BiOKEETON. — On the General Problem of Stellar Collision. l6S 



This evidence seems to be indisputable. The stars appear to be some- 

 thing hke the molecules of a heated gas, in motion in all directions, and 

 hke these necessarily sometimes coming into impact. But many stars will 

 come within the effective attraction of other stars, and this attraction will 

 be enormously more effectual than mere chance in producing collision. 

 Thus the star Alpha Cygni is almost directly approaching our sun, and it 

 is extremely likely that in one or two hundred thousand years it will be for 

 thousands of years the nearest star, by many times in the heavens, durmg 

 which time, by their mutual attractions, the Sun and this star will probably 

 be deflected several diameters towards each other out of their respective 

 independent paths. That stars are thus brought within each others' 

 attraction is borne out by the fact observed by Mr. J. W. Wilson, of Eugby, 

 that the constituents of the double- star Castor are moving in hyperbolic 

 orbits, and it would be well worthy of careful observation to ascertain if 

 any other of the binary stars are thus connected. I will not prolong this 

 discussion, for it appears certain that cosmical collisions must occur. That 

 they do occur on a small scale is evident by the stupendous number of 

 meteorites which strike the earth every year, and Proctor's idea of the small 

 craters on the Moon's surface is ~ that they have been formed by meteors 

 falling on its surface during the Moon's viscous condition ; clearly what 

 occurs on a small scale, analogy suggests should also occur on a larger 

 one. 



Having thus shown that cosmical collisions are necessary events with 

 such a system as the galaxy has been proved to be, I shall attempt to show 

 that, except in the collision of bodies of very different volumes, complete 

 collision is of extreme improbabihty, or, in other words, that almost all 

 considerable cosmical impacts will be partial collisions. If we suppose 

 two bodies of very great mass, and of excessively minute volume, collision 

 can only occur when by their motions the two are tending to occupy the 

 same point in space at the same time. In most other cases the bodies 

 will tend to take hyperbolic orbits with a common focus, hence they 

 escape each other. Suppose each to retain the same mass, but the 

 volume of each to be expanded beyond the common focus, collision is of 

 course inevitable, and it is clear that the impact must be partial. Those 

 parts of each which lie in each other's path will mutually destroy each 

 other's motion, whilst the remainder of each of the two bodies will pass on 

 in orbits more curved than before, but which may still be hyperbolic ; or, if 

 the original proper motion of the bodies were small, or the part struck off 

 of large mass, the new orbits may be elliptical, and one or both of the parts 

 will return and remain in permanent orbits, as double stars. I may men- 

 tion that I have already demonstrated, I believe, with sufficient clearness, 



