CHAPTER IX 



THE EVOLUTION OF ROTATING NEBULAE 

 GENERAL THEORY 



202. In the last chapter we examined the sequence of changes which 

 would occur in a mass of gas left to its own gravitation at rest in space. We 

 found that matter once in existence would either disperse into space or 

 contract continually. Masses which disperse into space would have but a 

 transitory existence ; the permanent bodies in the heavens must be supposed 

 to be contracting. 



We accordingly think of the permanent astronomical bodies as beginning 

 existence in a state of extreme rarity. If one such mass existed alone in the 

 universe, it would tend to assume a spherical form if devoid of rotation, or a 

 spheroidal or pseudo-spheroidal form if endowed with a small amount of rota- 

 tion. Observation, however, does not encourage the view that the whole 

 universe originated out of a single mass of gas ; we shall find it more profit- 

 able to think of a number of separate and detached nebular masses as forming 

 the earliest stage in the process of cosmic evolution. 



Whether these masses ought to be thought of as being originally endowed 

 with motion, either of translation or of rotation, we do not know. In any 

 case they must in time be set into motion by their mutual gravitational 

 attractions. As the masses move under these attractions there will be occa- 

 sions in which two of the masses will pass fairly close to one another, and the 

 tidal couples raised in this way will necessarily set both masses into slow 

 rotation, the mechanism being that which has already been considered in 

 131. Rotations set up in this way would doubtless be of very small amount 

 at first, but they will increase with the shrinkage of the mass in accordance 

 with the law of conservation of angular momentum. 



In this way we are led quite naturally to the consideration of a number 

 of separately-moving and rotating gaseous masses as providing the initial 

 material for our problem of cosmogony. The problem of tracing out the 

 history of the astronomical universe is seen to be closely related to the abstract 

 problem of following out the sequence of events in a rotating mass of gas. 



203. This abstract problem has already been solved in certain ideal cases. 

 Primarily it has been solved for a mass of gas in adiabatic equilibrium, this 

 being determined by the relation p = kp? in which k and 7 are supposed con- 

 stant throughout the mass. 



