1S77.1 "^^ [Kirk-wood. 



now generally accepted, the sun's heat is produced bj' the falling together 

 or condensation of the matter of which its mass is composed. But the I'ate 

 of solar radiation and also the mechanical equivalent of heat are known. 

 With these as data it may be easily calculated that a contraction of the 

 sun's radius amounting to one mile in 40 years would be sufficient to keep 

 up tlie present supply of heat. At former epochs, when the volume "waa 

 greater and the density less, a more rapid contraction was necessary to 

 keep up the supply. If the sun, or rather the solar system original!}' 

 existed as a nebulous mass, with a radius equal to half the distance of the 

 nearest fixed stars, the total amount of heat generated by contracting to its 

 present dimensions would have kept up a supply, equal to that now dis- 

 pensed, for about 20 millions of years. This period, it will be observed, 

 includes the entire physical history of the solar system, from Neptune down 

 to Mercury. It must be liable however, to considerable uncertainty, as it 

 assumes the radiation of Iieat to have been uniform. 



Before attempting a comparison between the ages of the sun and certain 

 of the fixed stars it should be premised that from the epoch of incipient 

 solidification, or rather of incipient transition from the gaseous to the liquid 

 form, the quantity of motion in the contracting mass, and consequently the 

 amount of radiant heat, must gradually diminish. Many of the nebulae 

 and some of the fixed stars have not reached this epoch ; while the sun, 61 

 Cygni, and the companion of Sirius, as will be shown hereafter, have 

 probably passed it. Our first comparison will be that of 



The Sun and Alpha Centauri. 



The larger component of the double star Alpha Centauri is of the first 

 magnitude ; the smaller one, of the second. The color of both has been 

 described as dark orange. According to Hind,* the system completes a 

 revolution about its centre of gravity in 8o years ; the mean distance between 

 the components being 23.49 times the radius of the earth's orbit — somewhat 

 greater than the distance of Uranus from the sun. 



Of all the fixed stars whose distances have been measured. Alpha Centauri 

 is the nearest to us. Its annual parallax is j%^^% ths of a second, which 

 corresponds to a distance 7336 times that of Neptune from the sun : 

 — so completely isolated in space is our planetary system. 



The apparent magnitude of Alpha Centauri is greater than that of any 

 star in the Northern Hemisphere ; and of those South of the equator but 

 visible in our latitude, Sirius alone surpasses it in splendor. Its mass is 

 nearly the same with that of the sun ;f while the intrinsic liglit and heat of 

 the system are nearly three times greater. It may be inferred from these 

 facts that the sun has probably passed the epoch of greatest heat, and that 

 it is farther advanced in its physical history, or, in other words, is an older 

 star, than Alpha Centauri. 



* Monthly Notices of the R. A. =;. for January, 1877. These elements differ 

 materially from those previously found. 



t Hind's recent value is greater than the sun's mass; all former estimates less. 



