ASTRONOMY: H. SHAPLEY 
527 
The tabulation of the material needs no general explanation. The 
photographs now available for Messier 3 do not give a large interval 
of photovisual magnitude, and consequently, in order to test for the 
presence of blue stars in this cluster, recourse was had to the short period 
variable stars. The last average, then, depends entirely on the colors 
deduced for a number of these variables^ and, representing a group that 
is probably peculiar for blueness, is entitled to diminished weight. 
The data in Table II show that in these four clusters the average color 
index is decidedly larger for the brighter stars. Not only is this true 
in the averages but a detailed examination of the magnitudes involved 
shows that with few exceptions relatively high luminosity is always 
accompanied by greater redness. (The exceptions may indicate real 
anomalies among the members of the cluster, or may be due to the 
superposed non-cluster stars.) Preliminary consideration of the colors 
and magnitudes in three additional globular clusters — Messier 4, 9 and 
14 — shows that the same relation holds for them as that deduced numer- 
ically in Table II. 
The condition clearly indicated by the above results is that the total 
light emission of the bluer stars (small color indices), for which the 
surface temperature is presumably in excess of 10,000 degrees Centi- 
grade, is less than that of many of the redder stars (large color indices), 
TABLE II 
Average Magnitudes and Colors in Four Globular Clusters 
PHOTOVISUAL 
AVERAGE 
NUMBER 
PHOTOVISUAL 
AVERAGE 
NUMBER 
MAGNITUDE 
COIOR-INDEX 
OF STARS 
MAGNITUDE 
COLOR-INDEX 
OF STARS 
Messier 3 
Messier 13 
12.71 
4-1.34 
10 
12.28 
+ 1.28 
13 
13.34 
+ 1.16 
10 
13.00 
+0.92 
23 
13.91 
+0.88 
10 
13.50 
+0.96 
23 
14.24 
+0.88 
9 
14.10 
+0.73 
76 
14.51 
+0.71 
10 
14,70 
+0.65 
110 
14.78 
+0.73 
11 
15.30 
+0.39 
222 
(15.20 
+0.05 
14) 
15.70 
+0.34 
27 
16.45 
+0.32 
28 
Messier 5 
Messier 15 
12 AG 
+1.52 
6 
13.04 
+ 1.35 
6 
12.93 
+ 1.22 
6 
13.52 
+1.17 
7 
13.24 
+ 1.20 
6 
14.19 
+0.83 
7 
13.66 
+ 1.02 
6 
14.87 
+0.86 
7 
14.42 
+0.59 
6 
15.18 
+0.68 
7 
14.72 
+0.44 
7 
15.54 
+0.42 
7 
14.83 
+0.29 
7 
15.65 
+0.32 
6 
