232 CARNEGIE INSTITUTION OF WASHINGTON. 



applicability of spectroscopic methods to the determination of absolute magni- 

 tudes and parallaxes would be assured for all stars with spectral types between 

 B and M, or about 99 per cent of all those with known spectra. 



Reference was made in the last annual report to the investigation by Mr. 

 Lindblad of the influence of absolute magnitude on the more refrangible 

 portion of the spectrum of stars of types B and A. This work was completed 

 during the autumn, and the methods developed should prove of especial value 

 in the case of very faint stars which can not be photographed with slit spectro- 

 graphs of considerable dispersion. It is of interest to note that the spectro- 

 scopic method for deriving absolute magnitudes of A-type stars as outlined 

 above is in full agreement with the results of Mr. Lindblad in indicating that 

 the intrinsically fainter stars are characterized by lines which are relatively 

 diffuse, and that this characteristic serves as an important criterion for the 

 determination of their magnitudes. 



Distribution of the Space-Velocities of Stars and Stream Motion. 



An extensive investigation by Mr. Stromberg of the distribution of the space- 

 velocities of stars of spectral types F to M was completed recently. The 

 material used consisted of about 1,300 stars of known proper motion, parallax, 

 and radial velocity. Spectroscopic parallaxes were used in most cases. The 

 three components of velocity were computed from these data and the distri- 

 bution of these velocities for different groups of stars relative to the galactic 

 system was studied in various ways. With the aid of a very general form of 

 frequency-function, of the Fourier's series type, in which the coefficients were 

 determined directly from the number of velocity-vectors within different 

 limits, it became possible to construct curves of equal frequency for these 

 vectors in the plane of the galaxy and the planes at right angles to it. In this 

 way a graphical representation of the distribution of space-velocities was 

 obtained. 



One of the principal purposes of the investigation was to decide which of 

 the two modern theories of stellar motions, the two-drift theory of Kapteyn 

 or the ellipsoidal theory of Schwarzschild, best represents the observed dis- 

 tribution of velocities. The following are some of the main conclusions bear- 

 ing on this question. 



(1) The giant stars of spectral types F to M form a single group with an 

 ellipsoidal velocity distribution, the elongation of the ellipsoid being greatest 

 for the F stars and decreasing with the later types. The distribution is nearly 

 spherical for stars of the late K and M types. 



(2) Among the fainter F-type stars there is another group, the motion of 

 which coincides closely with that of the Taurus group. This includes about 

 20 per cent of the fainter stars of this type of spectrum. 



(3) The dwarf stars form a distinctive group as regards their motions. The 

 distribution of velocities is ellipsoidal, but the directions of the axes of the 

 ellipsoid, as well as the position of its center, differ from those of the giant 

 stars. 



(4) There is a marked asymmetry in the distribution of the velocities, the 

 stars of high velocity, regardless of spectral type or absolute magnitude, 

 showing an avoidance of motion towards the quadrant of the sky between 

 galactic longitudes 0° and 90°. 



