46 The Outline of Science 



hypothesis, therefore, that our universe is a spiral nebula has 

 much to be said for it. If it be accepted it greatly increases our 

 estimate of the size of the material universe. For our central, 

 K-MN-shapcil .system is calculated to extend towards the Milky 

 Way for more than twenty thousand times a million million miles, 

 and about a third of this distance towards what we have called 

 the poles. If, as we suppose, each spiral nebula is an independent 



FIG. 28 



The above distances are merely approximate and are sub- 

 ject to further revis ; on. A " light-year" is the distance that 

 light, travelling at the rate of 186,000 miles per second, 

 would cover in one year. 



stellar universe comparable in size with our own, then, since there 

 are hundreds of thousands of spiral nebula?, we see that the size of 

 the whole material universe is indeed beyond our comprehension. 

 In this simple outline we have not touched on some of the 

 more debatable questions that engage the attention of modern 

 astronomers. Many of these questions have not yet passed the 

 controversial stage; out of these will emerge the astronomy of the 



