Bickerton. — On Cosmic Impact. 517 



stars, the temperature of the coalesced part would be the 

 same. 



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

 tatiug power of the coalesced part will depend upon the mass 

 and size of that part ; it will increase as the mass increases, 

 and diminish as the size increases, the mass remaining 

 constant. 



17. Heat is 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 bodies 

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

 stable, the velocity not overcoming the attraction of gravita- 

 tion ; in a small graze the body will expand indefinitely in 

 consequence of the small attractive power of the coalesced 

 mass, and every expanding particle will have so high a velocity 

 that it will in general become an independent wanderer in 

 space. Consequently space will be spread with free molecules. 



New Stars and Planetary Nebula. 



18. The mass of gas produced by such an impact will ob- 

 viously expand temporarily into a hollow shell of gas. Her- 

 schel tells us this is the condition of planetary nebulas. 



19. A partial impact of stars will therefore generally pro- 

 duce in less than an hour an intensely heated body that will 

 expand enormously without, for a time, much diminution of 

 heat. It will consequently become very bright indeed, and 

 will continue to expand with diminishing intensity until it be- 

 comes a planetary nebula, and will then disappear by dissi- 

 pating completely into space. 



20. That is, a new star has been born that increases in 

 intensity until the general parallelism of motion of the mole- 

 cules causes a lessening number of impacts between the mole- 

 cules. As molecules only radiate immediately after encounters, 

 the luminosity will diminish, and will go on diminishing until 

 it disappears. In special cases the planetary nebula may be 

 fairly permanent. In other cases a permanent star may ap- 

 pear in the centre of the nebula. 



21. The molecules on the far side of the nebula (or third 

 body formed by the two colliding stars) will be retreating 

 from us ; those on the near side will be advancing towards us. 

 The spectrum of such a body will consequently be crossed by 

 broad bright bands with a maximum in the centre and 

 gradually dying imperceptibly away. If this body has any 

 motion in space, as it probably will have when the two col- 

 liding stars are unequal, the line of maximum intensity, 

 though in the centre of the band, may be displaced from its 

 true position. 



