THE EVOLUTION OF THE UNIVERSE 13 



to-day an open question whether there are "other 

 universes than ours." Many people think that the 

 word ' ' universe * ' means the * ' whole ' ' of existing things , 

 and the word is generally used in that sense outside 

 of modern astronomy. But that is not the literal 

 meaning of it, ^ and astronomers choose to call a vast 

 collection of stars which form one immense family — 

 that is to say, which control each other's movements 

 by gravitation — a universe. There may be other 

 great systems of stars which are so far from ours 

 that they are practically independent. Many dis- 

 tinguished astronomers to-day think that we do 

 dimly see such "island universes," as they call 

 them, shining faintly, as blots of light, on the dark 

 background of space. I will say more about this 

 presently. 



Our universe, then, is a collection of hundreds, and 

 possibly thousands, of millions of worlds; if we call 

 each star and its planets a world. They are separated 

 by millions of miles of space from each other. They 

 are stupendous globes of white-hot metal and gas, at 

 a temperature of something between 3,000° C. (red 

 stars) and 30,000° C. (bluish-white stars). Our sun, 

 860,000 miles in diameter, is a fair average, or a little 

 above the average, in size, as far as we know. But 

 some are so much more intense in their light that, 



^The Latin Word "universum" is often said to mean "the 

 unity of all things"; but it does not contain the word "all." It 

 means a unified system. 



