300 PHYSICAL SCIENCE 



nebulae are on a far vaster scale — possibly a 

 million times as great as the dimensions of 

 Neptune's orbit. On this scale, gravitation 

 would overcome the diffusive effect of gaseous 

 pressure, and detached masses might become 

 stars. 



Mr Jeans has investigated mathematically the 

 history of a mass of gravitating gas. It would, 

 of course, form a sphere when at rest ; but if, 

 in the changes and chances of its nebulous life, 

 it be set in rotation, the sphere will broaden out 

 round the equator and flatten at the poles. 

 While it is contracting under gravity, the 

 angular momentum must keep constant, and 

 therefore the speed of rotation must increase. 

 The shape gets flatter and flatter, till the nebula 

 resembles a double convex lens. And such, 

 indeed, is the form of some of the nebulae : one 

 such is shown in Fig. 41. 



At still greater speed, the lens-nebula must 

 break up at the edge, and detach isolated globes 

 of matter. This is clearly the meaning of the 

 many spiral nebulae, one of which is shown in 

 Fig. 42. They are casting forth future stellar 

 worlds. Each isolated globe is shown by calcula- 

 tion to be about equal in mass to that of the 

 average star, and large enough to develop into 

 a system such as our sun and his attendant 

 planets. 



Measurements of the actual movement of the 

 arms of spiral nebulae indicate that they are 

 indeed matter flying out from the nucleus. The 

 arms in Fig. 42, for example, are moving fast 

 enough to complete a revolution in 45,000 

 years. 



