208 ANNUAL EEPOBT SMITHSONIAN INSTITUTION, 1933 



enable us to calculate the distance of the sun from the center and 

 the total mass of the system. The distance of the sun from the 

 center, on the assumption that the main part of the mass is concen- 

 trated near the center, comes out as about 40,000 light-years 

 and the total mass of the galaxy as about that of 250,000,000,000 

 suns. If the orbital speed were smaller or the mass less concentrated 

 to the center, these dimensions would be somewhat reduced, though 

 the order is not changed. Please remember that this distance and 

 the mass are calculated, on the assumption of a rotation of the 

 galaxy, wholly from the radial velocities of relatively few stars 

 without any reference to measures of stellar distances or star counts. 



It will be of interest to compare these dimensions with those 

 arrived at in our discussion of the structure of the galaxy and 

 based on the distances of the globular clusters. The diameter 

 arrived at was 200,000 light-years, which, with the sun halfway 

 between center and edge, makes its distance from the center 50,000 

 light-years, a very satisfactory agreement of geometrical and dy- 

 namical values. Scares and Van Rhyn, from the result of star 

 counts, estimated a total of 30,000,000,000 stars in the galaxy. Since 

 dark nebulae obstruct our view, particularly in the direction of the 

 center in Sagittarius, the numbers may well be much greater, while 

 there is in addition the mass of the cosmic cloud and the bright and 

 dark nebulae, so that the dynamical estimate may not be unreason- 

 ably higher than the geometrical. 



The evidence seems overwhelmingly in favor of a rotation of the 

 galaxy, but before unreserved acceptance two difficulties should be 

 mentioned. In the first place, practically all the velocity data on 

 which the observational test was made covered barely three-fifths of 

 the way around the galactic plane. Wliile it seems highly probable 

 that the lacking velocities from the southern sky will confirm the 

 northern results, we should accept the evidence with some reserve 

 until the actual observations are available. The same difficulty, of 

 the insufficiency of data in the southern sky, is felt in almost every 

 general astronomical investigation and to my mind money spent in 

 the increase of astronomical equipment in the Northern Hemisphere 

 would be much more useful if it could be transferred to the South- 

 ern. The second difficulty lies in the shearing effect of the differen- 

 tial rotation on the permanence of such aggregations of stars as the 

 local cluster and the Milky Way clouds. If we take the local cluster 

 as 5,000 light-years in diameter, the differential rotation will cause 

 the inner edge of the cluster to make 8 revolutions while the outer 

 part is making 7. Obviously a compact cluster will be sheared into 

 an elongated form, so that the presence of star clouds in the galaxy 

 must be regarded only as transitory eddies in a whirlpool which 

 form and dissipate continually. 



