Page:Popular Science Monthly Volume 14.djvu/619

Rh number of them all mixed together, and free, as it were, to find their own level. Nor is this all. Astronomers have not only determined that the sun is a star, and have approximately fixed his place in nature as regards size and brilliancy, but they have compared the spectrum of this star, this sun of ours, with those of the other bodies which people space, and have thus begun to lay the foundations of a science which we may christen Comparative Stellar Chemistry. Dealing with the knowledge already acquired along this line, we may say roughly that there are four kinds of stars recognizable by their spectra.

We have first the brightest and presumably hottest stars, and of these the spectrum is marvelously simple—so simple, in fact, that we say their atmospheres consist in the main of only two substances—a statement founded on the observation that the lines in the spectra are matched by lines which we see in the spectra of hydrogen and calcium; there are traces of magnesium, and perhaps of sodium too, but the faintness of the indication of these two latter substances only intensifies the unmistakable development of the phenomena by which the existence of the former is indicated.

So much, then, for the first class; now for the second. In this we find our sun. In the spectra of stars of this class, the indications of hydrogen are distinctly enfeebled, the evidences by which the existence of calcium has been traced in stars of the first class are increased in intensity, and, accompanying these changes, we find all simplicity vanished from the spectrum. The sodium and magnesium indications have increased, and a spectrum in which the lines obviously visible may be counted on the fingers is replaced by one of terrific complexity.

The complexity which we meet with in passing from the first class to the second is one brought about by the addition of the lines produced by bodies of chemical substances of moderate atomic weight. The additional complexity observed when we pass from the second stage to the third is brought about by the addition of lines due in the main to bodies of higher atomic weight. And—this is a point of the highest importance—at the third stage the hydrogen, which existed in such abundance in stars of the first class, has now entirely disappeared.

In the last class of stars to which I have referred, the fourth, the lines have given place to fluted bands, at the same time that the light and color of the star indicate that we have almost reached the stage of extinction. These facts have long been familiar to students of solar and stellar physics. Indeed, in a letter written to M. Dumas, December 3, 1873, and printed in the "Comptes Rendus," I thus summarized a memoir which has since appeared in the "Philosophical Transactions":

Il semble que plus une étoile est chaude, plus son spectre est simple, et que les éléments métalliques se font voir dans l'ordre de leurs poids atomiques.

Ainsi nous avons: