ASTRONOMY: H. SHAPLEY 
439 
the sun is in the general direction of the constellation Carina. The 
approximate galactic longitude of this center (which is to be taken as the 
center of the local cluster rather than that of the galactic system) is 
indicated in figure 1 by the short Kne across the galactic equator. A 
greater frequency of stars is to be noted in this direction, especially of the 
fainter objects plotted in figures 2 and 3. 
We recall that Stromberg's researches,^ through showing that stars of 
types F, G, and K appear to have motions related to the same center 
as that determined for the B-type stars, may be taken as strongly sup- 
porting the hypothesis of a local cluster which involves all types of stars. 
The observed decrease of stellar density with distance from the sun, for 
all the principal spectral classes, is also almost conclusive evidence that 
the phenomena of the local cluster are not confined to the B stars alone. 
It appears highly probable, therefore, that the incHnation and extent 
now under investigation refer to the cluster as a whole, and not only 
to these blue giant stars which, because of their uniformity in absolute 
brightness and their apparently high concentration to dynamically 
central planes, are of such high value in outHning a stellar system. 
The available volumes of the new catalogue of spectra cover less 
area than shown in figure 1, but the extension to the seventh magnitude 
(fig. 2) adds definitely to our knowledge of the cluster. Stars of the 
seventh magnitude are 1.6 times as far away as those of the sixth when 
the intrinsic brightness is the same. Increasing the depth of our survey 
by that amount has therefore brought in many B-type stars of the galac- 
tic field, objects concentrated to the galactic equator, and it has not 
introduced a great many stars of the local system. Hence, there is a 
suggestion in figure 2 that we are attaining the edges of the local cluster 
when we go out to the B-type stars of the seventh magnitude. 
Extending the survey much deeper into space by including the stars 
fainter than magnitude 7 (fig. 3), we find that outside the relatively 
small domain of the local cluster the galactic equator has definitely 
estabhshed itself as the central circle for B-type stars. 
The very high concentration of these distant stars to the Milky Way 
is a striking illustration of the great depth of the galactic system when 
compared with its extent perpendicular to the galactic plane. Prob- 
ably the most important inference from figures 2 and 3 is, however, 
that the galactic field is continuous in the immediate environs of the 
local cluster — that the latter is not a large group or cloud distinctly 
and distantly isolated in space from other stellar regions. Similar evi-. 
dence of a surrounding and interminghng galactic field may be deduced 
from the distribution of Cepheid variables, N-type stars, planetary 
nebulae, and similar special classes of sidereal objects. 
