136 Yery — Stellar Revolutions within the Galaxy. 



ter, with destruction of linear motion and the formation of a 

 condensed nucleus. With such a nucleus, assuming a perigalac- 

 tic approach to 0*01 of the apogalactic distance, or a short- 

 radius swing around some dominant galactic agglomeration, 

 the galactic period of revolution of a star, under the attraction 

 of a mass M = 20,000,000 XM,, may approximate 25,000,000 

 years. Mr. Maxwell Hall, by combining the positions, proper 

 motions, and parallaxes of a small number of stars for which 

 these quantities have been satisfactorily determined, and 

 assuming circular orbits around a general center of motion, 

 obtained for the length of the "Annus Magnus" 20,000,000 

 years.* A revision with improved.'data gave for the period of 

 general revolution 13,150,000 years,t the center of motion 

 being in Andromeda. Kapteyn considers the center of the 

 Galaxy to lie in the direction of Lacerta ; and Easton has 

 given reasons for placing this center in Cygnus. The presump- 

 tion that the orbits are ellipses rather than circles, and ellipses 

 of large eccentricity, which require longer periods, follows 

 from the principles adopted in the present paper, while the 

 solar motion favors Easton's position ; and as it is not necessary 

 to suppose that the nearer stars are moving around the same 

 center, arguments from the structure of the Galaxy itself 

 deserve the most weight. 



(5) When the proper motions of the stars are charted, the 

 divergence from the apex and convergence to the anti-apex of 

 the sun's way immediately attract attention ; but after allowing 

 for this, there is no very obvious preponderance in the residual 

 directions of the true proper motions, either with or across the 

 galactic plane. Upon the hypothesis that the stellar move- 

 ments have originated in dispersals produced by explosions, this 

 result indicates that the explosive forces have acted indiscrimi- 

 nately in all directions. The primary forces which have given 

 the condensed part of the Milky Way its structure have acted 

 in a plane ; but the motions of the outlying stars have probably 

 had a different and secondary origin, and this is further indi- 

 cated by the advanced types of their spectra. 



(6) According to the assumption that the stars have been 

 thrown off from centers of dispersal distributed along the 

 galactic axes of condensation, and that explosive division of 

 stellar masses in the galactic zone is still in progress, the Milky 

 Way must include both stars of great mass, as yet undivided, 

 and large numbers of small stars, together with star clusters 

 resulting from recent division, as well as dark meteor swarms 

 not yet ready for stellar existence. The concentration of mass 

 in a narrowly limited central region may be great, but we are 



*''0n the Sidereal System," Mein. Roy. Astr. Soc, xliii, 196, 1876. 

 t Monthly Notices, Roy. Astr. Soc, xlvii, 539, 1887. 



