42 THE FITNESS OF THE ENVIRONMENT 



consist of hydrogen or helium or a mixture 

 of the two gases. There is evidence that some 

 of these stars possess very high rotational 

 velocities. 



II. Yellow stars, including the sun, whose 

 spectra indicate the presence of hydrogen and 

 numerous metals, — sodium, iron, calcium, 

 magnesium, etc. The lines which show the 

 presence of hydrogen in the stars of this type 

 vary in intensity. The current belief is that 

 those stars which appear to possess more 

 hydrogen are the hottest. The stars of this 

 type are less hot than those of type I. 



III. Reddish stars whose spectra show little 

 or no sign of the presence of hydrogen, but 

 indicate that of chemical compounds, in- 

 cluding hydrocarbons. The presence in these 

 spectra of the lines of sodium, iron, calcium, 

 and magnesium is clearly established. Stars 

 of this type are evidently the coolest of lumi- 

 nous dense bodies. 



This classification is, of course, provisional 

 and unsatisfactory, and probably sometimes 

 results in bringing together relatively unlike 

 stars and in separating such as are very much 

 akin. Moreover, subdivisions in the classifica- 

 tion are necessary and hard to make. Other 

 better but more complex classifications appear 

 to exist, but they suffer only in less degree 



