THE STORY OF NINETEENTH-CENTURY SCIENCE 



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 Kel- 

 vin, such a culmination appears likely to occur within a 

 period of five or six million years. 



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 

 miniature are furnished by the earth and the smaller of 

 its companion planets. But there are larger bodies of 

 the same type out in stellar space veritable "dark 

 stars " invisible, of course, yet nowadays clearly recog- 

 nized. 



The opening up of this " astronomy of the invisible " 

 is another of the great achievements of our century, and 

 again it is Bessel to whom the honor of discovery is due. 

 "While testing his stars for parallax, that astute observer 

 was led to infer, from certain unexplained aberrations of 

 motion, that various stars, Sirius himself among the 

 number, are accompanied by invisible companions, and 



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