THE ENVIRONMENT 41 



aggregation. These include luminous dense 

 bodies like the sun and stars ; non-luminous 

 dense bodies like the earth, the moon, the 

 planets, and invisible partners of certain stars ; 

 nebulae, comets, and meteorites. The larger 

 of these bodies are separated by vast extents 

 of space which contain only rare meteorites, 

 perhaps minute traces of gaseous material, 

 and cosmic dust. There can be little doubt 

 that other types of bodies do not commonly 

 occur in that portion of the universe which is 

 open to astronomical investigation. Both the 

 enormous collections of astronomical data 

 which are now at hand and the beginnings 

 of clear knowledge of cosmic processes justify 

 this belief. Of what may lie beyond the 

 visible stars we can, of course, know nothing. 



The nature of the stars is revealed to us 

 chiefly by study of their spectra, according 

 to which they have been roughly classified, 

 by Vogel * for example, quite simply into 

 three principal types. 



I. White stars in which there is marked 

 evidence of the presence of hydrogen, or, in 

 some instances, helium. The stars of this 

 class undoubtedly are extremely hot, the 

 helium stars probably especially so. Their 

 atmospheres seem to be very dense and to 



1 See Arrhenius's "Lehrbuch," pp. 23-27. 



