THE MOVEMENTS OF THE STARS. 



Past, and the motion is the same in amount and direction for all 

 of them (within the limits of observational error) ; these must 

 clearly be associated. When we have picked out the stars of 

 this group by their characteristic movements, and turn to see 

 where they really are in the sky, we find that they form a long, 

 rather open, chain stretching over quite a large arc in the sky — 

 like a row of skirmishers advancing together. 



Turning for a moment to the other set, we find that they are 

 characterised by extremely small motions, scarcely detectable. 

 We cannot infer that there is any relationship between these. 

 The small motion may, and probably does, mean that all of 

 these stars are extremely distant ; the actual movements may 

 be quite diverse and unconnected, but distance has diminished 

 the scale so that we can scarcely observe their differences of 

 motion. We have, therefore, detected a " moving cluster " of 

 helium stars in the Perseus region, each moving with the same 

 velocity across a background of much more distant stars of the 

 same type. 



Another case of this kind is afforded by the Great Bear. Of 

 the well-known stars forming the Plough, the five middle ones 

 share just the same motion. The tip star of the tail and one of 

 the Pointers do not belong — their presence is only accidental — 

 but the other five are moving in exactly the same direction with 

 the same speed as accurately as we can detect. But there is a still 

 more curious fact : the Dog-star, Sirius, is also a member of this 

 system. We happen to know the motion of Sirius thoroughly 

 — not merely its apparent progress across the sky, but its actual 

 linear motion in three-dimensional space — and it fits in exactly. 

 The evidence is the more convincing because the system happens 

 to be proceeding in a direction which is very unusual. (We 

 shall see later that some directions of motion are much more 

 common than others.) Very few stars are taking a course at all 

 approximating to that of the Great Bear system ; so when we 

 find Sirius going in just this direction with just the right speed 

 it is a pretty clear case. There are a few other stars in different 

 parts of the sky which also seem to belong to this system, but 

 we are not so certain of them as we are of Sirius. You now 

 see that the constellations in the sky do not correspond well 

 with the real relations of the stars. Taking the Great Bear, we 

 have had to cut off two stars which are not really of that system, 

 whereas Sirius, which is quite the other side of the sky, 

 apparently in the Great Dog, must really (according to physical 

 relations) count as part of the Great Bear. If only this had 

 been known to the old mythologists I am sure they would 



D 



