364 CLASSICS OF MODERN SCIENCE 



In watching the appearance of these special enhanced Hnes in 

 stellar spectra we have a third series of phenomena available, and 

 we find that the results are absolutely in harmony with what has gone 

 before. Thus : 



Gaseous stars. .Highest temperature. . Strong helium and faint en- 

 hanced lines. 



Metallic stars. . Feeble helium and strong en- 



Medium temperature. . hanced lines. 



Carbon stars.. No helium and strong arc lines 



Lowest temperature. . Faint arc lines. 



It is clear now, not only that the spectral changes in stars are as- 

 sociated with, or produced by, changes of temperature, but that the 

 study of the enhanced spark and the arc lines lands us in the possi- 

 bihty of a rigorous stellar thermometry, such lines being more easy 

 to observe than the relative lengths of spectrum. 



Accepting this, we can take a long stride forward and, by carefully 

 studying the chemical revelations of the spectrum, classify the stars 

 along a line of temperature. But which line? Were all the stars in 

 popular phraseology created hot? If so, we should simply deal with 

 the running down of temperature, and because all the hottest stars 

 are chemically alike, all cooler stars would be alike. But there are two 

 very distinct groups of coolest stars ; and since there are two different 

 kinds of coolest stars, and only one kind of hottest stars, it cannot 

 be merely a question either of a running up or a running down of 

 temperature. 



Many years of very detailed inquiry have convinced me that all 

 stars save the hottest must be sorted out into two series — those getting 

 hotter and those, like our sun, getting cooler, and that the hottest stage 

 in the history of a star is reached near the middle of its life. 



The method of inquiry adopted has been to compare large-scale 

 photographs of the spectra of the different stars taken by my assist- 

 ants at South Kensington ; the complete harmony of the results ob- 

 tained along various lines of other work carries conviction with it. 



We find ourselves here in the presence of minute details exhibiting 

 the workings of a chemical law, associated distinctly with temper- 

 ature; and more than this, we are also in the presence of high tem- 

 perature furnaces, entirely shielded by their vastness from the 

 presence of those distracting phenomena which we are never free 



