Part 53 (1/2)
[Footnote 1362: _Phil. Trans._, vol. ci., p. 306.]
[Footnote 1363: _Conn. des Temps_, 1816, p. 213.]
[Footnote 1364: _OEuvres_, t. vi., p. 581.]
[Footnote 1365: _Mem. dell' Ist.i.t. Lombardo_, t. xii., p. 164; _Rendiconti_, t. vii., p. 77, 1874.]
[Footnote 1366: W. Forster, _Pop. Mitth._, 1879, p. 7; Fabry, _etude sur la Probabilite des Cometes Hyperboliques_, Ma.r.s.eille, 1893, p. 158.]
[Footnote 1367: _Mem. R. A. Soc._, vol. xxix., p. 335.]
[Footnote 1368: _Month. Not._, vol. xxiii., p. 203.]
CHAPTER XII
_STARS AND NEBULae_
That a science of stellar chemistry should not only have become possible, but should already have made material advances, is a.s.suredly one of the most amazing features in the swift progress of knowledge our age has witnessed. Custom can never blunt the wonder with which we must regard the achievement of compelling rays emanating from a source devoid of sensible magnitude through immeasurable distance, to reveal, by its distinctive qualities, the composition of that source. The discovery of revolving double stars a.s.sured us that the great governing force of the planetary movements, and of our own material existence, sways equally the courses of the farthest suns in s.p.a.ce; the application of prismatic a.n.a.lysis certified to the presence in the stars of the familiar materials, no less of the earth we tread, than of the human bodies built up out of its dust and circ.u.mambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained the generic partic.i.p.ation of stellar light in the peculiarity by which sunlight, spread out by transmission through a prism, shows numerous transverse rulings of interrupting darkness. No sooner had Kirchhoff supplied the key to the hidden meaning of those ciphered characters than it was eagerly turned to the interpretation of the dim scrolls unfolded in the spectra of the stars. Donati made at Florence in 1860 the first efforts in this direction; but with little result, owing to the imperfections of the instrumental means at his command. His comparative failure, however, was a prelude to others' success. Almost simultaneously, in 1862, the novel line of investigation was entered upon by Huggins near London, by Father Secchi at Rome, and by Lewis M. Rutherfurd in New York.
Fraunhofer's device of using a cylindrical lens for the purpose of giving a second dimension to stellar spectra was adopted by all, and was, indeed, indispensable. For a luminous point, such as a star appears, becomes, when viewed through a prism, a variegated line, which, until broadened into a band by the intervention of a cylindrical lens, is all but useless for purposes of research. This process of _rolling out_ involves, it is true, much loss of light--a scanty and precious commodity, as coming from the stars; but the loss is an inevitable one.
And so fully is it compensated by the great light-grasping power of modern telescopes that important information can now be gained from the spectroscopic examination of stars far below the range of the unarmed eye.
The effective founders of stellar spectroscopy, then (since Rutherfurd shortly turned his efforts elsewhither), were Father Secchi, the eminent Jesuit astronomer of the Collegio Romano, where he died, February 26, 1878, and Sir William Huggins, with whom the late Professor W. A. Miller was a.s.sociated. The work of each was happily directed so as to supplement that of the other. With less perfect appliances, the Roman astronomer sought to render his extensive rather than precise; at Tulse Hill searching accuracy over a narrow range was aimed at and attained.
To Father Secchi is due the merit of having executed the first spectroscopic survey of the heavens. Above 4,000 stars were pa.s.sed in review by him, and cla.s.sified according to the varying qualities of their light. His provisional establishment (1863-67) of four types of stellar spectra[1369] has proved a genuine aid to knowledge through the facilities afforded by it for the arrangement and comparison of rapidly acc.u.mulating facts. Moreover, it is scarcely doubtful that these spectral distinctions correspond to differences in physical condition of a marked kind.
The first order comprises more than half the visible and probably an overwhelming proportion of the faintest stars. Sirius, Vega, Regulus, Altair, are amongst its leading members. Their spectra are distinguished by the breadth and intensity of the four dark bars due to the absorption of hydrogen, and by the extreme faintness of the metallic lines, of which, nevertheless, hundreds are disclosed by careful examination. The light of these ”Sirian” orbs is white or bluish; and it is found to be rich in ultra-violet rays.
Capella and Arcturus belong to the second, or solar type of stars, which is about one-sixth less numerously represented than the first. Their spectra are quite closely similar to that of sunlight, in being ruled throughout by innumerable fine dark lines; and they share its yellowish tinge.
The third cla.s.s includes most red and variable stars (commonly synonymous), of which Betelgeux in the shoulder of Orion, and ”Mira” in the Whale, are noted examples. Their characteristic spectrum is of the ”fluted” description. It shows like a strongly illuminated range of seven or eight variously tinted columns seen in perspective, the light falling from the red end towards the violet. This _kind_ of absorption is produced by the vapours of metalloids or of compound substances.
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 Cla.s.s 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 a.n.a.lyzed 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 a.n.a.logy 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 a.s.sociation has undoubtedly a predominant influence upon colour.
Nevertheless, the crude notion thrown out by Zollner 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 ngstrom, in 1868,[1375] subst.i.tuted 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 cla.s.sification 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 a.n.a.logy, 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 ma.s.siveness.[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 const.i.tution, 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 s.p.a.ce are composed either of swarms of meteorites or of ma.s.ses 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 nebulae 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_ ent.i.tled Group vii. The ground-work of this cla.s.sification was, however, insecure, and has given way. Certain spectroscopic coincidences, avowedly only approximate, suggesting that stars and nebulae 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 a.s.sociation 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 ma.s.s 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 ill.u.s.trated by a set of original spectrographic plates, while precision was added to the adopted cla.s.sification 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 nebulae;[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.