some higher unknown principle. Whether they originate from one or many forms of matter has still to be determined; nor is there any certainty as to whether elements or compounds are concerned in their production.
Stars of Secchi’s third type, forming the class just now under consideration, are divided by Miss Maury into three groups. Antares and β Andromedæ belong to the first of these (“Group xvii.”). They show all the characteristic bands, slightly marked, and transparent enough to allow every detail of the linear spectrum to be clearly visible. Betelgeux (α Orionis) is the pattern of the next group. Its prismatic light makes a beautiful and wonderful effect. The usual multiple absorption is exerted upon it, but with a delicately balanced power. The blue rays retain appreciable vivacity; the flutings are not so deep as to obscure the underlying rays; they are finely shaded, yet exquisitely distinct. Ten were measured by Vogel and Dunér, the strongest of which are the fifth and seventh, with their steep sides at λ 5453 and λ 5169 respectively. Battalions of dark lines show through them. On the Harvard plates Miss Maury counted 463, mostly composite, between the sodium D and the calcium H. The latter and its associate, K, have shrunken somewhat from their giant dimensions in Arcturus.[388] Iron absorption predominates. The rays significant of it are more prominent than in the sun, and some have unilateral shadings—a feature also visible in the spectra of sun-spots and of metallic oxides, and indicative probably of a decline in heat.[389] Professor Keeler remarked that the lines in Antarian stars “are essentially those of the solar spectrum, but the relative intensities are not the same, and the general aspect of the spectrum is quite different from that of the spectrum of the sun. The strong lines are mostly those of iron—apparently the low temperature lines. Their relatively greater strength in the star spectrum gives to some well-known solar groups (notably the b group) quite an unfamiliar aspect.”[390]
Hydrogen-absorption is much more effective in Betelgeux than in any other spectrum of the fluted kind. It is, however, distinctly, though feebly represented in all by the four lowest members of the Huggins series. The rest are either absent or shrouded in overlying vapours.
In ρ Persei, a star capriciously variable between 3·4 and 4·2 magnitudes, the bands are perceptibly deeper than in α Orionis. On the other hand, the metallic rays seem rather less numerous and intense. But this diminution may be more apparent than real. The comparative faintness of the light they interrupt would partly account for it, and the added density of the associated flutings would help towards effacement.
Their still greater opacity in α Herculis occasions effects of contrast with the vividly tinted bright zones, described as “singular and magnificent” by Father Secchi, one of their earliest observers (see Plate XI. Fig. 1). Professor Keeler, whose study of fluted spectra was based on photographs of high dispersion showing manifold details, found the dark groovings, plentiful in α Orionis, to be present only as a comparatively scanty survival in α Herculis. Miss Maury reached a similar conclusion. Yet Dr. Vogel was struck with the richness in absorption-lines of this spectrum and the analogous one of β Pegasi.[391] Both these stars vary irregularly, α Herculis from 3·1 to 3·9, β Pegasi from 2·2 to 2·7 magnitude. Next to Antares, the brightest specimens of this class in the southern hemisphere are γ Crucis and β Gruis. Mr. McClean obtained spectrographs of all three at the Cape in 1897.[392]
Uncertainty has often been expressed as to the true nature of the luminosity in the open spaces of fluted spectra. Are they simply intervals of unshaded photospheric radiance, or is their brilliancy reinforced by the addition of bright lines? Just where they meet the black edges of the shafts of absorption, their splendour exceeds, in the opinion of some observers, what could be produced by contrast alone. It seems, nevertheless, unlikely that rays of emission should occur in these positions and nowhere else in the same spectra. Moreover, illusory impressions of the kind, both visual and photographic, are common and pertinacious. Nothing, however, is more inimical to truth than dogmatic denial; we must be ready to admit much that we should beforehand have deemed impossible, even the reality of far-fetched coincidences; for anticipation is often belied by fact. Professor Keeler, whose mind was singularly free from prepossessions, found it “impossible to avoid the conclusion” that in the spectrum of α Herculis “the edges of the zones bordering on the dark bands are bright—much brighter, that is, than the average continuous spectrum—and that they are due to a real predominance of emission at the regions of the spectrum in which they occur. In the case,” he wrote further, “of stars like α Orionis, of a less pure type, such a conclusion could not be safely drawn; yet the superior brightness of the spectrum at these places is obvious, and it can be traced even in second-type stars. May there not, after all, be bright regions in the solar spectrum, such as Draper supposed he had found in the places of the bright oxygen lines? And what is the relation between the dark bands in third-type stars and the bright zones which border on them?”[393]
Questions more easily asked than answered. They suggest doubts, not at once to be set at rest, as to the nature of so-called “continuous” spectra. May they not in certain cases include several maxima of radiation? The possibility is at least not excluded of individual differences in this respect between stellar photospheres. Yet of true gaseous emissions there seems to be no trace in Antarian stars. The admission of its presence could not be made for one without being extended to all; and many spectra of the class are clearly exempt from abrupt intensifications.
Chemical recognition has not been carried far in them. The familiar lines of only five substances—iron, calcium, magnesium, sodium, and hydrogen—are entirely unmistakable. The rest await more searching scrutiny. It will be of especial interest to determine whether titanium and its usual associate, vanadium, retain in these objects any share of their importance in sun-spots. Helium-absorption, too, which occasionally emerges to view in spot-spectra, might be looked for with some prospect of success, but is likely to be inconspicuous. The fundamental problem, however, in this connection relates to the origin of the flutings. It ought to prove capable of a definite and, so to speak, a simultaneous solution. For the members of the system show the coherence of a structural design. They form a marshalled array, an interdependent order. Their occurrence piecemeal need not then be expected. They will be recognised together or not at all.
Stars with fluted spectra have a fixity significant of immeasurable remoteness. Yet two—Antares and Betelgeux—are of the first order of apparent brightness. Their real magnitude must hence be prodigious. An approximate estimate of it can be arrived at in the case of Antares, which has an ostensible parallax of 0·021″, corresponding to a light journey of 155 years. This, at least, was the outcome of Mr. Finlay’s measurements at the Cape; but it is so small as to lie but slightly outside the margin allowed for their probable error. Its genuineness can then only be assumed for the purpose of fixing ideas; since the star may be indefinitely further off, while unlikely to be appreciably nearer, than this minute annual shift asserts it to be. Under this reserve we may compare Antares with our standard star, α_{2} Centauri, which, as we have seen, is equivalent to comparing it with the sun. The result is to show that four hundred suns, in its place, would barely supply the light we receive from the alter ego of Mars.[394] And this is only what remains after a heavy absorption-toll has been levied—a toll of probably twice the amount paid by the sifted solar beams. The photospheric extent of Antares may then be set down as at least eight hundred times that of the sun, while the immense sphere it covers may be held, on good grounds, to have a comparatively high mean density. Even on the basis of equality in this respect, its mass would exceed that of the sun more than 22,000 times, while gravity at the surface of this unimaginable globe must possess at least twenty-eight times its solar power. Similar reasonings apply to Betelgeux, only with still further enlargement in measure of the conclusions they lead to. For Betelgeux is a more brilliant luminary than Antares, and its immobility in the sky is, if anything, more nearly absolute.
The processes of interior circulation in such bodies are extremely difficult to realise. The intensity of radiation must depend—other things being equal—upon the promptitude of delivery at the photospheric level of heated stuff from within. This must increase with the force of gravity, which is the driving power on ascending and descending currents; but it must, on the other hand, fall off as the sphere they traverse grows more compact. The manner in which the balance is struck in each individual star between these opposing influences transcends every rational conjecture. We can only see that it must vary widely, and that its variations necessarily affect the photospheric composition and the radiative characteristics of the globes in which they prevail.
Antarian stars obey no special law of distribution. They are scattered at large over the heavens, and usually in isolated positions. They show as a rule no tendency to gather into groups. A collection of nine specimens, located in the intervening space between the two grand clusters in Perseus, is perhaps unique. Discovered by Mr. Espin in 1891,[395] this nest of red stars appears like a garnet clasp linking together a pair of diamond aigrettes. They are of about the eighth magnitude, but may nevertheless, since they are assuredly vastly remote, be most majestic orbs. Not that we should ascribe to all stars of this spectral type the colossal dimensions of Betelgeux and Antares. The same degree of variety may be supposed to exist among them as among solar stars. Yet the analogy may not hold. It is conceivable that very great mass is a pre-requisite for the development of a fluted spectrum.
VII. CARBON STARS
Stars of Secchi’s fourth type, also known as “carbon stars,” are the most exclusive of stellar families. They hold remarkably aloof from every other. They have indeed traceable relationships; but the genealogy obscurely indicated by them needs authentication.
Mr. Espin published in 1898 a catalogue of 237 carbon stars,[396] and about a dozen objects of the kind have since been detected. None are as bright as the fifth, and only seven exceed the sixth magnitude.[397] Their inconspicuousness probably arises, not from any deficiency of intrinsic light-power, but from the overwhelming absorptive action of their atmospheric envelopes. Thus only a small part of their original radiations attain to outer space; none of the shorter wave-lengths escape; the spectra are cut off short a little below the place of the blue calcium line. Intense visual redness is a consequence. These stars glow sanguine in the field of the telescope; they are variously compared to “drops of blood,” to carbuncles, garnets, or rubies. By a rough estimate, 12 per cent are strikingly variable, the proportion being nearly the same as for third-type stars. Scarcely any, however, shine steadily; if attentively watched, they can be perceived to flicker and fluctuate more or less extensively.[398] Some of their changes are indeed so lasting as to suggest a permanent drop or rise (as the case may be) in the photometric scale. The circumstance that instability of light ordinarily accompanies redness of colour in stars is most curious and significant.
The fundamental characteristic of fourth-type spectra is the presence of three deep bands, degraded towards the violet, sharp towards the red. They are a negative copy of the emission-bands displayed by comets. Five or six additional dusky stripes, so far unidentified, are of less distinctive construction. The general effect of these spectra differs from that of the fluted sort chiefly in two ways. First, the columns of absorption are broader and more massive; they are of Doric rather than Ionic proportions. Secondly, they are illuminated from the opposite direction; the chiaroscuro is inverted. Their variations of relative intensity in different objects have been proposed as subsidiary classification-marks, but cannot be much insisted upon. Such individualities—as Professor Dunér pointed out—do not imply radical distinctions; and they are so prevalent and so various that, by closely attending to them, “one might easily get as many subdivisions as there are stars.”[399]
The linear spectrum in carbon stars is seen with difficulty through the cloak of the bands. It is, however, none the less important. Sodium and iron contribute to it, but most of its constituents still lack interpretation. Hydrogen and helium alike fail to appear. No calcium lines are visible; indeed those in the blue and violet, which would most naturally be looked for, could not show through the dense veil of absorption shrouding the upper spectral reaches; so that their seeming absence is consistent with the presence of a calcium ingredient in the stellar atmospheres. Their carbonaceous strata nevertheless give them their special character. No other sidereal objects, except an imperfectly observed variable star, show a trace of the cometary analogy prominent in the fourth spectral type.
The carbon bands, which constitute its leading feature, were identified by Father Secchi in 1868, and he noticed besides certain bright lines, the reality of which, long discredited, has quite recently been confirmed. He noticed them, however, somewhat confusedly, for he at times failed to keep them apart from the illusory effect of vivid emission caused by the prismatic gleaming of the intercolumnar zones. Professor Hale in 1898, effectively aided by Mr. Ellerman, applied the photographic method with remarkable success to the investigation of these spectra. They offer no facilities to the camera. The use of ordinary plates is of course precluded by their deficiency in blue rays; only those rendered “orthochromatic” by suitable dyes avail for their delineation; and these are found practically inconvenient owing to irregularities in sensitiveness. A series of splendid pictures was nevertheless obtained with the aid of the Yerkes forty-inch refractor; but the arduousness of the undertaking can be estimated from the fact that, with a train of three prisms, exposures of nine hours were required to secure impressions comparable with those given by the spectrum of Betelgeux in twenty seconds.
The research embraced, to begin with, twenty-two stars, ranging from 5·4 to 8·2 magnitude.[400] Most of the spectrographs were limited to the region D to b; but a few extended to λ 4450, where dark blue merges into indigo, and one ranged far down in the crimson. This was derived from the brightest specimen of the type, numbered 152 in Schjellerup’s Catalogue of Red Stars. Situated in Canes Venatici, it shows a spectrum of such rare beauty as to justify the title of “La Superba,” bestowed upon it by Secchi in 1868. Its yellow section, photographed by Hale, is shown in Plate XI. Fig. 2. A surprising amount of detail is imprinted in it. On the original negatives over a hundred lines were measured where no more than three or four had been previously recorded.[401] Most of them are dark, but some are bright, among which, apparently, may be reckoned two yellow rays, compared by Secchi to “exquisite threads of gold.” They are prominent in our figure at wave-lengths λ 5593 and λ 5693, which, as Professor Hale was careful to point out, “agree very closely with” those of “two bright lines in the spectra of the Wolf-Rayet stars.”[402] He adds the caution that, several other similar approximate coincidences notwithstanding, “it is too soon to conclude that these classes of stars are related.” A fine group of vivid green lines was also photographed, and some blue rays were suspected. Although intensely red, 152 Schjellerup has not been observed to vary from 5·5 magnitude.
The typical star of Miss Maury’s “Group xxi.” is 19 Piscium. It shows a splendid four-zoned spectrum, vivified by the twinkling of emission rays (see Plate XII. Fig. 2). Secchi noted in it the shining of the same “threads of gold” previously seen in 152 Schjellerup,[403] and they may be considered as a feature common to all spectra of this class. The opinion to this effect expressed by M. Dunér carries great weight. It was in 1884 entirely adverse to the reality of the bright lines recorded by the Roman astronomer;[404] but he changed his view on improving his instrument. The materials for his invaluable Memoir of 1884 were collected with the Lund ten-inch refractor; in 1893 a Steinheil of fourteen inches aperture became available to him at Upsala. With it he at once undertook a revision of his former work,[405] which, although hampered by serious interruptions, progressed steadily down to 1898. M. Dunér was not disappointed in his hopes of seeing more and better with the larger instrument; and he chronicles as of primary importance “the fact that he was able to detect without difficulty bright lines in various spectra, which at Lund were either invisible, or at least could not be discovered.” Professor Hale’s photographic registration of them was thus visually authenticated by an observer of unrivalled experience, and was further verified with the great Lick telescope by Professors Keeler and Campbell under conditions so admirable as to leave little or no room for surviving doubts.[406]
It may then be regarded as an established fact that spectra of the fourth type include elements of direct emission. They are subsidiary, yet distinct, and seem to be unfailingly present in all members of the class. Three characteristics may provisionally be ascribed to these curious bright lines. In the first place, they are of entirely unknown origin. Hydrogen and helium are equally (so far as published measures enable us to judge) alien to their production. Some of them may coincide with Wolf-Rayet lines, but if so, it is with Wolf-Rayet lines which themselves lie outside the range of terrestrial acquaintanceship. Not even the exotic light of nebulium or coronium can be seen to glimmer in carbon stars. Secondly, they are independent of luminous change. They do not betoken variability. They occur indifferently in objects of steady lustre and in those subject to wide vicissitudes. Nor has the slightest sign of inconstancy been detected in the rays themselves. They do not fade and flash capriciously or periodically. They shine equably—to all appearance—from year to year, and from decade to decade. Thirdly, the locus of their development is above the region of carbon absorption. The tinted rays evidently overlie the dark bands; they are seen projected upon them. The substances to which they are due must then be found at a higher level in the stellar atmospheres than the carbon vapour. As Professor Hale remarks,[407] the case is paralleled in the sun, where hydrogen and calcium rise to great heights, while a shallow layer of carbon-gas lies low at the base of the chromosphere. This arrangement of emissive and absorptive strata does not prevail—as we shall see later—in stars of all spectral classes. They are, on the contrary, markedly distinguished in this respect, and the distinction implies profound physical differences.
It was found possible at the Yerkes Observatory to form a sequence of eleven stars,[408] in the order of growing depth of carbon absorption (see Plate XII.). The transition from one of these objects to the next was so gradual as to suggest that they represented actual phases of development. This, however, is merely a convenient hypothesis. One of the earliest of the series is unique, according to Dunér, in the relative strength of its spectral bands. Those due to carbon are quite feeble, while one of untraced origin in the red is broad and black. This star, known as 280 Schjellerup, is scarcely brighter than the eighth magnitude, so that it can be dealt with to advantage only by the aid of powerful instruments. Passing on to 19 Piscium, we find one of the three carbon bands dim, the others—in the green and blue respectively—very wide and dark. Their unequal prominence constitutes a striking anomaly. It recalls the variations in relative brightness of the hydrogen lines recorded with surprise in nebulæ and sundry species of stars.
A spectrum intermediate between those of 19 Piscium and of 152 Schjellerup is shown by the variable star U Hydræ (132 Schjellerup.) Dunér noticed long ago the wonderful chromatic effect of its four brilliant zones,[409] set off by deep bays of absorption; and Secchi perceived in it a green, as well as a yellow pair of fine rays, the genuineness of which is more than probable. The star fluctuates irregularly from 4·5 to 6·3 magnitude, but is very rarely seen at its maximum brightness. Professor Hale’s spectrographs afforded evidence of a partial but very interesting resemblance between its spectrum and that of μ Geminorum, a fine example of the fluted description, and the agreement—as can be seen by inspecting Plate XV.—extends to the sun. “Further toward the red,” he tells us,[410] speaking of the banded varieties, “the spectra become very unlike, though even here there are certain important points of resemblance which must be carefully investigated.” Only their linear elements are naturally in question; the shadowing bands are totally unlike in the two classes. The coincidences detected, however, are of real importance as forging a link, even if a slight one, between stellar families that stood previously entirely apart.
The invisibility (up to the present) of hydrogen in carbon stars is not easily accounted for. The substance must enter into their composition; its diffusion is seemingly universal and profuse; why, then, is its manifestation, whether by emission or by absorption, suppressed in this particular class of objects? The same query may be put in regard to comets, and the same obvious, although perhaps insufficient answer presents itself, namely, that their stock of hydrogen has been consumed in the fabrication of hydrocarbons. It is worth noting besides that the only metals yet identified in these stars—sodium and iron—are precisely those perceived to glow in one or two exceptional comets. But this may be only a chance concurrence.
The rule of colour in carbon stars long seemed inviolable; yet there are exceptions to it. Two spectra of the kind well extended in the blue were photographed at Harvard College in 1891, and they belong, in fact, to white stars.[411] They are situated, one in Aquila, the other about three degrees north of ε Ceti, and are of the seventh and eighth magnitudes respectively. Their investigation ought to prove peculiarly instructive, for in them the type has developed under most unusual conditions. It is besides more completely exhibited. Sections of these spectra can be registered and examined which in other analogous objects are concealed by dense general absorption. It should, for instance, be possible to determine whether calcium lines are really or only apparently absent from fourth-type spectra.
Carbon stars, there is little doubt, are inordinately distant from the earth. None, we believe, have any measurable proper motion, and experiments on their annual parallaxes would certainly prove a waste of time and trouble. We have, then, no means of estimating their real brilliancy, but it must in some cases, and it may in all cases, be exceedingly great. These objects show a marked preference for the Milky Way.[412] They occur, however, in other parts of the sky as well. They are condensed towards the galactic plane, but not limited to it. They are unmistakably, yet far from exclusively, swayed by its attraction.
VIII. STARS WITH FLUTED SPECTRA SHOWING BRIGHT LINES
Although the display of bright lines in fluted spectra is a sure sign of extensive variability, it is not the light-changes of the stars thus characterised that here concern us. The two classes of phenomena are beyond question intimately related; but as a matter of pure convenience they have to be treated of apart.
Mira Ceti has the advantage over its fellows of rising to more brilliant maxima, and of having received longer attention and more careful study. Since, however, all the members of Class vi. are not copied from one pattern, investigations conducted too exclusively can only lead to partial knowledge; and, indeed, the varieties distinguishing the different specimens are precisely their most instructive feature. Thus they agree in showing some brightened hydrogen lines, but not in the selection of those to be brightened. Then helium rays are vivid in certain of these stars, dark in others, and there are further, less assured diversities. These cannot yet be explained by any single consistent theory, but they may be definitely ascertained and brought into some kind of orderly relationship.
The spectrum of Mira has a prescriptive right to be considered first. It is the model, deviations from which count as exceptional. The bands are profound, the radiations of hydrogen intense during fully one-third of the light-period of eleven months. Detected photographically at Harvard College in 1886, they are now looked for, and rarely missed, in every analogous object as it rises from quasi-extinction. The hydrogen stratum in Mira seems to be in a peculiar condition. It emits only the higher members of the Huggins series, the red and the green lines (C and F) being alike invisible. The blue and the indigo lines, on the other hand, shine with extraordinary brilliancy—a brilliancy “too great to be shown on a drawing or to be safely expressed by a number representing relative intensity”[413] (see Plate XIII. Fig. 1). The fifth hydrogen line, by a rule without exception in Mira variables, is hidden; but eight of its associates in the ultra-violet have been recorded. Very singular, indeed, is the partial presentation in this star of a closely-linked sequence of vibrations. It can hardly result from an extraordinary elevation of temperature; we can better conceive it as due to some subtle form of electrical action not yet evoked in the laboratory. The state of things as regards the hydrogen spectrum is the opposite of that prevailing in the reversing stratum of the sun. Here the upper radiations are suppressed; in Mira the series starts from its third term.
The star has been spectrographically investigated by Vogel, Sidgreaves, and Campbell. The Potsdam plates were exposed during the low maximum of January-February 1896,[414] when the variable scarcely exceeded fourth magnitude; and this is a circumstance to be borne in mind, since there is reason to suspect that emission may differ, not only in degree but in kind, at light crises of different intensities. However this be, only the hydrogen lines were perceptibly vivified in 1896. Of the dark lines measured by Dr. Vogel, many coincided with Fraunhofer rulings, but a goodly proportion seemed unfamiliar.
The maximum of 1897–98 was studied by Father Sidgreaves.[415] It was an improvement upon that observed by Vogel. Mira attained 3·2 magnitude on 30th November. The Stonyhurst plates were isochromatic; their range of sensitiveness extended from high up in the violet to near D in the yellow, and they continued to be exposed until 5th February, when the variable had sunk to the sixth magnitude. But the light remained essentially unchanged in quality, although reduced to one-thirteenth its original amount. Only the continuous spectrum in the blue had faded, relatively as well as absolutely, showing that the star grew redder in its decline.
Professor Campbell’s[416] observations at the maximum of October 1898 had the twofold advantage of being made with a magnificent apparatus and at an exceptionally bright phase. In more ways than one they mark a beginning. They both suggest relations and establish facts. The plates exposed with the Mills spectrograph attached to the great Lick refractor show only the region near the third hydrogen line (Hγ) in the fine detail needed for measures of precision. These were designed primarily for the determination of the star’s radial movement, which proved to be one of recession at a speed of 62 kilometres (38½ miles) per second. It may be regarded as constant. No part of it seems to be due to orbital motion round an invisible companion. It was, however, derived exclusively from the dark lines in the spectrum. The bright lines told a different tale. Four were compared—a hydrogen pair and a pair ascribed to iron—and all showed a much smaller displacement redward than the dark lines. The amount of the discrepancy, moreover, proved subject to fluctuations; but to fluctuations obviously depending upon intrinsic, not upon extrinsic causes. No attempt has been made to explain them on the hypothesis of variable motion. It may be accepted, on the evidence of lines physically in a normal state, that Mira—so far as appears yet—is a solitary body in course of withdrawal from the earth at a uniform rate of 38 miles a second.
Early in October 1898 the star reached 2·6 magnitude, and during the few weeks of its greatest brightness the blue and indigo hydrogen bands were perceived to be broken up each into three unequal components. This remarkable appearance falls into line with symptoms of disturbance in stellar spectra of other types, but had not previously been observed in a Mira variable. It is of very curious interest. In studying the “intensity curves” of the tripled line (see Fig. 19) the conviction becomes almost irresistible that here a “Zeeman effect” is in question. The polarisation test might decide. If the lines are distended and shattered by powerful magnetic action, then the lateral components and the central component must be polarised in planes at right angles to one another, and the rotating of a Nicol’s prism in the field should produce alternating extinction. Professor Campbell was prepared to make the experiment at the maximum of 1899, but the star unfortunately failed to replenish its due measure of light, and gave an imperfectly legible spectrum. Favourable opportunities, however, for applying this simple criterion must frequently recur, and they are well worth watching and waiting for. Positive results of the kind indicated would be of revolutionary importance; obscure phenomena would be illuminated; anomalies would be removed; a boundless region would be thrown open to investigation. The issue, it is scarcely too much to say, is vital to the progress of astrophysics.
The production of multiple hydrogen-lines in the spectrum of Mira may quite possibly be restricted to brilliant phases. Metallic emission almost certainly is. Thus the iron rays (λλ 4376, 4308) registered as bright in 1898 were, the one strongly dark, the other either dark or invisible in 1896 and 1897. This gives a hint of the diversities to be looked for in the future, and lends enhanced interest to minutiæ of observation which in themselves might seem trivial.
The spectrum of Mira includes a good many dark lines closely adjacent to, if not actually coincident with, rays of helium. None have been seen bright. Calcium absorption is very prominent. The line in the blue, which develops with increase of pressure, comes out as a black grooving; the giant pair in the violet are of surprising intensity. The less refrangible, as in all such stars, smothers the hydrogen emission of nearly its own wave-length; and this circumstance demonstrates some unexpected relations. The calcium-layer, plainly under considerable pressure, must be located, as in the sun, quite close to the photosphere. But the glowing hydrogen necessarily lies lower still, stoppage of light implying superincumbence of the arresting vapour, and there seems no room for it except in the very interstices of the photosphere itself. The overlaying of a light by a heavy substance is indeed anomalous, yet no other arrangement is consistent with the spectral phenomena of Mira and its congeners. Besides calcium, iron, magnesium, strontium, titanium, manganese, and chromium are easily recognised as absorptive constituents of its atmosphere.
The mode of hydrogen-radiation characterising Mira does not recur in all stars of its type. Some show the two lowest lines conspicuously bright, and they are often accompanied by the glimmering of the yellow helium ray. R Aquilæ is an example.[417] Other members of the class have F for their chief bright line, C being invisible, as in R Andromedæ and S Cassiopeiæ, or dim as in V Boötis. Hydrogen in these stars appears to exist in its nebular condition.[418] Analogous to them in some respects, R Cygni may in others be divergent. Its chief bright lines are F and D;[419] but the nature of the accompanying banded absorption appears somewhat indeterminate.[420] It might be definitely ascertained by a few well-timed observations. An important spectrographic investigation of χ Cygni was carried out by M. Eberhard at Potsdam in 1901.[421] It disclosed phenomena closely analogous to those detected by Campbell in Mira. Thus the maximum of lustre was attained, in the hydrogen series, by its fourth member (Hδ); iron lines, both bright and dark, were abundantly visible; above all, the absorption and emission-spectra were relatively displaced, just as in Mira in 1898. The bright lines, that is to say, were pushed towards the blue, while the dark lines deviated in an opposite sense, though very slightly, from their normal places. This surprising feature may then prove common to the whole of this class of stars, and doubtless depends upon some essential peculiarity of their constitution.
About two hundred variable stars with fluted spectra are known to emit bright lines, and this kind of spectrum is a distinctive badge of variability. Mrs. Fleming’s classification of them[422] is based mainly upon differences of hydrogen-emission. For the typical star of the first of her eleven groups she chose R Lyncis, in which Hβ and Ηγ are brilliant, while Ηδ—sometimes the brightest line in Mira—is scarcely visible. She then traced a continuous sequence of change to R Leonis, the exemplar of her last group, in which Hβ is imperceptible, Ηγ excessively faint, Ηδ conspicuous. But this order of relative lustre is not in R Leonis permanently maintained. The effacement of the green ray is only transient. In April 1895 Dr. Krüger, observing with the eleven-inch Bamberg refractor, found it to dominate the spectrum; and MM. Gruss and Laska saw in the same star, 6th May 1894, Hα doubtfully, Hβ and D_{3} unmistakably, although two nights later Hα shone alone, while on 28th May Hβ was similarly isolated. Such changes, inexplicable as they are, cannot be set aside as incredible. Their further investigation is most desirable. Meanwhile, the relative brilliancy of the hydrogen lines in variables evidently supplies a highly insecure basis for their arrangement.
Mira is the only member of its family which has been at all adequately studied. A good beginning has been made with χ Cygni; but about most of the remaining couple of hundred, particulars are wholly lacking. The great majority, having been registered in sweeping spectrographic surveys, were pigeon-holed for future reference, after brief inquiry into the history of their recorded light-changes; and in their pigeon-holes they have been mostly allowed to rest. Enough is known, however, to whet curiosity as to what remains unknown. Spectral changes of a remarkable kind affect these stars; their thorough verification and the unravelment of their tangled relationships are essential to progress. The work may be difficult, but it is of profound interest. The elucidation of the hydrogen-spectrum in one variable star may indeed open the door to unexpected and far-reaching discoveries.
The helium-spectrum is equally significant, but more evasive. The emergence of the yellow ray seems to accompany the brightening of the two lower hydrogen rays; but its shining may be comparatively transient. The important point, however, is that it does not seem to occur at all in stars showing, like Mira, a mutilated hydrogen series. Then there is the further question whether D_{3}, when it does shine out, shines alone. Are all its numerous associates invisible, or are they dark, as some of them appear to be in Mira? Finally, we know very little as yet about the lighting-up of metallic rays in such spectra. It is nevertheless certain that some regulating principle governs the selection of those brightened; and only by detailed study can the nature of that principle be ascertained. All this, and much more, needs prolonged and extended inquiry; but in a field that will yield ample return for the expended labour.