34 



PROFESSOR A. S. EDDIXGTOX, F.R.S., ON 



have given us an entertaining legend as to how so important a 

 part of the Great Bear has come to be situated between the 

 teeth of the Great Dos. 



In this connection you may be interested to know that the 

 constellation Orion, except for one corner star (Betelgeuse), is 

 probably a real physical system of stars, and not just an 

 accidental configuration. 



This Great Bear system extends our ideas of associations of 

 stars enormously. The sun is more or less between Sirius and 

 the stars of the Plough, so that the sun, and in fact many other 

 stars, must be actually interloping in the Great Bear system. 

 The tie is not between neighbouring stars, but between stars 

 almost arbitrarily picked out from the crowd, with numbers of 

 unassociated stars interspersed between. What may be the 

 nature of the tie admits, I think, of only one opinion — the 

 members must have started off from a common origin. Very 

 slight deviations from truly parallel motions have in the course 

 of ages made them spread wide apart ; but they still preserve 

 their common velocity almost unaltered, because nothing has 

 ever happened which could disturb them. 



But the nock of stars which has been most extensively 

 studied is in the constellation Taurus. Our knowledge of this 

 system is due to the late Professor Lewis Boss. Thirty-nine 

 stars have been recognized as belonging to the group, and no 

 doubt many fainter stars belonging to it will ultimately be 

 detected. These stars cover a considerable area in the sky. If 

 we mark their motions on a map or globe, we find that the 

 motions all converge to a definite point or vertex. By the 

 theory of perspective we know that that is what would happen 

 if the actual motions in space were along parallel lines. 

 Further, the apparent motions (which are rather large) are 

 nearly equal for all these stars. "We conclude that they form a 

 moving cluster of the kind we have been considering. 



Xow, from this fact we are able to measure the distances of 

 all the thirty-nine stars.* The distances range from 600 to 900 

 billion miles ; so that the cluster is about 300 billion miles deep ; 

 light would take fifty years to cross it from side to side. We 

 could not have measured the distances of these stars by the 

 ordinary method of parallax — they are too remote for that ; 



* This piece of geometry was described in the lecture with diagrams. 

 It is only necessary to know the position of the convergent point, the 

 positions and proper motions (in angular measure) of the stars, and the 

 spectroscopic radial velocity of one of the stars. 



