To the fourth order of stars belongs also a colonnaded spectrum, but reversed; the light is thrown the other way. The three broad zones of absorption which interrupt it are sharp towards the red, insensibly gradated towards the violet end. The individuals composing Class IV. are few and apparently insignificant, the brightest of them not exceeding the fifth magnitude. They are commonly distinguished by a deep red tint, and gleam like rubies in the field of the telescope. Father Secchi, who in 1867 detected the peculiarity of their analyzed light, ascribed it to the presence of carbon in some form in their atmospheres; and this was confirmed by the researches of H. C. Vogel,[1370] director of the Astro-physical Observatory at Potsdam. The hydro-carbon bands, in fact, seen bright in comets, are dark in these singular objects—the only ones in the heavens (save one bright-line star and a rare meteor)[1371] which display a cometary analogy of the fundamental sort revealed by the spectroscope.
The members of all four orders are, however, emphatically suns. They possess, it would appear, photospheres radiating all kinds of light, and differ from each other mainly in the varying qualities of their absorptive atmospheres. The principle that the colours of stars depend, not on the intrinsic nature of their light, but on the kinds of vapours surrounding them, and stopping out certain portions of that light, was laid down by Huggins in 1864.[1372] The phenomena of double stars seem to indicate a connection between the state of the investing atmospheres, by the action of which their often brilliantly contrasted tints are produced, and their mutual physical relations. A tabular statement put forward by Professor Holden in June, 1880,[1373] made it, at any rate, clear that inequality of magnitude between the components of binary systems accompanies unlikeness in colour, and that stars more equally matched in one respect are pretty sure to be so in the other. Besides, blue and green stars of a decided tinge are never solitary; they invariably form part of systems. So that association has undoubtedly a predominant influence upon colour.
Nevertheless, the crude notion thrown out by Zöllner in 1865,[1374] that yellow and red stars are simply white stars in various stages of cooling, obtained for a time undeserved currency. D'Arrest, indeed, protested against it, and Ångström, in 1868,[1375] substituted atmospheric quality for mere colour[1376] as a criterion of age and temperature. His lead was followed by Lockyer in 1873,[1377] and by Vogel in 1874.[1378] The scheme of classification due to the Potsdam astro-physicist differed from Father Secchi's only in presenting his third and fourth types as subdivisions of the same order, and in inserting three subordinate categories; but their variety was "rationalised" by the addition of the seductive idea of progressive development. Thus, the white Sirian stars were represented as the youngest because the hottest of the sidereal family; those of the solar pattern as having already wasted much of their store by radiation, and being well advanced in middle life; while the red stars with banded spectra figured as effete suns, hastening rapidly down the road to final extinction.
Vogel's scheme is, however, incomplete. It traces the downward curve of decay, but gives no account of the slow ascent to maturity. The present splendour of Vega, for instance, was prepared, according to all creative analogy, by almost endless processes of gradual change. What was its antecedent condition? The question has been variously answered. Dr. Johnstone Stoney advocated, in 1867, the comparative youth of red stars;[1379] A. Ritter, of Aix-la-Chapelle, divided them, in 1883,[1380] into two squadrons, posted, the one on the ascending, the other on the descending branch of the temperature-curve, and corresponding, presumably, with Secchi's third and fourth orders of stars with banded spectra. Whether, in the interim, they should display spectra of the Sirian or of the solar type was made to depend on their greater or less massiveness.[1381] But the relation actually existing perhaps inverts that contemplated by Ritter. Certainly, the evidence collected by Mr. Maunder in 1891 strongly supports the opinion that the average solar star is a weightier body than the average Sirian star.[1382]
On November 17, 1887, Sir Norman Lockyer communicated to the Royal Society the first of a series of papers embodying his "Meteoritic Hypothesis" of cosmical constitution, stated and supported more at large in a separate work bearing that name, published in 1890. The fundamental proposition wrought out in it was that "all self-luminous bodies in the celestial space are composed either of swarms of meteorites or of masses of meteoric vapour produced by heat."[1383] On the basis of this supposed community of origin, sidereal objects were distributed in seven groups along a temperature-curve ascending from nebulæ and gaseous, or bright-line stars, through red stars of the third type, and a younger division of solar stars, to the high Sirian level; then descending through the more strictly solar stars to red stars of the fourth type ("carbon-stars"), below which lay only the caput mortuum entitled Group vii. The ground-work of this classification was, however, insecure, and has given way. Certain spectroscopic coincidences, avowedly only approximate, suggesting that stars and nebulæ of every species might be formed out of variously aggregated meteorites, failed of verification by exact inquiry. And spectroscopic coincidences admit of no compromise. Those that are merely approximate are, as a rule, unmeaning.
In his Presidential Address at the Cardiff Meeting of the British Association in 1891, Dr. Huggins adhered in the main to the line of advance traced by Vogel. The inconspicuousness of metallic lines in the spectra of the white stars he attributed, not to the paucity, but to the high temperature of the vapours producing them, and the consequent deficiency of contrast between their absorption-rays and the continuous light of the photospheric background. "Such a state of things would more probably," in his opinion, "be found in conditions anterior to the solar stage," while "a considerable cooling of the sun would probably give rise to banded spectra due to compounds." He adverted also to the influential effects upon stellar types of varying surface gravity, which being a function of both mass and bulk necessarily gains strength with wasting heat and consequent shrinkage. The same leading ideas were more fully worked out in "An Atlas of Representative Stellar Spectra," published by Sir William and Lady Huggins in 1899. They were, moreover, splendidly illustrated by a set of original spectrographic plates, while precision was added to the adopted classification by the separation of helium from hydrogen stars. The spectrum of the exotic substance terrestrially captured in 1895 is conspicuous by absorption, as Vogel, Lockyer, and Deslandres promptly recognised in a considerable number of white stars, among them the Pleiades and most of the brilliants in Orion. Mr. McClean, whose valuable spectrographic survey of the heavens was completed at the Cape in 1897, found reason to conclude that they are in the first stage of development from gaseous nebulæ;[1384] and in this the Tulse Hill investigators unhesitatingly concur.
The strongest evidence for the primitive state of white stars is found in their nebular relations. The components of groups, still involved and entangled with "silver braids" of cosmic mist, show, perhaps invariably, spectra of the helium type, occasionally crossed by bright rays. Possibly all such stars have passed through a bright-line stage; but further evidence on the point is needed. Relative density furnishes another important test of comparative age, and Sirian stars are, on the whole, undoubtedly more bulky proportionately to their mass than solar stars. The rule, however, seems to admit of exceptions; hence the change from one kind of spectrum to the other is not inevitably connected with the attainment of a particular degree of condensation. There is reason to believe that it is anticipated in the more massive globes, despite their comparatively slow cooling, as a consequence of the greater power of gravity over their investing vaporous envelopes. This conclusion is enforced by the relations of double-star spectra. The fact that, in unequal pairs, the chief star most frequently shows a solar, its companion a Sirian, spectrum can scarcely be otherwise explained than by admitting that, while the sequence of types is pursued in an invariable order, it is pursued much more rapidly in larger than in small orbs. It need not, indeed, be supposed that all stars are identical in constitution, and present identical life-histories.[1385] Individualities in the one, and divergencies in the other, must be allowed for. Yet the main track is plainly continuous, and leads by insensible gradations from nebulæ through helium stars to the Sirian, and onward to the solar type, whence, by an inevitable transition, fluted, or "Antarian,"[1386] spectra develop.
The first-known examples of the class of gaseous stars—β Lyræ and γ Cassiopeiæ—were noticed by Father Secchi at the outset of his spectroscopic inquiries. Both show bright lines of hydrogen and helium, so that the peculiarity of their condition probably consists in the intense ignition of their chromospheric surroundings. Their entire radiating surfaces might be described as faculous. That is to say, brilliant formations, such as have been photographed by Professor Hale on the sun's disc,[1387] cover, perhaps, the whole, instead of being limited to a small portion of the photospheric area. But this state of things is more or less inconstant. Some at least of the bright rays indicative of it are subject to temporary extinctions. Already in 1871-72, Dr. Vogel[1388] suspected the prevalence of such vicissitudes; and their reality was ascertained by M. Eugen von Gothard. After the completion of his new astrophysical observatory at Herény in the autumn of 1881, he repeatedly observed the spectra of both stars without perceiving a trace of bright lines; and was thus taken quite by surprise when he caught a twinkling of the crimson C in γ Cassiopeiæ, August 13, 1883.[1389] A few days later, the whole range including D_3 was lustrous. Duly apprised of the recurrence of a phenomenon he had himself vainly looked for during some years, M. von Konkoly took the opportunity of the great Vienna refractor being placed at his disposal to examine with it the relighted spectrum on August 27.[1390] In its wealth of light C was dazzling; D_3 and the green and blue hydrogen rays shone somewhat less vividly; D and the group b showed faintly dark; while three broad absorption-bands, sharply terminated towards the red, diffuse towards the violet, shaded the spectrum near its opposite extremities.