STELLAR EVOLUTION. 307 



While some uncertainty must therefore prevail until further in- 

 vestigations have been completed regarding the exact stage at which the 

 liighest stellar temperatures are attained, there can be little doubt as 

 tc the path which is followed when through the long continued action 

 of gravitation a young star like Vega develops into a star like the Sun. 

 We are fortunate in possessing examples of a great number of inter- 

 mediate stages in this orderly progress ( Fig. 8 ) . As condensation con- 

 tinues, 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 hydrogen gradually gives place to a much shallower 

 atmosphere, in which hydrogen 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 cal- 



a 



i_ 





!'• I i 



Fig. 8. 

 Characteristic spectra of (a) white, (6) yellow, and (c) red stars (Huggins). 



cium, rise into prominence as the hydrogen lines fade. Meanwhile the 

 light of the star undergoes a change of color, completely losing its 

 Ijluish cast and assuming a distinctly yellow hue. There can be little if 

 any doubt that our own sun once passed through the successive stages 

 which are represented by the spectra shown in Fig. 8. The time which 

 has elapsed since it acquired its present size and density as the result of 

 the condensation of the great nebula in which the earth and the other 

 planets also had their origin, covers many millions of years. It is fortu- 

 nate 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 perfectly 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- 



