1867.] 



Constitution of the Sun and Stan 



33 



pand when the brush takes place. When this happens the final separation 

 of the two stars is imminent, and a new double star is on the point of being 

 permanently added to the sky. 



The astonishing appearances witnessed last year in T Coronse seem to 

 receive an easy explanation upon this hypothesis. They are exactly what 

 we should expect upon the occurrence of one of the last perihelion passages 

 that take place before two stars which are in the state of transition into a 

 double star finally separate. The outer parts of the atmospheres becoming 

 engaged would raise to incandescence the region in which hydrogen only 

 is found, thus transforming what had previously been its four dark lines 

 into intensely bright lines. At the same time the strata that lie further 

 down would be very sensibly heated, though not to incandescence — quite 

 enough, however, to lessen temporarily in a very material degree the extent 

 to which they at other times subdue the light of the photospheres. This 

 extent would of necessity have been very great, inasmuch as the enormous 

 dilatation of the atmospheres must greatly enfeeble the force of gravity upon 

 the outer strata of both stars. 



Again, it follows as a consequence of this hypothesis that the circum- 

 stances which most favour the formation of a double star are when the two 

 bodies that come into collision are of nearly equal mass. Such cases must 

 be rare ; but when they do occur, there is a very high probability that the 

 issue will be a double star. This appears to accouut for the fact that a 

 very remarkable proportion of double stars have constituents of nearly the 

 same magnitude. 



Another consequence is that when the stars are very unequal, the com- 

 panion will, as it plunges over and over again through the atmosphere of 

 the primary, be gradually deprived of several of its lighter gases ; so that 

 when it finally gets clear it will not emit the principal spectral lines of a 

 solitary star, but others which emanate from denser gases. This probably 

 accounts for the blue, violet, and green colours which are found in the mi- 

 nute companions of double stars. 



Another consequence is that the orbits of double stars will almost always 

 have a considerable ellipticity. 



Another consequence is that the conditions are likely not unfrequently 

 to arise which would separate the companion into two or more fragments ; 

 and that when this happens, the separate pieces will pursue paths which 

 are distinct from one another and not far apart. This seems to account for 

 such systems as y Andromedee. 



When the same conditions act with unusual violence they would pro- 

 bably break up the companion into numerous fragments ; and it is remark- 

 able that they would at the same time be likely to cause the primary to 

 throw off a number of rings. The fragments and the rings would move all 

 in the same direction and nearly in the same plane, and each fragment 

 would rotate rapidly in the direction in which it revolves in its orbit. 

 When the fragments, as must generally happen, are of inconsiderable mass 



TOL. XYI. D 



