764 TABLE 876.— MOTIONS OF THE STARS* 



The motions of the stars show various well-marked features, of which the ellipsoidal 

 distribution and the asymmetry are a consequence of the rotation of the galaxy ; the sig- 

 nificance of certain other features is not yet fully understood. If we assume the circular 

 velocity around the galactic center (Table 828) as our origin, and plot the individual mo- 

 tions of the stars of any group as vectors from this origin, the ends of these vectors do 

 not form a spherical distribution (as they would if the motions of the stars were at ran- 

 dom) but rather an elongated distribution which is more or less asymmetrical and in 

 which the area of highest concentration of the vector points is centered about the origin. 

 If for the moment we ignore the asymmetry, the distribution may be characterized as 

 roughly ellipsoidal and the approximate extent and shape of the distribution may be 

 inferred from the dispersions of the velocity components along each of the three principal 

 axes, ff a , ffb, and a c , in km/sec. 



The direction of the a-axis is called the direction of the preferential motion ; the two 

 opposite points on the sky at the extremities of this axis are called the vertices. The 

 a-axis for any group of stars is always nearly parallel to the plane of the galaxy. In the 

 case of most groups of stars fainter than eighth magnitude, it appears that the a-axis is 

 directed approximately toward the galactic center at longitude 325? Among stars brighter 

 than sixth magnitude the direction deviates from the direction of the galactic center toward 

 greater longitudes and the deviation is most marked in the case of the A stars, for which 

 the longitude of the vertex is close to 350? In every case the f-axis is directed toward 

 some point close to the galactic pole. The asymmetry referred to above characterizes 

 the distribution of the components parallel to the fc-axis. It is relatively slight when the 

 dispersions are small as with the A stars, but becomes very pronounced in the case of 

 groups with large dispersions, there being practically no large motions in the direction of 

 the galactic rotation (longitude 55°). 



The last column in the table contains the product of the mean stellar mass (in terms of 

 the sun's mass) and the square of the dispersion along the c-axis. This quantity (analo- 

 gous to kinetic energy) is practically constant for the various groups of the main sequence 

 but is much larger for the giant branch. 



The dispersions of velocities for the B stars, the c stars, and the Cepheids are of the 

 order of 10 km/sec and difficult to determine accurately. For long-period variables the 

 dispersions average about 50 km/sec and for the cluster-type variables 90 km/sec. 



A general card catalog of radial velocities is kept at Mount Wilson Observatory. It 

 now contains approximately 14,000 entries and will be published in the near future. The 

 proper motions of all stars brighter than magnitude 7.0 and of many fainter stars may be 

 found in the Albany General Catalog. The Transactions of the Yale Observatory contain 

 the proper motions of many thousands of stars down to magnitude 9.5 and north of 

 declination — 30° and two catalogs of the Cape Observatory contain 40,000 proper motions 

 in the zone — 40° to — 52? 



* Prepared by A. N. Vyssotsky, University of Virginia. 

 288 Astron. Journ., vol. 53. p. 94, 1948. 



SMITHSONIAN PHYSICAL TABLES 



