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CHAP. VI. THE CHEMISTRY OF THE STARS. 



THE recent advances in our knowledge which have come from the 

 combination and interaction of solar, stellar and laboratory research,, 

 carried on by the aid of instruments of much greater power than those 

 formerly used, have given us a firm chemical hold on all the groups of 

 stars in my classification of them. These groups were established by 

 discussing sequences of lines before the origin of the lines had been 

 made out ; as I have already said, a series of hieroglyphics is now re- 

 placed by chemical facts ; and we can now study the chemistry of the 

 stars, as well as their order in a system of classification. 



The first question which naturally arises is this : Do the chemical 

 elements make themselves visible indiscriminately in all the celestial 

 bodies, so that practically, from a chemical point of view, the bodies 

 appear to us of similar chemical constitution 1 This is not so. 



From the spectra of those stars which resemble the sun, in that 

 they consist of an interior nucleus surrounded by an atmosphere which 

 absorbs the light of the nucleus, and which therefore we study by 

 means of this absorption ; it is to be gathered that the atmospheres of 

 some stars are chiefly gaseous, i.e., consisting of elements we recognise 

 as gases here, of others chiefly metallic, of others again mainly com- 

 posed of carbon or compounds of carbon. 



Here then we have spectroscopically revealed the fact that there is 

 considerable variation in the chemical constituents which visibly build 

 up the stellar atmospheres. 



This, though a general, is still an isolated statement. Can we con- 

 nect it with another ? 



By means of one of the first principles of spectrum analysis referred 

 to in Chapter I, we know that the hotter a thing is the light of which 

 produces a continuous spectrum, the further does the- spectrum stretch 

 into the violet and ultra-violet. 



Hence the hotter a star is, the further does its complete or con- 

 tinmus spectrum lengthen out towards the ultra-violet, and, cwteris 

 paribus, the less is it absorbed by cooler vapours in its atmosphere. 



Now to deal with three of the main groups of stars, we find the 

 following very general result : 



Gaseous stars .. .. Longest spectrum. 

 Metallic stars . . . . Medium spectrum. 

 Carbon stars . . . . Shortest spectrum. 



