The study of stellar evolution; an account of some recent methods of astrophysical research

Front matter

THE DECENNIAL PUBLICATIONS OF THE UNIVERSITY )F CHICAGO

THE STUDY OF STELLAR EVOLUTION

HALE

Preface

As first planned, this book was intended to serve as a handbook to the Yerkes Observatory. Many inquiries regarding the observatory's work, made by the numerous visitors received there annually, seemed to call for a printed explanation of the purposes in view and the observational methods employed. Removal to California and new duties connected with the organization of the Mount Wilson Solar Observatory caused a modification of the project. I finally adopted the plan of describing a connected series of investigations, laying special stress on the observational methods employed, in the hope of explaining clearly how the problem of stellar evolution is studied. The advantage of using concrete illustrations drawn, in large part, from personal experience, and the desire that the book should be of special service to visitors at the Yerkes and Mount Wilson Observatories, are sufficient reasons, I trust, for the otherwise undue proportion of space devoted to these institutions.

The omission of such important subjects as the theories of temporary and variable stars; Sir George Darwin's discussions of evolution as affected by tidal friction; Vogel's and Pickering's photometric and spectroscopic studies, and the researches of the latter on the distribution of stars of various types; Campbell's investigations of stellar spectra, and, to mention no other work, his development of the spectrographic method of determining radial velocities, sufficiently indicate that I have made no attempt to deal with the general problem of stellar evolution, or to offer anything approaching an adequate description of the observational methods of astrophysics. The various researches described are chosen rather arbitrarily, in some cases with more regard for my personal

Preface acquaintance with the facts than because of their intrinsic importance. I trust, however, that although this method of treatment has necessarily resulted in a fragmentary exposition of the subject, the book will serve to show how the problem of stellar evolution is attacked along converging lines, leading from solar, stellar, and laboratory investigations.

I wish to express my thanks to Sir William Huggins ; to Messrs. Adams, Ellerman, Olmsted, and Ritchey of the Mount Wilson Solar Observatory; to Professors Barnard, Burnham, and Frost of the Yerkes Observatory; to Professor Campbell of the Lick Observatory; to Professor Pickering of Harvard College Observatory ; to Mr. Abbot of the Smithsonian Astrophysical Observatory; to Professor Lord of the Emerson McMillin Observatory ; and to Professor Ames, Mr. Jewell of Johns Hopkins University, for photographs which appear in the plates. I am also indebted to the Astrophysical Journal and the Publications of the Yerkes Observatory for many cuts, and to Messrs. Ticknor & Co. for permission to reproduce Langley's drawing of a typical sun-spot. I am under special obligations to my colleague, Mr. Ellerman, to whom are due most of the photographs of instruments, buildings, and landscapes which appear in the plates, besides many of the solar and stellar photographs taken from our joint papers in the Astrophysical Journal and elsewhere.

G. E. H.

Pasadena, California November, 1907

Contents

CHAPTER PAGE

I. The Problem of Stellar Evolution - - - 1

II. The Student of the New Astronomy - - - 9

Chapter I The Problem Of Stellar Evolution

It is not too much to say that the attitude of scientific investigators toward research has undergone a radical change since the publication of the Origin of Species. This is true not only of biological research, but to some degree in the domain of the physical sciences. Investigators who were formerly content to study isolated phenomena, with little regard to their larger relationships,. have been led to take a wider view. As a consequence, the attractive qualities of scientific research have been greatly multiplied. Many a student, who could see in a museum only a wilderness of dry bones, now finds each fragment of profound interest if the part it plays in a general scheme of evolution can be made clear. The color and structure of any animal or plant, the minute modifications which distinguish one variety from another, take on new significance when considered as evidences of development. Their appeal to the microscopist, or to anyone who finds delight in intricacy of structure or beauty of form, is quite as great as before. But to the student whose interest is not aroused by such details, perhaps from lack of technical knowledge, or from the feeling that these matters are trivial as compared with the larger problems of science, such minor peculiarities must appear in a new light. Their true significance becomes apparent, and the importance of studying them, once perhaps underestimated, now requires no demonstration.

In astronomy the idea of evolution goes back to a very early period. In a crude and grotesque form the traditions of the earliest peoples invariably struggle to account for the origin of the Earth and its inhabitants. On a much higher plane stand the speculations of the Greek philosophers and of those who have followed them in the centuries preceding our own time. All schools of astronomers, dealing in some instances with purely philosophical and theoretical considerations, and in others basing their conclusions upon known facts of observation, have sought in their turn to explain the origin of the solar system and the larger relationships that obtain in the universe as a whole. In the eighteenth century these speculations reached their climax in the nebular hypothesis of Laplace, which still remains as the most serious attempt to exhibit the development of the solar system. Attacked on many grounds, and showing signs of weakness that seem to demand radical modification of Laplace's original ideas, it nevertheless presents a picture of the solar system ^which has served to connect in a general way a mass of individual phenomena, and to give significance to apparently isolated facts that offer little of interest without the illumination of this governing principle.

It will be seen, therefore, that the idea of evolution and development is by no means new to the astronomer. But it may nevertheless be maintained that it has occupied a more important position since Darwin published his great work. ^In 1859, the very year of the publication of the Origin of Species, Kirchhoff first succeeded in determining the chemical composition of the Sun by the aid of the spectroscope. His fundamental discovery marked the entrance of this instrument into the field of astronomical research and established on a firm basis the new science of astrophysics. The importance of spectroscopic investigations in their relationship to evolution was soon made clear. Within a single decade the study of stellar spectra by Huggins, Rutherfurd, and Secchi had shown that the stars may be divided into several classes, characterized by distinctive peculiarities in their luminous

The Problem of Stellar Evolution 3

emission and marking definite stages in an orderly process of development. Following close upon this pioneer work came the capital discovery by Huggins of the gaseous nature of the nebulae, and the relationship of these celestial clouds to the stars which they enshroud. In these filmy masses of luminous gas it appeared probable that the stars had their origin, taking form after long ages of condensation, through processes regarding which our ideas are still vague and ill defined. Belief in such a mode of development has been greatly strengthened through the results of recent investigations, and especially through the discovery by Keeler that of 120,000 nebulae strewn over the heavens fully one-half are distinctly spiral in form. This far-reaching conclusion, coming at the end of the nineteenth century, is furnishing materials for those who seek, through modification of the nebular hypothesis, to provide a sound and sufficient explanation of the development of suns like our own.

We are now in a position to regard the study of evolution as that of a single great problem, beginning with the origin of the stars in the nebulae and culminating in those difficult and complex sciences that endeavor to account, not merely for the phenomena of life, but for the laws which control a society composed of human beings. Any such consideration of all natural phenomena as elements in a single problem must begin with a study of the Sun, the only star lying near enough the Earth to permit of detailed investigation. The knowledge thus derived may then be applied in researches on the nebulae, and in the elucidation of spectroscopic observations of those stars which represent the early period of stellar existence. According to present views, the state of development attained by the Sun is that of maturity, if not of decline. After it come the red stars, which represent the last stages of luminous stellar life. Even the extinction of light due to continued cooling is not sufficient to exclude altogether from the astrophysicist's study those dying stars which represent a condition lying between that of a glowing sun and a dead planet like the Earth or the Moon. Through one of its many remarkable properties, the spectroscope enables us to detect the presence, and sometimes to determine the dimensions, of vast bodies which have resulted from the cooling of former suns. It will be the object of this book to show how the student of astrophysics attacks this problem of stellar evolution, through the development of special instruments and methods of research, and the accumulation and discussion of observations.

It must not be forgotten that such a study comprises only the earliest and simplest elements in the general problem of evolution. The province of the student of astrophysics may be said to end with an understanding of the production of a planet like the Earth. It remains for the geologist to explain the changes which the surface of the Earth has undergone since the constructive process left it a rocky crust. The conditions which brought about the formation of the oceans, the effects of the long-continued action of winds and waves, and the vast changes in surface structure that have resulted from internal disturbances and the operation of volcanic phenomena, afford limitless opportunity to the student of evolution in this other aspect. Closely related to these changes, and presenting difficulties far greater than those experienced by the astrophysicist, comes the problem of accounting for the origin and development of plant and animal life. The preservation of the earlier forms of life, principally through the agency of sedimentary deposits, affords the paleontologist the means of connecting the links in the evolutionary chain. Thus we are brought to our own era, where countless living objects continue to supply material for new inquiries. Both in the examination of existing species and their relationships, and in those experimental researches on variation which offer

The Peoblem of Stellar Evolution 5

such promising opportunities to the investigator, the evohitionist may secure data for further advances. Outside the immediate domain of the natural sciences, in regions of activity where still greater complexity prevails, the student may seek to trace out evidences of unity and development in the mental and moral relationships of the peoples of many countries and of many generations.

It is a noteworthy fact, of prime significance to all investigators who find special interest in attempting to enter new and unoccupied fields, that some of the most important developments of recent years have taken place in those regions which lie between the boundaries of the old established sciences. Thus the union of physics and chemistry has opened up the extensive field of physical chemistry, where advances of the greatest value are being made. In the same way the application of physical methods and the principles of physical chemistry to the experimental study of physiology has resulted so successfully as to give hope for even more remarkable developments in the near future. In astronomy, the introduction of physical methods has revolutionized the observatory, transforming it from a simple observing station into a laboratory, where the most diverse means are employed in the solution of cosmical problems. The fact that physics is common to these and other intermediate branches of science affords striking proof of its fundamental importance. An investigator who has been confined to the traditional methods of a department of science where physics has as yet played little part, may therefore find in physical methods a powerful means of advancing his subject.

The suggestive value, to investigators in other departments, of any species of scientific research which involves new methods and principles is perhaps greater at the present time than ever before. Even those methods of research which can find no direct application in other subjects are frequently capable of suggesting modifications or adaptations involving related principles. The development and use of new methods is quite as likely to advance a subject as the prosecution of extensive investigations by existing means. For this reason the investigator is ever on the alert to seize and utilize suggestions derived from any source.

The interest of the student of astrophysics is no longer confined simply to celestial phenomena. For astrophysics has become, in its most modern aspect, almost an experimental science, in which some of the fundamental problems of physics and chemistry may find their solution. The stars may be regarded as enormous crucibles, in some of which terrestrial elements are subjected to temperatures and pressures far transcending those obtainable by artificial means. In the Sun, which appears to us not merely as a point of light like the stars, but as a vast globe whose every detail can be studied in its relationship to the general problem of the solar constitution, the immense scale of the phenomena always open to observation, the rapidity of the changes, and the enormous masses of material involved, provide the means for researches which could never be undertaken in terrestrial laboratories. Hence it is that astrophysics may equally well be regarded as a branch of physics or as a branch of astronomy. A telescope may be defined as an instrument for revealing celestial phenomena, or it may be likened to the lens which the physicist uses in his laboratory to concentrate the light of an electric spark on the slit of his spectroscope. To the student of astrophysics whose interests are not confined to a single branch of science, the subject is likely to make a double appeal, no less strong on the physical and chemical than on the astronomical side.

In entering upon our consideration of the study of stellar development, we may think of the subject in either one of two ways. Some will prefer to regard it as the general prob-

The Problem of Stellar Evolution 7

lem of stellar evolution, in its broad application to the universe at large. But others will find it easier to conceive of the question as an investigation of the Sun, tracing it, through analogies afforded by stars in earlier stages of growth, from its origin in a nebula to those final chapters which, though not yet written for the Sun itself, may be read in the life-histories of the red stars. Viewed from whichever standpoint, the task of the investigator remains the same, since in either case it is concerned with stellar origin, development, and decay.

It must, of course, be remembered that the processes of stellar development ordinarily advance so slowly that a lifetime would be far too short to permit any permanent change to be observed in a star. Temporary stars flash into view and fade rapidly away; but these represent an abnormal condition, typical of some catastrophe rather than of a natural course of change. The spiral nebulae, though their appearance leaves little doubt of extremely rapid motion and constant change of form, are so far removed, and constructed on so vast a scale, that no actual differences in structure have been detected in photographs of the same object, taken at intervals of many years. In the processes of creation a thousand years is but a day, and we must be content to base our stellar histories upon analogy.

Fortunately, the data needed for the construction of these histories are easily found. Our problem is like that of one who enters a forest of oaks, and desires to learn through what stages the trees have passed in reaching their present condition. He cannot wait long enough to see any single tree go through its long cycle of change. But on the ground he may find acorns, some unbroken and some sprouting. Others have given rise to rapidly growing shoots, and saplings are at hand to show the next stage of growth. From saplings to trees is an easy step. Then may be found, in the form of dead limbs and branches, the first evidences of decay, reaching its full in fallen trunks, where the hard wood is wasting to powder.

Scattered over the heavens are millions of stars, each representing a certain degree of development. The cloud forms of the nebulae tell us of stellar origins; the white, yellow, and red stars illustrate the rise and decline of stellar life; and the Earth itself affords a picture of what may remain after light and heat have been extinguished.

Chapter II The Student Op The New Astronomy

The traditional conception of the astronomer, while still applicable (with sundry limitations) in certain modern instances, does not accurately apply to the student of stellar evolution. According to the old view, the astronomer, soon after the setting of the Sun, retires to a lofty tower, from whose summit he gazes at the heavens throughout the long watches of the night. His eye, fixed to the end of a telescope tube, perceives wonders untold, while his mind sweeps with his vision through the very confines of the universe. The lineal descendant of the seers and soothsayers of the Chaldeans, he dwells apart, finding little of interest in the ordinary concerns of the world, so occupied are his thoughts with celestial mysteries.

Now there can be no doubt that the study of stellar evolution brings a degree of pleasure and enthusiasm which it would be difficult to surpass. The joys of the pioneer, the excitement that comes to him who looks for the first time upon an unknown land, the intense satisfaction of discovery, all belong to the successful investigator. Moreover, mere gazing through a telescope, as distinguished from the painstaking work of modern astronomers with micrometer or photographic plate, is still competent to reveal new or changing phenomena, and important discoveries are yet to come to the alert and careful observer. It is pleasant to picture the surprise and delight of Galileo when he first perceived spots on the supposedly immaculate surface of the Sun. His little instrument, much less perfect than a modern spy-glass, could reveal none of that intricate structure and exquisite detail that are at once the joy and the despair of present-day snn-spot observers. But he had discovered a new and important fact, the basic principle of the science of astrophysics: he had shown that with suitable optical aid the physical structure of the heavenly bodies might be investigated. Prior to this time astronomy had concerned itself only with the positions and motions of the stars; now it became evident that each of these luminaries might present peculiar and distinguishing phenomena worthy of the most searching investigation. Discovery followed discovery in rapid sequence. The mottled face of the Moon, formerly without meaning, was suddenly revealed in unsuspected landscapes of valley, plain, and mountain, resembling, in curious degree, the variegated surface of the Earth. Jupiter, who had seemed to travel alone through the heavens, was found to possess four companions, whose revolutions about him forcibly suggested the revolutions of the planets about the Sun. The mysterious ansae, inclosing between them the globe of Saturn, were soon made out to be the more conspicuous elements of a vast incircling ring, unlike anything of earlier experience. With the growth of the telescope more marvels were brought to light, until it seemed, in sound reason, as though the universe would never cease to yield new knowledge to the explorer of its boundless wastes. ^ Thus was established that conception of the astronomer that still persists, long after a new astronomy has come into being. Gazing through a telescope, as has been said, is still competent to bring discoveries ; for change is the very essence of celestial phenomena, and persistent watching must detect important facts, on which broad generalizations may be founded. But the eye and the telescope have been supplemented by various instrumental aids which, in their multiplication, have transformed the occupations of the astronomer. The micrometer, in its application to the accurate measure-

The Student of the New Astronomy 11

ment of place and form, permits changes to be detected which are beyond the perception of the eye. The photometer, in its precise determinations of brightness, has shown that stars whose light never varies are rather the exception than the rule. The photographic plate, used in conjunction with the telescope, has proved itself to be more sensitive than the human retina, in that it is capable of adding up into a visible record the invisible radiations received during an exposure of many hours. Finally, to mention but one more of the telescope's new adjuncts, the spectroscope has introduced a new and revolutionary principle into astronomy, permitting the chemical and physical analysis of the most distant stars.

Hence it is that the present-day student of astrophysics does not correspond to the traditional idea of the astronomer. His work at the telescope is largely confined to such tasks as keeping a star at the precise intersection of two cross-hairs, or on the narrow^ slit of a spectrograph, in order that stars and nebulae, or their spectra, may be sharply recorded upon the photographic plate. His most interesting work is done, and most of his discoveries are made, when the plates have been developed, and are subjected to long study and measurement under the microscope. His problems of devising new methods of calculation or reduction are as fascinating as the invention of new instruments of observation. Much of his time may be spent in the laboratory, imitating, with the means placed at his disposal by the physicist and chemist, the various conditions of temperature and pressure encountered in the stars, and watching the behavior of metals and gases in these uncommon environments. If, in the conviction that new and promising means of research are always awaiting application, he would advance into still unoccupied fields, he must devote himself to the design and construction of new instruments, to supplement the old. Kept thus in touch with the newest phases of physical and chemical investigation, the countless applications of electricity, the methods of modern engineering, and the practical details of workshop practice, his interest in these things of the world is likely to be quite as broad as that of the average man. His sympathy w^ith research in every branch of science must increase and strengthen as his conception of the great problem of evolution is developed by his own investigations of its earliest phases. And the pleasure and enthusiasm derived from his studies must become, not like the vague passion of the mystic, whose inability to see clearly leads him to pursue strange gods, but such as every successful searcher after truth must experience, whether he deal with the vast dimensions and distances of the heavenly bodies, or with the minute but no less marvelous phenomena of microscopic life and form.

Now, w^hile it cannot be too strongly emphasized that the student of stellar evolution can have no sympathy with the mystic, whose habit of thought must be the very antithesis of his own, yet it is true that the imagination, when properly exercised and controlled, is to be regarded as his best aid to progress. The question of control is so important that it may well be mentioned first. For nothing has done more injury to science than the play of imaginations subject to no control, on the part of men who enjoy in the public press the rank of scientific authorities. Thus great sun-spots become the innocent cause of earthquakes or tornadoes, not to speak of their effect upon the price of wheat. Comets, once the unerring portents of war and pestilence, still carry the brands of conflagration, and threaten at each apparition to destroy the Earth. Mystic properties are ascribed to the center of the universe, and a well-known planet, because it is incorrectly assumed to be stationed there, is dogmatically asserted to be the only possible abode of human life. There is a fine field here for humor and amusing speculation, as the author of the "Moon Hoax," and other more recent writers, have

The Student of the New Astronomy 18

shown us. But humor is not always intended: the pronunciamentos go forth in the name of science, and are so accepted by a host of intelligent persons, who naturally believe that the supposed authorities have reached their conclusions by scientific methods. Thus there arises a false conception of science, and a popular demand for wonders, which is not easily satisfied by acquaintance with the less sensational facts.

But though dangerous when unrestrained, the imagination, when rightly exercised, is the best guide of the astronomer. His dreams run far ahead of his accomplishments, and his work of today is part of the development of a plan projected years ago. He perceives that only a few generations hence many of the instruments and methods of his time are to be replaced by better ones, and he strains his vision to obtain some glimpse, imperfect though it be, into the obscurities of the future. As he sits in his laboratory, surrounded by lenses and prisms, gratings and mirrors, and the other elementary apparatus of a science that subsists on light, he cannot fail to entertain the alluring thought that the intelligent recognition of some well-known principle of optics might sufiice to construct, from these very elements, new instruments of enormous power. He learns of some advance in engineering or in the art of the glass-maker, and dreams of new possibilities in its application to the construction of his telescopes or the equipment of his laboratory. He reads of discoveries in physics or chemistry, and at once his mind is busy in its endeavor to apply the new knowledge to the solution of long-standing cosmical problems.

But here, again, we see the need of control ; for with such a multiplicity of interests, and such constant stimulus to the imagination, the danger of mere dilettantism is obvious. With scores of problems suggesting themselves for solution, and with attractions on every hand, each rivaling the other in its apparent possibilities of development, the chief difficulty is to choose wisely. It is not a question of searching for something to do, but of picking out those things which are most worthy of pursuit. Here the importance of having a definite and logical plan of research becomes apparent. Such a plan may involve a single investigation, continued along systematic lines over a long period of years, or it may comprise several investigations, carried on simultaneously. In a large observatory each piece of work acquires increased importance if it is selected, not at random, or solely because of its intrinsic value, but rather because of the part it plays in a single logical scheme of research. Its intrinsic importance need not be in the least diminished by its relationship to other work, while the illumination which its results cast on the other investigations of the scheme can hardly fail to improve them, and may even reveal the chief source of their meaning. Moreover, the same research, if carried on elsewhere, might prove of small value, in the absence of such suggestions and modifications as are sure to come from the related investigations. We shall have occasion to revert to this question in discussing a plan of attack on the general problem of stellar evolution.

CHAPTEK III THE SUN AS A TYPICAL STAR

Befoee proceeding to the more detailed portions of our discussion, let us examine the present condition of the bodies with which we are to deal, and briefly trace out those elements of relationship which it will be our purpose later to describe more fully. Let us begin with the consideration of a single object, which we may afterward compare with other objects less easily observed because of their greater distance from the Earth.

The photographic reproduction in Plate II represents the Sun, as seen with an ordinary telescope. So far as could be judged from this picture, the Sun might be described as a luminous sphere, brighter in its central part than near its circumference, and marked with dark spots, irregularly distributed over the surface. On closer examination it will also be seen that there are certain bright regions, which are most easily noticed near the edge of the Sun. The dark spots are the well-known sun-spots, first discovered by Galileo, while the bright regions are the faculae, which have also been known since the invention of the telescope. At times of total eclipse, when the bright body of the Sun is covered by the dark body of the Moon, shielding our atmosphere from the usual brilliant illumination, red flames, sometimes reaching heights of several hundred thousand miles, may be seen rising from a continuous sea of flame, which completely incircles the Sun. These are the prominences, and the continuous mass of flame from which they rise is the chromosphere (Plate III).^ Extending far beyond these flames into space,

1 See the remarks on anomalous dispersion, p. 148.

sometimes to a distance of millions of miles, is the corona, which shines with a silvery luster somewhat inferior in brightness to that of the full Moon (Fig. 2, Plate IV).

An analysis of the light of the Sun, made with the spectroscope, has shown the presence of the vapors of iron, sodium, magnesium, calcium, hydrogen, and many other substances known to us on the Earth. In fact, it has been remarked that if the Earth were heated to the temperature of the Sun, the light emitted by its vapors would resemble closely, when analyzed with the spectroscope, the light emitted by the Sun. Thus the chemical composition of the Earth and the Sun is much the same, although we have evidence of the existence in the Sun of a large number of substances not yet found on the Earth. This same means of analysis has led to the discovery that the chromosphere, and the prominences which rise out of it, are composed of the vapor of calcium and of the light gases helium and hydrogen. The sun-spots, too, have also been found to have a characteristic chemical composition; while the corona emits rays which probably indicate the presence in it of very light and tenuous gases.