THE MOVEMENTS OE THE STARS. 



31 



star) ; this would contain about thirty stars — we actually know 

 the identical stars contained, or most of them. On a smaller 

 scale this would be represented by thirty tennis-balls distributed 

 through a volume of space as large as the earth. Ima.gine thirty 

 tennis-balls wandering about in the whole interior of the earth ; 

 that represents the fine-grainedness and sparsity of stars in a 

 typical "portion of space. There is plenty of room for the stars to 

 move without much fear of collisions. We are often inclined to 

 think of the celestial 1 todies as moving under nicely-balanced 

 forces, each with its own path arranged to prevent disaster ; 

 however that may be in the solar system, there is no need for 

 regulation of the traffic in interstellar space. There is any 

 amount of room for each star to take its own course, and the 

 duty of a look-out man would be a sinecure. 



That being the case, are we to regard each star as an hide- 

 pendent islet in space, unrelated to any other ? That is the 

 grand question of stellar astronomy. With thirty tennis-balls 

 distributed through the whole terrestrial globe, can we imagine 

 anything more unlikely than that any connection should bind 

 one to another ? Is each star an independent entity ; its birth, 

 its motion and its history having no relation to any other ? Or 

 are there signs of some community of origin by which we can 

 group the stars into relationship ? In fact, is the universe a 

 chaos or a system ? I use the word chaos in no depreciatory 

 sense, for it is one of the beautiful discoveries of science that 

 out of chaos proceed some of the most simple and uniform laws 

 of Nature. 



Various suggestions were at one time made that the stars 

 revolved around some central sun. Alcyone in the Pleiades 

 was suggested for this centre, for reasons which seem to have 

 been more sentimental than scientific ; but we now know there 

 is no simple arrangement of that sort. Very recently, however, 

 there have been discovered anomalies in the way stars are 

 moving, which, however they may be interpreted, forbid us to 

 think of the universe as a pure chaos of stars. There seems to 

 be some sort of association between even the most widely 

 separated stars. 



The clue to these associations is in the movements of the 

 stars. In the early days of astronomy the fixed stars were 

 regarded as marking out a definite background against which we 

 could record the movements of the wandering stars or planets. 

 They were like the figures on the dial of a clock by which we 

 tell how the hands are moving. But in 1718 Halley, just 

 before he became Astronomer Eoyal, made the discovery that 



