Page:Popular Science Monthly Volume 39.djvu/677

Rh particular star, has arrived. According to some views, the sun is an older star than Sirius; according to others, it is younger. The question whether a sun is still approaching the climax of its radiative energy, or has passed that point and is descending the scale, is an important one as regards the ultimate destiny of inhabited worlds revolving around it. It is upon the revelations of the spectroscope, assisted by photography, that we must depend for the advance of our knowledge of the condition of the various orders of suns. Upon spectroscopic evidence the stars are ranged into four principal classes distinguished by the kind and extent of the absorption which their light undergoes in passing outward from their photospheres through their gaseous envelopes or atmospheres. Sirius stands as the most brilliant representative of the first class, sometimes called the white stars, in whose spectra the lines of hydrogen are very conspicuous while other lines are few and faint. Some of the stars of this class have a splendid blue tint in their light. More than half of all the stars whose spectra have been studied belong to the first class. Vogel thinks they are the youngest stars, and that their youthful fires represent the most intense development of solar heat. If they are the youngest, then, on account of their great number as compared with the other stellar classes, it is evident that the universe has not yet passed the high noon of life; in other words, that the starry system, taken as a whole, is still in its prime; if indeed it has as yet attained the summit of its development as a community of suns. But the correctness of Vogel's assumption that the stars of the first class are all younger than those of the other classes is seriously questioned, and, as Prof. Young says, "it is very far from certain that a red star is not just as likely to be younger than a white one as to be older."

The second class is represented by Capella and our own sun. These stars are evidently surrounded by a far more complicated envelope than is the case with the white stars. Metallic vapors suspended above their photospheres in great variety serve to absorb a large part of their radiation, so that the spectrum of their light as it comes to us presents an enormous number of black lines, showing that it has been sifted through the vapors of iron, calcium, platinum, and many other metals, each of which, existing at a lower temperature than the fiercely glowing surface beneath, has arrested from among the passing rays those peculiar to itself. Some of the hydrogen lines also exist in the spectra of stars of the second class, but they are no longer conspicuous above the others. The second-class stars often called the solar and sometimes the yellow stars, are far less numerous than the white stars, the proportion, so far as known, being about one to six.

In the third class we come to the red stars, the majority of the stars of that color, as well as most of the variable stars, falling