IGU STELLAR EVOLUTION. 



to the path which iis followed when through the long continued action 

 of gTa\dtation a young star like Vega develops into a star like the sun. 

 We are fortunate in possessing examples of a great nuni])cr of inter- 

 mediate stages in this orderly progress (PI. VIII, fig. 1). As condensa- 

 tion continues, and as the vapors which constitute the star continue to 

 crowd upon each other, the stellar nucleus becomes denser and denser 

 and the vast atmosphere of h^^drogen gradually gives place to a nuich 

 shallower atmosphere, in which h3'drogen is still conspicuous, though 

 it no longer predominates in a very striking manner over the other 

 elements. The spectral lines of such elements as iron, magnesium, 

 sodium, and calcium, rise into prominence as the hj^drogen lines fade. 

 Meanwhile the light of the star undergoes a change of color, com- 

 pletely losing its bluish cast and assuming a distinct!}" j^ellow hue. 

 There can be little if any doubt that the development of our own ,sun 

 must cause it to pass through some such stages as are represented l)y 

 the spectra shown in PI. VIII, tig. 1. The time which has elapsed 

 since it acquired its present size and density as the result of the con- 

 densation of the great nebula in which the earth and the other planets 

 also had their origin, covers mau}^ millions of j^ears. It is fortunate 

 for the study of stellar evolution that the stages through which the 

 sun once passed are all exemplified in existing stars, which for unknown 

 reasons began their stellar life at widely different times. 



It will be profitable to consider for a moment some of the remark- 

 able phenomena which are presented to us by the sun, not only because 

 of their intrinsic interest, but also because it is perfecth^ safe to assume 

 that similar phenomena, sometimes on a much greater scale, would be 

 presented by other stars, were they not at so great a distance from the 

 earth as to reduce them to mere points of light, even in the most power- 

 ful telescope. The sun has a diameter of 860,000 miles and, as its dis- 

 tance from the earth is onl}" 93,000,000 miles, an extremely small frac- 

 tion of the distance of the other stars, it is possible to observe and to 

 study in detail its extraordinary phenomena, which are incomparably 

 more violent than anything observed on the earth. When we sjjcak of 

 the sun w^e speak collectively of a great number of phenomena, some 

 of which extend for millions of miles from the sun's visible disk. Chief 

 of these is the corona, a vast filmy atmosphere so rare that it offers 

 little or no resistance to the passage of a comet, as it sweeps around the 

 sun under the action of gravitation and returns into the space from 

 which it came. The polar streamers of the corona (PI. VIII, fig. 2) sug- 

 gests the action of magnetic forces and offer material for long continued 

 study of this, the most mysterious of all the solar appendages. At the 

 base of the corona, rising out of a sea of Hanie Avhich coniph^el}' 

 encircles the sun, are tiie prominences, some of which occasionally 

 attain a height of nearly 400, 0()(» miles. Like the corona, the promi- 

 nences are hidden ))y the brilliant iiluniiuation of our own atmospherCj 



