[1367] Mem. R. A. Soc., vol. xxix., p. 335.
[1368] Month. Not., vol. xxiii., p. 203.
CHAPTER XII
STARS AND NEBULÆ
That a science of stellar chemistry should not only have become possible, but should already have made material advances, is assuredly 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 assured 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 space; the application of prismatic analysis 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 circumambient vapours.
We have seen that, as early as 1823, Fraunhofer ascertained the generic participation 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 associated. 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 passed in review by him, and classified 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 accumulating 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 class 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.