STELLAR EVOLUTION JEANS. 157 



mingled with it spatially while yet preserving sufficient uniformity 

 of motion among their members to enable us to recognize them as 

 distinct groups of stars.] 



Let us now turn to a study of the lives of individual stars. To 

 the naked eye the stars appear as mere points of light of varying 

 brightness. The telescope adds little except possibly diiferences of 

 color. The spectroscope appears at first to add a wealth of new 

 information, but a detailed study of stellar spectra discloses the un- 

 expected fact that all stellar spectra, apart from a few exceptions, 

 fall into one single linear series. Photographs of the spectra of all 

 stars, in which varying exposures have been made to compensate for 

 varying brightnesses, can be arranged uniquely in a consecutive 

 order in which each spectrum differs only imperceptibly from its 

 neighbor. All the complicated diversities of stellar spectra appear 

 to be determined, in the main, by one single variable. This is be- 

 lieved, with good reason, to be the temperature of the star's surface. 



Positions on this linear series are specified by reference to six 

 selected points denoted by the letters B, A, F, G, K, M, in this order. 

 The order given is that of decreasing surface temperature. Stars 

 having B spectra are of bluish color with a surface temperature of 

 10,000° C. or more. Stars of type M are red with a surface tem- 

 perature of only 3,000° C. Our sun is of type G, with a surface tem- 

 perature of about 6,000° C. 



We might also arrange the stars in order of brightness. The dis- 

 tances of many stars are known, and for these we can calculate the 

 " absolute brightness " or " luminosity " — i. e., the amount of light 

 omitted as compared with our sun. Since the masses of the stars 

 are all approximately the same, it might be expected that the order 

 of " luminosity " would prove to be substantially the same as that 

 of surface temperature, but this does not prove to be the case. Eight 

 years ago it was found by Hertzsprung and H. N. Russell that the 

 red M stars fell into two widely different classes, one class having 

 abnormally high luminosity, and the other abnormally low. The 

 ratio of luminosities in the two classes is of the order of 10,000 to 1, 

 and since the surface temperatures are the same, this ratio must imply 

 a corresponding ratio in the areas of the radiating surfaces. Thus 

 the two classes of M stars must have volumes in a ratio of about 

 1,000,000 to 1, for which sufficient reason they have been designated 

 " giants " and " dwarfs." From a comprehensive discussion by 

 Russell, recently confirmed by Adams and Joy, it is clear that the 

 demarcation between " giants " and " dwarfs " extends, although 

 with diminished intensity, through the types K, G, and F, while at 

 types A and B the classes coalesce. 



Lately Shapley, by determining the distances of the globular 

 clusters, has greatly increased our knowledge of stellar luminosi- 



