526 
ASTRONOMY: H. SHAPLEY 
among the fainter magnitudes blue stars were conspicuously predomi- 
nant. Since all the stars of the cluster are at sensibly the same distance 
from the earth, the apparent difference in brightness between red and 
blue stars represents an actual characteristic of the brighter objects of 
the system. This result, however, is not in harmony with the relations 
heretofore found between color and intrinsic brightness in the less con- 
densed bright clusters and in other selected groups of stars, and it is 
of importance to seek verification of the phenomenon in additional 
globular clusters. If it is proved that in these remote, gigantic stellar 
systems the most luminous objects are chiefly of the redder, low-temper- 
ature spectral types, and consequently of very great dimensions relative 
to the thousands of other stars each cluster contains, the result may have 
a significant bearing on theories of the evolution of stars. For instance, 
a satisfactory hypothesis of the evolutionary sequence of spectral and 
color classes would need to account explicitly for the ancestral relation- 
ships of these extremely voluminous second-type (red) stars and the 
comparatively small bodies of the bluer types. 
Measures of the magnitudes and color indices are now available for 
stars in three other globular clusters. For none of them is the work 
completed in all its detail, but the preliminary values, taken in con- 
junction with final results for the Hercules cluster, are sufficiently defi- 
nite to justify a report regarding the present problem. The obser- 
vational material and the final discussion will appear in extended form 
in the Contributions from the Mount Wilson Solar Observatory. 
The names of the clusters and their positions for the epoch 1900.0 are 
given in Table I. 
TABLE I 
DESIGNATION 
RIGHT 
ASCENSION 
DECLINATION 
GALACTIC 
LATITUDE 
GALACTIC 
LONGITUDE 
Messier < 
\ 3 
5 
13 
15 
N.G.C.5272 
5904 
6205 
7078 
13^ 38°^ 
15 13 
16 38 
21 25 
+28^53' 
+ 2 27 
+36 39 
+11 44 
8° 
333 
26 
33 
_j_77o 
+45 
+40 
-29 
A number of stars that undergo periodic light variations have been 
found in these clusters. Such objects were avoided in collecting the 
results of which a summary appears in Table II. The inclusion of the 
variables would scarcely change the averages, however, for the plates 
were taken in pairs and only extremely rapid light changes could enter 
as systematic errors in the colors. Stars near the center were also 
omitted from the present treatment in order to avoid possible errors 
similar to those suspected for the dense central part of Messier 13.^ 
