Sidereal science has a great future before it. The prospects of its advance are incalculable; the possibilities of its development virtually infinite. No other branch of knowledge attracts efforts for its promotion, at once so wide-spread, so varied, and so enthusiastic; and in no other is anticipation so continually outrun by the brilliant significance of the results achieved.

For the due appreciation, however, of these results, some preliminary knowledge is required, and is possessed by few. To bring it within the reach of many is the object aimed at in the publication of the present volume. Astronomy is essentially a popular science. The general public has an indefeasible right of access to its lofty halls, which it is the more important to keep cleared of unnecessary technical impediments, since the natural tendency of all sciences is to become specialised as they advance. But literary treatment is the foe of specialisation, and helps to secure, accordingly, the topics it is applied to, against being secluded from the interest and understanding of ordinarily educated men and women. Now, in the whole astonishing history of the human intellect, there is no more astonishing chapter than that concerned with the sidereal researches of the last quarter of a century. Nor can the resources of thought be more effectually widened, or its principles be more surely ennobled through the vision of a Higher Wisdom, than by rendering it, so far as possible, intelligible to all.

The question whether nebulæ are external galaxies hardly any longer needs discussion. It has been answered by the progress of discovery. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way. A practical certainty has been attained that the entire contents, stellar and nebular, of the sphere belong to one mighty aggregation, and stand in ordered mutual relations within the limits of one all-embracing scheme—all-embracing, that is to say, so far as our capacities of knowledge extend. With the infinite possibilities beyond, science has no concern.

The chief reasons justifying the assertion that the status of the nebulæ is intra-galactic, are of three kinds. They depend, first, upon the nature of the bodies themselves; secondly, upon the stellar associations of many of them; thirdly, upon their systematic arrangement as compared with the systematic arrangement of the stars.

The detection of gaseous nebulæ not only directly demonstrated the non-stellar nature of a large number of these objects, but afforded a rational presumption that the others, however composed, were on a commensurate scale of size, and situated at commensurable distances. It may indeed turn out that gaseous and non-gaseous nebulæ form an unbroken series, rather than two distinct classes separated by an impassable barrier.

The study of the stars inevitably leads us to consider the advancing movement in the midst of them of the sun and its attendant train of planets. There can be no reasonable doubt—and the thought is an astounding one—that we are engaged on a voyage through space, without starting-point or goal that we can know of, but which may prove not wholly uneventful. Its progress may possibly bring about, as millenniums go by, changes powerfully influential upon human destinies; nay, an incident in its course may, at any time, by the inscrutable decree of Providence, terminate the terrestrial existence of our race, and consign the records of its civilisation, in dust and cinders, to the arid bosom of a dead planet. A curious sense of helplessness, tempered, however, by a higher trust, is produced as we thus vividly realise, perhaps for the first time, how completely we are at the mercy of unknown forces—how irresistibly our little ‘lodge in the vast wilderness’ of the universe is swept onward over an annual stretch of perhaps five hundred millions of miles, under the mysterious sway of bodies reduced by their almost infinite distances to evanescent dimensions.

But, as things are constituted, the translation of the sun's household is a necessity, albeit one of startling import to ourselves. The stellar system is maintained by the balance of forces, and motion is the correlative of force.

About two hundred and fifty stars have been formally registered as variable, and many more are open to the like suspicion. Gore's ‘Revised Catalogue’ includes 243 entries, besides 39 provisional additions; Chandler's nearly contemporaneous list enumerates 225 objects. Of these 160 are reckoned as ‘periodical,’ the rest as ‘irregular’ or ‘temporary.’ Periodical stars are further divided into those with ‘long,’ and those with ‘short’ periods. Nor is the distinction by any means arbitrary. The stars seem to separate of themselves into two principal groups, undergoing fluctuations in cycles of respectively less than fifty, and between two and four hundred days. The paucity of stars with periods of intermediate lengths is shown graphically in fig. 11, where the height of the curve represents the numbers of stars subject to changes proportionate in duration to the horizontal distance from left to right.

Variations requiring several months for their completion differ both in degree and kind from those run through in a few days. They are of much greater amplitude, ranging over five to eight instead of, at the most, two magnitudes; they are accomplished with less punctuality; and they are frequently attended by symptoms of atmospheric ignition entirely foreign to quicker vicissitudes. Most important of all, they affect bodies of peculiar constitution. Nearly all long-period variables are red stars with banded spectra; those of short period are white or yellowish in colour, and display Sirian or solar spectra.

The stars, speaking broadly, are suns. But what is a sun? We can only reply by taking function into consideration. A sun is a great radiating machine, and the obvious criterion for admission to the order is fitness for this office. Qualification to be a centre of light and heat is the dominant characteristic of each of its true members. Now the solar emissive activity is concentrated in a shining shell of clouds known as the ‘photosphere,’ which the entire energies of the organism (so to speak) seem directed to maintain and renew. And with reason, since its efficiency as a radiator depends upon the perpetuation of the condensing process by which this brilliant surface is produced.

The possession of a photosphere must then be regarded as an essential feature of the suns of space. But such a structure can only be formed in an incandescent atmosphere, the action of which modifies, more or less powerfully, the light emitted from it. The spectroscope can then alone decide whether a given sidereal object be, in the proper sense, a sun. For it is not so much the quantity as the quality of its radiations that determines the point. They must be such as can be supposed to emanate from condensed and vividly glowing matter bathed in cooler, though still ignited, vapours. That is to say, they must be primarily unbroken from end to end of the rainbow-tinted riband formed by prismatic dispersion, while showing the secondary effects of absorptive encroachments.

Are the stars subject to growth and decay? We might almost as well ask, Are they subject to the laws of nature? There can be in either case no doubt about the reply. We are perfectly assured, both from reason and revelation, that a time was when they were not, and that at some future date they will have ceased to be. And we may further confidently affirm, guided by the analogy of all other creative processes with which we are acquainted, that their present condition has been gradually attained and will gradually become modified.

Each has then a life-history. It is what it is, because it has been what it was. Nor is it conceivable that all should have arrived simultaneously at the same stage of development. A contemporaneous universal origin can by no means be assumed as a postulate; and even if it could, the rate of progress of individual stars must have been indefinitely varied. There is hence a strong probability that the present state of some represents the past of others, the future of many more. Among the hosts of heaven we may expect to find stars in embryo, stars half formed yet chaotic, full-grown stars in orderly and equable working order, stars still effective as radiators though of declining powers, and stars on the verge of decrepitude. Their comparative study ought then, under certain conditions, to enable us to compile, as it were, the typical biography of an average star.

The fifth spectral class is at present the most restricted of all, but its numbers are being rapidly augmented both by photographic and by improved visual means. The objects belonging to it are distinguished by the display in their spectra of isolated bright lines on a more or less perfectly continuous background, sometimes, however, also interrupted by dark lines or bands. They present us then with a triple combination—a direct gaseous spectrum, a reversed gaseous spectrum, and a spectrum due to glowing solid or liquid matter, all simultaneously made manifest by the unrolling, as it were, of a single scroll, yet each originating under very different conditions. The investigation of what those conditions are constitutes one of the most important tasks of physical sidereal astronomy.

The state of bright emission is in some stars normal, in others it only supervenes as part of a great general increase of light. This is the case with many ‘temporary’ and periodical stars, their blazing atmospheric constituents being almost invariably hydrogen and helium. Objects, on the other hand, showing bright lines with approximate constancy, can be discriminated into two varieties, according as they give or withhold evidence of the presence in them of hydrogen.

The first specimen of a ‘gaseous star’ was made known by Father Secchi's discovery, August 19, 1866, of the green line (F) of hydrogen conspicuously bright in γ Cassiopeiæ, the middle star of five of the second and third magnitudes grouped into the shape of a W on the opposite side of the pole from the Great Bear.

The fantastic variety of nebular forms was long a subject of wonder, scarcely tempered by a speculative effort. Inchoate worlds, disclosed with astonishing profusion by Herschel's telescopes, seemed like mere ‘sports of nature’ in the sidereal spaces. Nebulæ were to be found in the semblance of rings, fans, brushes, spindles; they abounded in planetary, cometary, elliptical, branching varieties; nebulous shields, embossed with stars, or tasselled like the ægis of Athene, displayed themselves, as well as nebulous dises, rays, filaments, triangles, parallelograms, twin and triple spheres. One nebula, thought to resemble the face of an owl, was named accordingly; another suggested a crab; a third a swan; a fourth (the great Orion formation) became known as the Fish Mouth nebula, from its supposed likeness to the gaping jaws of a marine monster. Fancy ranged at large through this wide realm, attempting to familiarise itself with the strange objects contained in it by finding for them terrestrial similitudes.

Within the last few years, however—indeed, it may be said, since the completion of the Rosse reflector in 1845—nebular inquiries have entered upon a new phase. A ‘glimmering of reason’ has begun to hover over what long appeared a scene of hopeless bewilderment. With improved telescopic means—above all, with the aid of photography—structure has become increasingly manifest among all classes of nebulæ; structure, not of a finished kind, but indicating with great probability the advance of formative processes on an enormous scale, both as regards space and time.

The light-changes of double stars are commonly of a fitful and indecisive kind. They may affect one or both members of stationary pairs; but visibly revolving stars, as a rule, conspire to vary, if they vary at all. The alternating fluctuations of γ Virginis, discoverable only by close attention to the swaying balance of lustre between the components, are in this respect typical. Each may be described as normally of the third magnitude, and each in turn declines by about half a magnitude and recovers within a few days, yet so that the general preponderance during a cycle of several years, remains to the same star. The existence of this double periodicity was recognised in 1851 by M. Otto Struve, who, however, despaired of investigating it with success in a latitude where the stars in question never rise more than 30° above the horizon.

Their circulation is in the most eccentric of ascertained stellar orbits (see fig. 30). The ellipse traversed by γ Virginis in 180 years is, in fact, proportionately somewhat narrower than the path round the sun of Encke's comet, so that the stars will in 1926 be separated by fully seventeen times the interval of space between them in 1836, when they merged into a single telescopic object. Their inequalities of light seem to have developed as they approached each other; at least, they first began to be noticed by Struve in 1818, and they at present tend to become obliterated, whether to revive with regained proximity towards the close of the twentieth century, future observations must decide.

About five hundred clusters are at present tolerably well known to astronomers, and a large number besides, their character rendered ambiguous by distance, are probably included among both ‘resolvable’ and ‘unresolved’ nebulæ. Such aggregations may be broadly divided into ‘irregular’ and ‘globular’ clusters. Although, as might have been expected, the line of demarcation between the two classes is by no means sharply drawn, each has its own marked peculiarities.

Irregular clusters are framed on no very obvious plan; they are not centrally condensed, they are of all shapes, and their leading stars rarely occupy critical positions. The stars in them are collected together, to a superficial glance, much after the fashion of a flock of birds. Alcyone, it is true, seems of primary dignity among the Pleiades, and the Pleiades may be regarded as typical of irregular clusters; yet the dominance, even here, of a central star may be more apparent than real.

The arrangement of stars in clusters is, nevertheless, far from being unmethodical, even though the method discernible in it be not of the sort that might have been anticipated. It seems, indeed, inconsistent with movements in closed curves, and suggests rather the description of hyperbolic orbits. Obviously, however, its true nature must be greatly obscured to our perception by the annulment, through perspective, of the third dimension of space, whereby independent groupings, flattened down side by side, are rendered scarcely, if at all, distinguishable.

The elliptical and irregular classes of nebulæ are illustrated by such splendid examples that we have thought it well to devote a chapter to their separate consideration. One member especially of each towers above the rest, like Ajax among the Argive host, its rival alone excepted, and the two are so different that it is not easy to award the palm of superiority to either. Needless to say that we allude to the objects in Andromeda and Orion, the types respectively of the elliptical and irregular plans of nebular construction.

The former (M 31) is the only real nebula which can readily be detected with the unaided eye, and it is the only one, accordingly, which was discovered in pre-telescopic times. Al Sûfi was familiar with the ‘little cloud’ near the most northern of the three stars in the girdle of Andromeda; and its place was marked on a star-map brought from Holland to Paris by De Thou, and believed to date from the tenth century. Simon Marius, who was the first to turn a telescope upon it, December 15, 1612, called it ‘stellam quandam admirandæ figuræ,’ and compared its dull and pallid rays to those of a candle shining by night through a semi-transparent piece of horn. Yet this strange phenomenon was only rescued from neglect by Boulliaud, whose attention was directed to it by the passage of the comet of 1664 across that part of the sky.

The shining face of each one of the innumerable suns aggregated into the vast system of the galaxy is, as we have said, veiled by absorbing vapours; but in widely varying degrees. The light of Vega, though indelibly stamped with the characteristic lines of hydrogen, reaches our upper air without sensible general modification. Sunlight is not only charged with significant inscriptions, but throughout toned down and mellowed; while in Aldebaran, the process of stoppage in the blue has been carried so far as to leave the red rays visibly predominant. In Aldebaran, too, the first symptoms begin to appear of a generic change in the manner of absorption through the emergence of dark bands in addition to the dark lines of the spectrum.

This change is full of meaning. Isolated rays of definite wave-lengths, forming in the spectrum what we call ‘lines,’ bright or dark, are emitted only at very high temperatures. They represent perhaps the fundamental vibrations of the ‘atoms’ of each different substance. But these, at lower grades of heat, are not free to thrill separately. Bound together into ‘molecules,’ they give rise, by their associated vibrations, to complex systems of light-waves, dispersed into sets of prismatic ‘flutings,’ each with a sharp and a nebulous side. The fluted spectra thus constituted are derived from chemical compounds such as oxides and chlorides, as well as from ‘elementary’ substances, both metallic and non-metallic, at moderate degrees of heat.

When the relative positions of the stars are compared at considerable intervals of time, they are in many cases found to have undergone small, but unmistakable changes of a seemingly capricious character. These are termed ‘proper motions,’ to distinguish them from merely nominal shiftings due to the slow variation of the points of reference which serve to define the places of all the heavenly bodies as seen projected on the inner surface of an imaginary concave sphere. Proper motions are by no means easy to get at. Only from the most delicate observations, and with stringent precautions for bringing those at distant dates under precisely similar conditions, can they be elicited with satisfactory accuracy. Otherwise, some trifling systematic discrepancies in the compared catalogues, or accidental errors of computation, might pass for genuine effects of movement, with disastrous influence upon sidereal investigations. Hence, proper motions cannot generally be regarded as established unless, in addition to the terminal observations showing a sufficiently marked change of place in the course of thirty, fifty, or one hundred years, at least one intermediate observation is at hand to prove that the suspected motion has proceeded uniformly in the same direction, and is accordingly not the creation of personal or instrumental inaccuracy.

Although not one of the millions of telescopic stars can, with any show of reason, be supposed at rest, less than five thousand of the stellar army are at present securely credited with measurable and progressive displacements.

Sir William Herschel conceived it to be the supreme object of astronomy ‘to obtain a knowledge of the construction of the heavens;’ and this, in his view, would be accomplished by the ‘determination of the real place of every celestial body in space.’ Thus limited, the problem would be completely solved could the absolute distance be ascertained of every object telescopically or photographically discernible in the sky. But even the attainment of this unattainable point would never have satisfied Herschel's restless spirit. The real scope of his inquiries went far beyond it. They had an historical, as well as a statistical aim. ‘Looking before and after,’ they embraced the past and future, no less than the present of the Cosmos.

Modern investigators are of the same mind. The heavens are regarded by them from a physiological, rather than from a purely anatomical point of view. Mere knowledge of structure, however accurate, will not content them. The vital functions of the organism, the mutual dependence of its parts, the balance of internal forces tending towards destruction and preservation, the dimly-apprehended aim of its divinely sustained activity, engage their eager attention. The heavens live and move, and the laws of their life and motion involve the material destiny of man. It is impossible that he should be indifferent to them.

Even, however, if our instinctive interest in the working of the machine were less keen, we should be driven to search out the dynamical relations of its parts by the impossibility of otherwise arriving at a true knowledge of their geometrical relations.

From multiple stars the transition is easy to star-clusters. These seem to embody completely the idea contained in germ in the former class of objects. They are collections, often on the grandest scale, of sunlike bodies small and large, united in origin and history, acted upon by identical forces, tending towards closely related ends. The manner and measure of their aggregation, however, vary widely, and with them the cogency of the evidence as to their organic oneness. There are innumerable cases in which it absolutely excludes doubt; there are some in which it is rather persuasive than convincing. It is not then always easy to distinguish between a casual ‘sprinkle’ of stars and a genuine cluster. Nor can the movement-test, by which so many physical have been discriminated from optical double-stars, be here applied. Internal displacements of a circulatory character have not yet become apparent in any cluster, and there is only one with an ascertained common proper motion.

This is the immemorial group of the Pleiades, famous in legend, and instructive, above all others, to exact inquirers—the meeting-place in the skies of mythology and science. The vivid and picturesque aspect of these stars riveted, from the earliest ages, the attention of mankind; a peculiar sacredness attached to them, and their concern with human destinies was believed to be especially close and direct.

Speculations as to an identity of nature between nebulæ and comets are no novelty; they presented themselves, as they could hardly fail to do, to the mind of Sir William Herschel; but some consistency was first given to them by the recent experimental researches of Mr. Lockyer. It is true that the results of light analysis are far from being decisive in their favour. The spectra of the two classes of bodies are fundamentally unlike. No gaseous nebula gives a trace of the carbon-bands which characterise nearly all comets; and no comet has yet furnished any direct evidence of the presence of hydrogen among its constituents. Moreover, nebulæ (apart from the stars contained in them) seem to emit no genuinely continuous light, while cometary nuclei glow in the ordinary manner of white-hot solid and liquid substances. Traces of a spectroscopic analogy can indeed be shown to exist; but they are met with only in the secondary elements of each spectrum. The resemblance seems only incidental; the dissimilarity essential.

This does not, however, detract from the closeness of a physical analogy, the deep import of which cannot be too forcibly dwelt upon. Both comets and nebulæ consist of enormous volumes of gaseous material, controlled by nuclear condensations, whether of the same or of a different nature in the two genera we need not now stop to inquire. Both, there is the strongest reason to believe, shine through the effects of electrical excitement.

The most arduous among the problems of stellar astronomy was, singularly enough, the first to be attacked. It was attacked, indeed, before the possibility was even remotely discerned that stellar astronomy might come to be regarded as a substantive branch of science. In the hope, not of penetrating the inscrutable secrets of the remote sphere of the fixed stars, but of solving doubts about the motion of the earth, Copernicus, Tycho, and Galileo led the way in the long series of experiments on the apparent displacements of the stars resulting from our own annual travels round the sun. The interest of the question whether such displacements existed or not was for them of a wholly ‘parochial’ kind; it lay in the test they afforded as to the reality of the terrestrial revolutions. Should the stars be found to shift ever so little by the effect of perspective, then the heliocentric theory could no longer be gainsaid; if, on the contrary, they ignored sublunary circlings, the ‘pill’ (as Kepler termed it) to be swallowed by Copernicans was indeed a huge one. For the distances to which the fixed stars had, in that case, to be relegated, seemed in those times monstrous and incredible; and monstrous and incredible they would appear still, were we not forced by irrecusable evidence to believe in them.

From the beginning to the end (so far) of the history of these inquiries, it may be taken almost as an axiom that the largest ostensible parallaxes have been obtained by the worst means.