ON STELLAR DIMTIUBUTION AND MOVEMENTS. 247 



In studying the movements of the stars, we have perforce to leave 

 out the remoter parts of space and confine attention mainly to stars of 

 the central system, and perhaps only the inner parts of it, where the 

 apparent movements are appreciable. The remarkable result appears 

 that the stars move with a strong preference in two opposite direc- 

 tions in the galactic plane. As Professor Kapteyn snowed at the 

 British Association meeting in 1905, there are two favoured directions 

 cf motion; and I believe there is moderately good evidence that this is 

 caused by two aggregates of stars of more or less independent origin 

 passing through each other, and so, for the time, being completely 

 intermixed. Be that as it may, there can be no doubt that one parti- 

 cular line in the galactic plane is singled out; the stars move to and fro 

 along it in preference to any transverse direction. It is a significant 

 fact that the line lies in the galactic plane ; it establishes some kind 

 of connection between the phenomena of stellar distribution and the 

 phenomena of stellar motion ; but the relation is of an unexpected 

 character, and is not easy to interpret. 



We must in passing take note of the 'Moving Clusters,' which 

 have lately attracted attention. These are groups of stars possessing 

 equal and parallel velocities though often widely separated in space. 

 The most important is the Taurus cluster, 1 which contains about forty 

 known members. Of the others, the Ursa Major stream a now known 

 to be by no means confined to the one constellation, has about thirteen 

 members; a Perseus cluster 3 of Helium stars has seventeen members, 

 and there is a very large but ill-defined moving cluster in Scorpius. 

 In each cluster the component stars move with practically identical 

 velocities, as it were ignoring the other non-cluster stars, which are 

 actually interspersed between them. The chief interest of these 

 clusters lies in the fact that in the case of the Taurus and Ursa Major 

 groups their peculiarity of motion has afforded an unexpected means 

 of determining their distances and absolute luminosities ; we thus obtain 

 precise knowledge about stars which are far too remote for the ordinary 

 methods of measurement to be successful. These clusters however 

 also throw an interesting light on the great problem of the origin of 

 the individual velocities of stars, as we shall see later. 



There is a whole class of stars, those of the Orion or Helium type 

 of spectrum, which stand out exceptionally. These seem not to share 

 in the motions of the two star-streams, and in fact have very little 

 motion of any sort, either individual or systematic. They are known 

 to be stars of enormous intrinsic brilliancy, so that a large proportion 

 of those visible to us are very far away ; perhaps the fact that most of 

 them lie beyond the part of the universe we generally consider in dis- 

 cussing stellar motions explains why their movements are exceptional. 

 Those Orion stars Hint are nearest have a great tendency to form 

 moving clusters, such as those of Perseus, Scorpius, the Pleiades, and 



1 I . Bos-, A-tron. J own , No. 604. 



2 Ludcndorff, Ast. Nach., No. 4313-14; Hertzsprung, Astro ph t/sical Jotirna!, xxx., 

 p. 135. 



s Benjamin Boss, Asiron, Joiirn., No. 620; Eddington, Monthly Xotices, R.A.S., 

 1st!., p. 43. 



