14 The Outline of Science 



different types of stars, and then he is able to deduce more or less 

 accurately the distance of a star of a known type from its faint' 

 ness. He, of course, has instruments for gauging their light. 

 As a result of twenty years work in this field, it is now known 

 that the more distant stars of the Milky Way are at least a 

 hundred thousand trillion (100,000,000,000,000,000) miles away 



no. 



Our sun is in a more or less central region of the universe, 

 or a few hundred trillion miles from the actual centre. The re' 

 mainder of the stars, which are all outside our Solar System, are 

 spread out, apparently, in an enormous disc-like collection, so 

 -t that even a ray of light, which travels at the rate of 

 180,000 miles a second, would take 50,000 years to travel from 

 one end of it to the other. This, then is what we call our 

 universe. 



Are there other Universes? 



Why do we say "our universe"? Why not the universe ? It 

 is now believed by many of our most distinguished astronomers 

 that our colossal family of stars is only one of many universes. 

 Hy a universe an astronomer means any collection of stars which 

 are close enough to control each other's movements by gravita- 

 tion; and it is clear that there might be many universes, in this 

 sense, separated from each other by profound abysses of space. 

 Probably there are. 



1 'or a long time we have been familiar with certain strange 

 in the heavens which are called "spiral nebula?" (Fig 4). 

 We shall see at a later stage what a nebula is, and we shall see 

 that some astronomers regard these spiral nebula? as worlds "in 

 the making." lint some of the most eminent astronomers believe 

 that they are separate universes "island-universes" they call 

 them or great collections of millions of stars like our universe. 

 There are certain peculiarities in the structure of the Milky Way 

 which lead these astronomers to think that our universe mav be 



