PROGRESS OF MODERN ASTRONOMY 



metallic vapors; and sundry red stars, with banded 

 spectra indicative of carbon compounds; besides the 

 purely gaseous stars of more recent discovery, which 

 Professor Pickering had specially studied. Zollner's 

 famous interpretation of these diversities, as indicative 

 of varying stages of cooling, has been called in question 

 as to the exact sequence it postulates, but the general 

 proposition that stars exist under widely varying condi- 

 tions of temperature is hardly in dispute. 



The assumption that different star types mark vary- 

 ing stages of cooling has the further support of modern 

 physics, which has been unable to demonstrate any way 

 in which the sun's radiated energy may be restored, or 

 otherwise made perpetual, since meteoric impact has 

 been shown to be under existing conditions, at any 

 rate inadequate. In accordance with the theory of 

 Helmholtz, the chief supply of solar energy is held to 

 be contraction of the solar mass itself ; and plainly this 

 must have its limits. Therefore, unless some means as 

 yet unrecognized is restoring the lost energy to the 

 stellar bodies, each of them must gradually lose its lus- 

 tre, and come to a condition of solidification, seeming 

 sterility, and frigid darkness. In the case of our own 

 particular star, according to the estimate of Lord 

 in, such a culmination appears likely to occur 

 within a period of five or six million years. 



The Astronomy of the Invisible 



But by far the strongest support of such a forecast as 

 this is furnished by those stellar bodies which even now 

 appear to have cooled to the final stage of star develop- 

 ment and ceased to shine. Of this class examples in 



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