134 Very — Stellar Revolutions within the Galaxy. 



from the boundary of its assumed sphere of motion to its 

 present position, and the velocity acquired will be approxi- 

 mately 4:7 km., or more than twice that assigned from inv^esti- 

 gations of the solar apical motion. 



With a galactic mass of 10^, and the same dimensions as 

 before, the sun will travel over the path from apoo^alacteum to 

 its present position in about 19,000,000 years, but the final 

 velocity is less than 15 km. per sec, or probably a little less 

 than the actual one, still not so much smaller but that the 

 increased acceleration produced by central condensation may 

 bring the velocity up to the required amount. 



Twenty million stars of the same mass as the sun, after 13J 

 million years, will give a velocity of 21 km. per sec. with uni- 

 form distribution, and more with central condensation. 



Extreme condensation is improbable. Professor I^ewcomb 

 has shown* that, for equal areas of sky, the brightness of the 

 sky in the Milky Way is more than twice as great, but not as 

 much as three times as great, as that near the galactic pole. If 

 the general light of the nocturnal sky is due to small stars, this 

 observation seems to indicate that the stellar distribution is 

 more nearly uniform than is commonly supposed. 



Some 20 years ago Professor ISTewcombf calculated that 

 500,000,000 stars with masses equal to the sun's mass, and uni- 

 formly spread out in a thin circular layer or disk, 284,000x10'" 

 km. in diameter, will produce a velocity of 40 km. per sec. in 

 a body falling from infinite distance to the center of the sys- 

 tem. Such a disposition of attracting masses can hardly be 

 considered a probable scheme of present stellar distribution, 

 and I presume its proposer will prefer to modify it in the light 

 of his own more recent photometric observations. The same 

 mass distributed through a sphere of equal diameter has 50 

 per cent greater attraction, and central condensation would 

 still farther increase the mean velocity. 



Lord Kelvin;}: assumes a uniform stellar sphere with a radius 

 five times as great as mine. This may be a better value than 

 mine for an outer boundary of the galactic system, but com- 

 paratively few stars attain it, and the velocity possessed by our 

 sun does not point to so large a recession. Yogel and Scheiner 

 obtained an average velocity in the line of sight of 16'5 km. 

 from fifty-one stars.g Professor W. W. Campbell, after separa- 

 ting the solar motion, obtained for the numerical average of the 

 velocities of 280 stars in the line of sight =bl7*05 km. per sec, 



* Astropbysical Journal, xiv, 297, Dec. 1901. 

 f "Popular Astronomy," 2d ed.. p. 501. 

 iPhil. Mag. (6), ii, 16i, 1901 ; also ill, 1, 1902. 



i^ Schemers " Treatise on Astronomical Spectroscopy, '■ Frost's translation, 

 p. 350, 1894. 



