BiCKERTON. — Evidence in favour of Impact. 469 



8. All impacts brought about in this way by deflection will 

 be of a grazing character ; consequently, nearly all stellar col- 

 lisions will be of a grazing character. 



9. The average velocity of stars at impact will be hundreds 

 — in many cases thousands — of miles a second. The average 

 proper motion will not appreciably affect the velocity at im- 

 pact. Thus a proper motion of ten miles will only add one to 

 a colliding velocity of one hundred. 



10. A mere graze of the atmosphere of stars obviously will 

 not cause them to coalesce. As a mean result when more 

 than a third of each of two equal bodies collide, coalescence 

 will ensue, but this will depend on the original proper motion. 

 Were nine-tenths of 1830 Groombridge to collide with a similar 

 star the remaining tenth would not be stopped in its course ; 

 it would pass on in space, the bulk of the two stars temporarily 

 coalescing. 



11. The effect of the collision will be to intensely heat the 

 colliding part. 



12. The heating effect of a graze of two stars, of two star- 

 clusters, or two nebulae, or even of a star plunging through a 

 star-cluster, &c., will not appreciably extend to the parts not 

 colliding. To emphasize this fact such impacts have been 

 called " partial." 



13. Partial impacts generally result in the formation of 

 three bodies ; the parts of each whose momentum is destroyed 

 by impact remain behind, and the two cut stars pass on in 

 space. 



14. Partial impacts of a third of two equal stars having 

 considerable original proper motion would make the two into 

 three equal bodies ; two of them would travel in space in 

 opposite directions, the third would remain at rest between 

 them. If there had been no proper motion the three bodies 

 would coalesce ; but if less than a third be cut off each the 

 two bodies become three bodies orbitally connected. 



15. The temperature produced by an impact will depend 

 upon the velocity destroyed and upon the chemical constitu- 

 tion. High velocities and heavy molecules both tend to pro- 

 duce high temperature. Consequently the temperature will 

 not depend upon the amount of the graze. Were one-tenth or 

 one-hundredth grazed off the stars, the temperature of the 

 coalesced part would be the same. 



16. Although the temperature will be the same, the gra- 

 vitating-power of the coalesced part will depend upon its 

 mass. 



17. Heat is a molecular motion. In a small graze of any 

 given pair of stars the molecules will have the same velocity 

 as in a large graze ; but the gravitating force holding the body 

 together will be different. In a large graze the body may be 



