ON STELLAR DISTRIBUTION AND MOVEMENTS. 251 



Pickering's standard sequences running down to the faintest magni- 

 tudes now provide an excellent basis for work. At Greenwich a large 

 programme of photometry is being carried out, based on Pickering's 



standards, and may be expected to provide improved statistics, which 

 will be available in a year or two. 



Reference is often made to the phenomenon that certain classes of 

 stars do and others do not crowd towards the galactic plane. For 

 example, stars of the solar type of spectrum do not show the tendency 

 to congregate towards it, whereas stars of the Orion type are found 

 almost exclusively in it. These differences admit of an easy explana- 

 tion; everything depends on whether the class of stars coming into 

 consideration extends sufficiently far. If we take a class of feebly 

 luminous stars, or if we take the stars of large proper motion, we 

 necessarily limit ourselves to stars which are near the sun; there is 

 no reason why they should be most numerous near the galactic plane. 

 If, however, we take a class of stars which extend and are bright 

 enough to be seen a long way, then the bun shape of the universe 

 makes itself apparent. Clearly it is only if the class of stars extends 

 to the limits of the stellar system that the shape of those limits can 

 affect the apparent distribution. 



One further question: where does the Milky Way begin? Do 

 its clusters consist entirely of faint stars or are some of the bright 

 stars actually in it? The answer seems to be given clearly enough 

 by Newcomb. 6 By counting the abundance of the bright stars in 

 the parts of the sky where the Milky Way is bright, and again 

 in the faint portions and openings, he concludes that even stars 

 brighter than the sixth magnitude are associated with it; they are 

 sparse or dense according as the Milky Way is faint or concentrated. 

 It seems at first difficult to reconcile the existence of naked-eye stars 

 in the Milky Way with the remote position we have assigned to it, but 

 1 do not think there is any contradiction. Modern views admit a very 

 wide difference in the intrinsic brightness of stars, and even if the 

 parallax of the Milky Way is O'^OOl we might still expect it to contain 

 stars (probably of the Orion type) which are visible to the naked eye. 



We now turn from the distribution to the motions of the stars,' 

 and in particular to the two star-streams found by Kapteyn. Our 

 knowledge of stellar motions refers principally to the nearer parts of 

 the universe; but it is really very difficult to say to what part of the 

 stellar system the results of any investigation apply. Any particular 

 catalogue of proper motions includes a fairly compact bunch of near 

 stars together with a 'ragged edge' of enormous extent. It is one 

 of the questions for future research, How far do the two star-streams 

 extend into space? Any estimate that I can give now would be only 

 a rough guess; but I should think that at least half-a-million stars 

 belong to the two streams. 



The first researches on the star-streams were based on two important 



c The Stars : a Study of the Universe, p. 2G0. 



7 The first clear recognition that the motions of the stars deviated considerably 

 from a haphazard distribution is found in the researches of Kobold ; he moreover 

 detected a relation of the systematic motions to the galactic plane. 



