THE EVOLUTION OF THE UNIVERSE 15 



The millions of stars in the sky are of different ages. 

 Some are in the prime of life. Some (like our own) 

 are beyond middle age. Large numbers— astronomers 

 now generally hold — died long ago. 



This is one half, the latter half, of the evolution 

 of worlds. The next step is to trace the first half, 

 to discover their origin and early stages. Here 

 there is a good deal of dispute, but we may choose 

 a very clear starting point which is admitted by 

 all. If you leave your room full of fine dust at 

 night, you will find all this gathered together on 

 the floor in a layer the next morning. The floor 

 (or the earth) "attracts" it, we say. Now, sup- 

 pose an enormous cloud of this dust were in 

 space, millions of miles away from any solid globe 

 to attract it. Suppose this cloud were millions or 

 billions of miles in extent, and it consisted of a 

 mixture of particles of all the metals and gases. How 

 wotdd it behave? 



We should say that if the dust were quite evenly 

 distributed, and nothing entered it to disturb it, it 

 might remain as a cloud for indefinite ages. But, as 

 a matter of fact, the chances of such a cloud being 

 quite equally distributed are very slight. There 

 would be sure to be some parts denser than others. 

 Then we know what would happen. These thicker 

 parts or centres would begin to "attract" the dust 

 from all round them. Even if there were no denser 

 centres to begin with, there soon would be. As we 



