1S77.J uZO [Kirkwood. 



or 279 times that of the sun. The relative quantity of heat emitted by the 

 different bodies may be assumed to have the same ratio. 



But while the mass and distance of Sirius have been ascertained within 

 moderate limits of error, the degree of its condensation, as compared with 

 tliat of the sun, is still undetermined. On the hypothesis of equal density 

 the light emitted from the sun would be but ;fV th part of that radi- 

 ated from an equal portion of the star's surface. But, if equal areas of 

 the two bodies afford equal quantities of light, then the volume of Sirius is 

 4061 times that of the sun, and the mean density of the latter is 333 times 

 that of the former. It seems probable, therefore, that the principal com- 

 ponent has still chiefly, if not entirely, a gaseous constitution. 



As the light of Sirius, according to Sir John Herschel, is 324 times that 

 of an average star of the sixth magnitude, and as the satellite discovered by 

 Clarke is of the ninth or tenth magnitude, the light of the latter must be 

 much less than one thousandth part of that received from the principal star. 

 But according to Auwers the mass of the less component is equal to half 

 that of the greater. Is it possible to explain these remarkable facts on the 

 theory that the two bodies had a simultaneous origin in the same nebula, 

 or has their present proximity resulted from the proper motions of two orig- 

 inally independent stars V 



The conclusions apparently sustained by the facts here considered may be 

 summarized as follows : 



(1.) The history of the solar system is comprised within twenty or thirty 

 millions of years. 



(2.) From the fact of the larger component of Alpha Centauri radiates 

 twice as much light as the sun while the mass of the former is Jess than 

 that of the latter, we infer the probability that our solar system is the more 

 advanced in its physical history. 



(3.) 61 Cygni seems to have reached a greater degree of condensation 

 than the sun, since, on the hypothesis of equal density, the surface of the 

 larger member is one third that of the sun, while the intrinsic light is less 

 than one ninth. 



(4.) The companion of Sirius appears to have reached a stage of greater 

 maturity than the sun, while the contrary seems to be true in regard to the 

 principal star. 



Bloomington, Indiana, March 26, 1877. 



