370 CLASSICS OF MODERN SCIENCE 

 resemble these fleeting ones — new stars, as they are called — are those 

 discerned in the once mysterious nebulae which, so far from being 

 stars, were supposed not many years ago to represent a special order 

 of created things. 



Now the nebulae differ from stars generally in the fact that in their 

 spectra we have practically to deal with radiation alone ; we study them 

 by their bright lines ; the conditions which produce the absorption by 

 which we study the chemistry of the hottest stars are absent. 



A NEW VIEW OF STARS 



Here, then, we are driven to the perfectly new idea that some of the 

 cooler bodies in the heavens, the temperature of which is increasing 

 and which appear to us as stars, are really disturbed nebulae. 



What, then, is the chemistry of the nebulae? It is mainly gaseous; 

 the lines of helium and hydrogen and the flutings of carbon, already 

 studied by their absorption in the groups of stars to which I have al- 

 ready referred, are present as bright ones. 



The presence of the lines of the metals iron, calcium, and probably 

 magnesium, shows us that we are not dealing with gases merely. 



Of the enhanced metallic lines there are none ; only the low temper- 

 ature lines are present, so far as we yet know. The temperature, 

 then, is low, and lowest of all in those nebulae where carbon flutings 

 are seen almost alone. 



A NEW VIEW OF NEBULA 



Passing over the old views, among them one that the nebulae were 

 holes in something dark which enabled us to see something bright 

 beyond, and another that they were composed of a fiery fluid, I may 

 say that not long ago, they were supposed to be masses of gases only, 

 existing at a very high temperature. 



Now, since gases may glow at a low temperature as well as at a 

 high one, the temperature evidence must depend upon the presence of 

 cool metallic lines and the absence of the enhanced ones. 



The nebulae, then, are relatively cool collections of some of the 

 permanent gases and of some cool metallic vapors, and both gases 

 and metals are precisely those I have referred to as writing their 

 records most visibly in stellar atmosphere. 



