EVOLUTION OF THE ST A US 



2 33 



great dimensions. This is the particular reason why a few astronomers 

 suggest that the spirals may be distant systems of stars. They say that 

 our own stellar system, if viewed from a great distance, might be seen 

 to have a spiral structure : that it 

 would be fairly circular in general 

 outline if viewed from the poles 

 of the Milky Way, or greatly elon- 

 gated or spindle-shaped if the ob- 

 server were in the plane of the 

 Milky Way. We illustrate this 

 point by means of well known 

 spirals viewed broadside, and ob- 

 liquely, and of the spindle-shaped 

 nebula?, which we do not doubt 

 are spirals seen edgewise. Easton, 

 the principal modern student of 

 Milky Way structure, has even 

 gone through the laborious task of 

 assigning the stars, as seen from 

 our viewpoint near the supposed 

 center of the stellar system, to their 

 assumed places in a spiral struc- 

 ture. But I need scarcely say that 

 the subject is too vast for solution 

 now, or in the near future. If our 

 stellar system is one of a hundred 

 thousand or more spiral nebulae, we 

 have at once the problem of deter- 

 mining the place of our stellar sys- 

 tem in the larger universe of sys- 

 tems, necessarily beginning with 

 the motion of our system as a whole „ „ m „ „ „ 



J Fig. 11. The Spiral Nebula H. \. 



with reference to the great num.- 41, panum Venaticorum, seen edgewise, 

 bers of surrounding systems. ' Photographed at the Lick Observatory. 



Star Streams 



It was supposed, until ten years ago, that the stars are moving ap- 

 proximately at random, both as to direction and as to speed. In 190-1 

 Kapteyn announced, on the contrary, that the stars have decided prefer- 

 ences for motions toward two opposite points in the sky; one point in 

 the northern edge of Orion, in the Milky Way; and the other point 

 exactly opposite to this. Investigations by many others have in all cases 

 confirmed Ivapteyn's discovery. Kapteyn did not mean to say that the 

 individual stars are moving parallel to a straight line joining these two 



