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.

Sidereal science is, on its geometrical side, of modern development; on its physical side, of modern origin. The places of the stars, as referred to certain lines and points on the surface of an imaginary hollow sphere, are obtained now on essentially the same principles as by Hipparchus, only with incomparably greater refinement. And refinement is everything where the stars are concerned. Significant changes among them can only be brought out by minute accuracy. To a rough discernment their relative situations are immutable; and systematic inquiries into their movements hence became possible only when the grosser errors were banished from observation. Bessel's discovery of Bradley's exactitude gave the signal for such inquiries. It became worth while to re-observe stars already so well determined that discrepancies might safely be interpreted to mean real change.

Thus it is only within the last sixty or seventy years that the stars have been extensively catalogued for their own sakes, and no longer in the undivided interests of planetary or cometary astronomy. The scope of such labours now widens continually. For the objects of them are all but innumerable, and the nineteenth century has brought to bear on its large schemes of scientific ambition heretofore undreamt-of facilities for executing them by combination.

The strong presumption that the law of gravitation would prove truly universal has been fully borne out by investigations of stellar orbits. Binary stars circulate, it can be unhesitatingly asserted, under the influence of the identical force by which the sun sways the movements of the planets, the earth the movements of the moon. It is true that this does not admit of mathematical demonstration, but the overwhelming improbability of any other supposition amounts practically to the same thing. The revolutions of the stars are hence calculable, because conducted on familiar principles; their velocities have the same relation to mass, their perturbations may lead to similar inferences as in the solar system.

Observations, however, must precede calculations; and they are rendered arduous in double stars by the extreme minuteness of the intervals to be measured. Many revolving pairs never separate to the apparent extent of a single second of arc; yet this fraction of a second may represent, in abridgment, a span of some thousands of millions of miles. Infinitesimal errors, magnified in this proportion, become of enormous importance, and often impenetrably disguise the real aspect of the facts.

For determining the relative situations of adjacent stars, two kinds of measurement are evidently needed. The first gives their distance apart, the second the direction of the line joining them as regards some fixed line of reference.

We have seen, in the last chapter, that stars varying their light in periods of less than fifty days stand apart in several important respects from those undergoing slower changes. The distinction is accentuated by the tendency apparent in each class to group its members as far as possible from the frontier-line of separation from the other. Thus, long periods for the most part exceed three hundred days, while a large majority of short periods fall below ten. Thirty-eight stars in all are reckoned as variable within fifty days; of these thirty-two complete an oscillation in less than twenty, twenty-seven (including two with imperfectly ascertained periods) in less than ten days. A comparison of figures 17 and 18 shows that, among short periods taken en masse, those of three to four days predominate; those of five to eight days when Algol variables are excluded.

Variables of short period are, as we have said, nearly all white or yellow stars. A very few are reddish; and one—W Virginis—is suspected to possess a banded spectrum. R Lyræ, a star of about 4·5 magnitude, with a superb spectrum of the third type, is nominally variable in forty-six days, but its changes are so trifling and so imperfectly rhythmical as to suggest that its proper place is with β Pegasi and α Herculis among stars affected by abortive periodicity.

The further resolvability of a great many double stars is perhaps the most curious result of modern improvements in the optical means of observing them. With every addition to the defining power of telescopes, the visible complexity of stellar systems has increased so rapidly as to inspire a suspicion that simple binary combinations may be an exception rather than the rule. The frequency with which what appeared to be such have yielded to the disintegrating scrutiny of Mr. Burnham and others, suggests at any rate the presence of an innate tendency, and seems to show that the duplicity of stars is no accident of nebular condensation, but belongs essentially to the primitive design of their organisation. Although we can never become fully acquainted with all the detailed arrangements of stellar systems, we are then led to suppose them far more elaborate and varied than appears at first sight. Each, we cannot doubt, is adapted by exquisite contrivances to its special end, reflecting, in its untold harmonies of adjustment, the Supreme Wisdom from which they emanate.

The continuance of the process of optical dissociation, begun by the splitting-up of an apparently simple star, sometimes shows the primary, sometimes the satellite, not unfrequently both primary and satellite, to be very closely double. Ternary systems are accordingly of two kinds. In one, the smaller star consists of two in mutual circulation, and concurrent revolution round a single governing body; in the other, an intimately conjoined pair guides the movements of an unattended attendant.

The Milky Way shows to the naked eye as a vast, zoneshaped nebula; but is resolved, with very slight optical assistance, into innumerable small stars. Its stellar constitution, already conjectured by Democritus, was, in fact, one of Galileo's earliest telescopic discoveries. The general course of the formation, however, can only be traced through the perception of the cloudy effect impaired by the application even of an opera-glass. Rendered the more arduous by this very circumstance, its detailed study demands exceptional eyesight, improved by assiduous practice in catching fine gradations of light. Our situation, too, in the galactic plane is the most disadvantageous possible for purposes of survey. Groups behind groups, systems upon systems, streams, sheets, lines, knots of stars, indefinitely far apart in space, may all be projected without distinction upon the same sky-ground. Unawares, our visual ray sounds endless depths, and brings back only simultaneous information about the successive objects met with. We are thus presented with a flat picture totally devoid of perspective-indications. Only by a long series of inductions (if at all) can we hope to arrange the features of the landscape according to their proper relations.

To the uncritical imagination, the Milky Way represents a sort of glorified track through the skies—