Chapter 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.