1868.] 



Constitution of the Sun #nd Stars. 



53 



effect of both forces will be at each revolution to shorten the ellipses in 

 which the stars move, and at the same time to augment or reduce the peri- 

 helion distance, according as the effect of the normal or tangential com- 

 ponent of the resistance preponderates. If the normal force carry the 

 day, the stars will at successive passages gradually work themselves clear 

 of one another, a result which may be very much promoted by the beha- 

 viour of the atmospheres. 



90. If what I here venture to offer as a surmise with respect to the prox- 

 imate cause of stellar heat and the origin of double stars, is what really 

 took place, we must conclude the sky to be peopled with countless hosts of 

 dark bodies so numerous, that those which have met with such collisions 

 as to render them now visibly incandescent, must be in comparison few 

 indeed. In the majority of those cases in which adequate collisions have 

 taken place, the two stars must have emerged from the catastrophe, moulded 

 into one, dilated by the conflagration to an enormous size*, and rotating. 

 Occasionally, however, the circumstances of the collision must have favoured 

 the disentanglement of the two stars from one another by a predominating 

 influence in these cases of the normal force acting in the way that has been 

 traced in the last paragraph. Wherever this happens, there is a prospect 

 that a double star may form. The heat into which much of the previous 

 vis viva of the two components has been converted will dilate both to an 

 immense size, and thus enable the two stars gradually in successive peri- 

 helion passages to climb, as it were, to the great distance asunder, which 

 we find in the few cases in which the final perihelion distance can be rudely 

 estimated, a length comparable with the intervals between the more re- 

 mote planets and the sun. While this is going on, the ellipticity of the 

 orbits is at each revolution decreasing ; but if the stars succeed in getting 

 nearly clear of one another's atmospheres before the whole ellipticity is 

 exhausted, the atmospheres will begin to shrink in the intervals between 

 two perihelion passages more than they expand when the atmospheres get 

 engaged, and will thus complete the separation of the two stars. When 

 once this has taken place, a double star is permanently established. 



91. It is a striking confirmation of this view to find that the astonishing 

 phenomena witnessed last year tin T Coronas were precisely what we should 

 expect to arise towards the end of the process which has been described. 

 The stars having been intensely heated by previous perihelion passages, and 

 having begun to shrink, would, at ordinary times, present a spectrum sub- 

 dued by an abundance of very dark lines ; but immediately after one of the 



* If any dependence is to be placed upon the records of Sinus's having formerly been a 

 ruddy star, it would appear to argue that Sirius when lie last met with a collision was heated 

 only in the outskirts of his enormous mass : that these parts were so dilated as to render 

 him a ruddy star, but that the store of heat laid up was so small that even within the little 

 term of human history he has so cooled down as to have during it shrunk into an intensely 

 white star. 



t This was written in the spring of 1867. 



