NEBULiE 127 



into stars (if so converted) is, of course, quite un- 

 known ; but if such conversion take place, it 

 probably takes place in some such way as is sur- 

 mised by the various nebular hypotheses. 



When we come to the post-natal character of 

 stars we are on surer but still rather speculative 

 ground. 



The spectroscope shows that stars have varying 

 degrees of heat, and that, according to the heat, 

 the chemical constitution of the star varies. 

 The hottest stars (which, upon one nebular 

 hypothesis, may be supposed to be stars new-born 

 from the nebula) are known as " gaseous " stars ; 

 these give a very long spectrum, and consist 

 almost entirely of hydrogen and helium and some 

 unknown gases. A group of stars less hot, known 

 as metallic stars, give a spectrum of medium length, 

 which shows that they contain, besides gases, cer- 

 tain metals, e.g, calcium or magnesium, in a semi- 

 dissociated form. And a third group of stars less 

 hot still have a short spectrum, which shows fewer 

 gases, and metals now in the condition in which 

 they exist at the comparatively low temperature 

 of the electric arc. (It should be explained, 

 perhaps, that many elements, when exposed to great 

 heat, give spectra varying with the degree of heat. 

 Thus iron has a flame spectrum, an arc spectrum, 

 and a spark spectrum, all distinct.) 



