Secchi's classification of the stars was unwarped by any speculative fancy. It was purely formal; it aimed only at providing distinct compartments for the convenient arrangement of a multitude of differently characterized items of information. Then, by degrees, the closeness of the gradations between one class and the next came to be noticed; partitions melted away; the methodized array showed itself to be in movement; and the bare framework took shape, under the auspices of Zöllner and Vogel, as a cosmic pedigree. The white stars were set forth as the progenitors of yellow, yellow of red stars; and the insensibly progressive reinforcement of the traits of relationship between the successive types went far towards demonstrating some partial, if not a complete, correspondence of the indicated order with the truth of things. It has since been found necessary to divide the first stellar class into helium and Sirian stars; and here, too, essential diversity shades off imperceptibly into likeness approximating to identity. All the groups hang together; the entire scheme is on an inclined plane of change. Helium stars, as they condense, pass into Sirian, these into solar stars, which finally, reddening through the increase of absorption, exhibit the badge of post-meridional existence in fluted spectra. The finality of the red stage is, indeed, very far from being absolute, but what lies beyond is matter of conjecture.
There are several good reasons for taking helium stars to be the 'youngest' or most primitive of the amazing assemblage that sparkle in the vault of heaven. The first is their affinity with nebulæ. Every star, perceived to be involved in folds or effusions of shining haze, has yielded—if bright enough for profitable examination—a spectrum of helium quality. Further, they are remarkably tenuous bodies. It has been ascertained with approximate certainty, from the investigation of stellar eclipses, that helium stars are commonly, perhaps invariably, of far slighter consistence than the sun. Radiation, however, is maintained by contraction; hence, orbs at the outset of their course must be, on the whole, the most diffuse. A third note of youth is membership of embryo systems, and this is affixed very markedly to helium stars. One-third certainly, probably one-half of those lately submitted to trial by Professors Frost and Adams proved to have spectroscopic companions. They are pairs believed to have been recently divided by the fission of a single parent-globe. And this is an operation which must, we should suppose, be undergone early, or not at all.
The spectra of helium stars are peculiar and suggestive. Those belonging to Miss Maury's earliest groups—many of them visibly nebulous—bear next to no traces of metallic absorption, showing instead lines of oxygen, of nitrogen, and of hydrogen in all its three series. The conditions, accordingly, needed to produce the 'cosmic' modification of hydrogen are realized in these inchoate bodies. What those conditions actually are we cannot tell, yet it may be confidently surmised that they will prove to be of an electrical nature. Hydrogen resembles the metals in being electro-positive; it collects at the negative pole during the electrolytic decomposition of water. There is, however, an unmistakable tendency in primitive sidereal objects to display absorption rays of electro-negative rather than of electro-positive elements. It is conceivable that hydrogen may be capable of altering its behaviour in this respect, and that the molecules radiating the Pickering and Rydberg series, in addition to the more familiar Huggins series, have, in fact, through some corpuscular re-arrangement, assumed the electro-negative quality properly characterizing a non-metallic substance. The association of this form of hydrogen with oxygen and nitrogen in early helium stars would thus be naturally related to the simultaneous quasi-disappearance from them of the spectral badges of metals.
The helium-line most distinctive of this stellar family is situated well up in the blue. It appertains to the same vibrational sequence with D3, which is also represented in Rigel, one of the more 'advanced' Orion stars. In Rigel, too, we meet a fairly prominent magnesium ray, lying below the blue helium emanation, while as yet iron is unapparent. Numerous fine, faint streaks, due to its absorption, only emerge when the Sirian type is fully reached, and they are mostly of the 'enhanced' kind. When the spark discharge is substituted for the arc as the source of illumination, certain lines in the resulting spectrum brighten relatively to the others, and these have been distinguished by Sir Norman Lockyer as 'enhanced.' Now, the rule is strikingly prevalent that the absorption rays in white stars are of this class; yet it can no longer be interpreted as indicating for them an excessively high temperature. Rather, it would seem that electrical conditions still imperfectly defined are in question, and their gradual removal or subsidence is, beyond doubt, largely instrumental in bringing about the transition to the solar stage. The effacement of helium-absorption is even more perplexing. No sooner does iron begin to show than it vanishes. There is still a faint trace of its 'blue' line in Vega; none survives in Sirius.
In spectra of the solar type two great bars of violet light are stopped out by calcium; otherwise metallic arc-lines predominate, while those of hydrogen are no longer so powerfully emphasized as in white stars. Moreover, the whiteness of the unveiled Sirian photospheres has become tinged with yellow owing to the development of a shallow envelope partly impermeable to blue rays. For this reason the comparative extension of their ultra-violet spectra affords, for stars of different types, no secure criterion of relative temperature. Sound in principle, it becomes inapplicable when the unknown factor of general absorption comes into play. The energy-curve of the solar spectrum, as it is, can be determined; the energy-curve of the solar spectrum, as it would be if unaffected by general absorption, has to be constructed from inference. But only photospheres bare to space give comparable results. Hence, there are no valid grounds for asserting that Sirius is hotter than the sun, or the sun than Betelgeux. It may be so, but the evidence at present available is inconclusive. The appearances expounded in this sense may bear quite different meanings.
The reasons for holding that solar mature into Antarian stars are of the same character, and of equal cogency with those tending to prove their own development from luminaries of Sirian type. There is a similar continuity of specimens. They can be ranged one after another in an unbroken series, in which, as we run down the line, primrose shades into orange, and orange into red; general absorption arrests an increasing percentage of the blue radiations, while specific absorption becomes strengthened by dusky channellings of titanium. Carbon stars are less easily located. Dr. Vogel regards them as co-ordinate with the Antarian class. The two varieties of red stars with banded spectra descend, in his opinion, from the common stock exemplified by our sun. Professor Hale also favours this view, some attendant difficulties notwithstanding. His photographs have certainly established for carbon stars links of relationship both with the Antarian and the solar families; yet the fact remains indisputable that the carbon type is, to a great extent, isolated from all the rest. Tokens of a genuine migration towards it are few and obscure.
The ultimate fate of both tribes of red stars can only be conjectured. Most of the objects constituting them vary in brightness, some to the verge of periodical extinction; and variability may be a symptom of interior dilapidation. But the organization of such bodies is profoundly enigmatical. They are exceptionally remote, and offer slight holding-ground for inquiry. No indications have been gathered as to their density or intrinsic light-power. Very little is known about their movements. They rarely form binary combinations, and those that they do form are almost always relatively fixed. No red star travels in a computed orbit; only one, η Geminorum, occurs on the long list of spectroscopic binaries. The revolutions of this curious system ought to prove, when thoroughly investigated, replete with interest and instruction.
Coupled stars present special opportunities to students of cosmogony. They are obviously contemporaries; they have started fair in the evolutionary race; identical influences have acted upon them; hence, differences in their standing can only result from dissimilarities in mass or composition. It is commonly taken for granted that a body containing less matter than its fellow must develop faster, and incur the final quenching sooner. But Sir William and Lady Huggins have adverted to the probability of the very opposite being the case. Powerful surface-gravity may, they consider, serve to hasten the transition from a Sirian to a solar spectrum; and we should then have giant suns like Capella, advanced in type while at a very early stage of condensation. This perhaps explains the remarkable spectral relations of contrasted stellar pairs. Always, so far as we yet know, the Sirian spectrum is yielded by the lesser star, the mass of which, judging by analogy, must be even smaller than would be indicated by the proportion of its faintness. It is true that the distribution of mass in binary systems is often widely different from what might have been anticipated. Certain purplish satellites, for instance, of undetermined spectral quality exercise a gravitative sway of surprising force. Some results of this kind lately obtained by Mr. Lewis and others are likely to prove of fundamental importance to theories of stellar evolution.
What we know of 'dark stars' has been mainly derived from the observation of stellar systems. They are assumed to be the denizens of a stellar Hades, dim wanderers amid the shades, who 'have had their day, and ceased to be' as suns. In the 'cold obstruction' of these viewless orbs the grand cosmical procession is held to terminate. Their presence attests the downward progress of decay, and gives logical completeness to the argument for development. Yet there are circumstances warning us against too full an assurance that their status is really that of skeletons at the feast of light. They are very frequently found to be in close attendance upon brilliant white stars. Thus intimately, if incongruously, coupled, they circulate and compel circulation in brief periods, as members of systems just, it might be said, out of the shell. What are we to think, for instance, of the obscure body spectroscopically discovered to control the revolutions of the chief star in the Orion trapezium? It is evidently comparable in mass with that imperfectly condensed luminary. Is it credible that it has already traversed all the stages of stellar existence, and cooled down to planetary rank? So violent an assumption should at least not be made without due consideration; and we may more prudently hold our judgment in suspense as to whether globes so circumstanced—and they abound—should be regarded as effete, or as abortive suns.[100]
Speculations on the exhaustion of stellar vitality have lately become inextricably involved with the complex problem of elemental evolution. A dim inkling has been acquired of the activity in the universe of obscure forces, availing, we can just see, to falsify many forecasts. The theory, among others, of the dissipation of energy needs to be revised or qualified. Nor was it propounded by Lord Kelvin with dogmatic certainty. He carefully noted the possibility that in 'the great storehouses of creation' reserves of energy might be provided by which the losses incurred through radiation could be, wholly or in part, made good.[101] The anticipated possibility is perhaps realized in the phenomena of radio-activity. But if we inquire how, we are met at the threshold by difficulties connected with the origin of helium. Helium appears to result from the disintegration of radium, its generation being accompanied by the setting free of enormous quantities of energy. Its copious presence, then, argues long-continued and lavish expenditure of heat and light. Yet it is as a constituent of highly primitive orbs that it is chiefly conspicuous. Gaseous nebulæ, too, include immeasurable supplies of it, while it is incompatible with whatever we seem to know about them to suppose that radium at any time entered into their composition.