Stellar velocity distribution 



43 



stars or perhaps also the parameter X 1 will he sufficient to bring the two ellipsoids 

 to coincide. 



40. As remarked before, the class B stars showed a very singular velocity 

 distribution. Campbell says that for these stars there seems to exist no distinct 

 vertex direction, further he points out that the average velocity of the same 

 stars in a region of the sky 7. = 12 /! to 18'' and 5 = 0° to — 70° seems to be smaller 

 than for the other parts of the sky. 



The equation (38) however gives us a possibility to study this case more closely 

 when all moments of the right membrum are considered. 



In spite of the small total number of stars, which will cause large mean errors 

 in the six unknown quantities, the complete solution of (38) will give an idea of 

 the position of the velocity ellipsoid. The solution gave the following values of 

 the moments : 



#200 = + 3 - 979 ' ^020 = + 4 - 084 ' #002 - + 2-438, 

 N'i 1 = + 0.211, N<> 0 , = - 2.143, N'^ 0 = + 0.167. 



From these values the position of the velocity ellipsoid and the length of its 

 axes were computed. This ellipsoid was found to be represented by a surface of revolu- 

 tion. The two equal axes have a length of 2.166 Sin. and the third — the shortest — 

 has a length of 0.956 Sm. and is directed towards a point: a. = 244°.6, § = + 12°. 2, 

 or about 35° from the pole of the Milky Way. This point lies in the square O s . 

 A mean of the dispersion in this and the surrounding squares gave a = 1.406 ± 0.176 

 Sm. from 32 stars. 



Now it may be observed that the distribution of Class B stars relative to the 

 galactic plane will cause a large uncertainty in the determination of the axis direc- 

 ted towards the pole of the Milky Way. I found it therefore convenient to make 

 a new solution assuming the moments 



This solution gave the following results: 



N 20o = + 3 - 486 ' #020 = + 4 - 008 ' #oo« = + i- 604 ' N ll 0 = + 0.082. 

 The last moment shows a very small correlation. The moment N£ 2 ö seems to 

 point to a large mean velocity in this direction, but, as the mean error is large, the 

 velocity distribution is quite circular in the plane of the Milky Way. To test the 

 reality of the flat shape of the ellipsoid I have computed the mean velocities for the 

 two groups of squares surrounding the plane and the pole of the Milky Way. 

 The following results were obtained : 



Class B stars surrounding 



the plane of the M. W. the pole of the M. W. 



Number of stars 188 59 



Mean velocity 1,936 + O.100 1.555 + 0.143 



Values computed from the velocity | j 1.867 ^ ^ 



ellipsoid constants I 1 2.002 



