l 



ASTRONOMY 



of the curves obtained on the simpler hypothesis 



(Fig. 5). The meaning of this is that the stars 



do not form a single system, but are to be regarded 



rather as forming two distinct systems. The 



motions in one or other of the systems are to be 



regarded as quite irregular, but the systems as a 



whole are in motion relative 



to one another. In relation 



to one of them our sun is 



moving at the rate of twenty 



miles a second, and with 



respect to the other at a little 



over eleven miles a second. 



It is found that the greater 



number of the stars which Fig. 5 Diagram showing 



the Combination of Two 



exhibit any motion belong- to theoretical curves to 



J Represent an Observed 



one or other of the two groups, curve 



or drifts, as they are now usually designated, but 

 the numbers in the drifts are unequal, being in 

 the ratio of 3 to 2. Now although the greater 

 number of the stars belong to the drifts, 

 there are others which stand apart and form 

 distinctive systems by themselves. The most 

 important of these is that formed by the B stars, 

 which are characteristic of the constellation of 

 Orion, though not confined to it. These are the 

 most massive and luminous stars in the heavens, 

 and of them I shall speak later. In addition to 

 these groups, several clusters of stars have been 



59 



