16 
ASTRONOMY: H. SHAPLEY 
clusters are relatively blue and that the colors g and k do not appear 
until we go several magnitudes fainter than the brightest stars. This 
condition of increasing redness with decreasing brightness holds with but 
few exceptions among the stars in the open and moving clusters, such as 
the systems of the Pleiades, the Hyades, Ursa Major, and Messier 67. 
In the great globular cluster in Hercules (Messier 13), however, the in- 
terval of the four brightest magnitudes contains red and blue stars in 
approximately equal numbers. 
In the following table the average magnitudes in Messier 13 are given 
for successive concentric regions and for different classes of color. The 
progression from blue to red is from left to right. As in the preceding 
paper of this series the data for the stars of the innermost regions of the 
cluster are not included, though the results would not be affected ma- 
terially by retaining them. Each magnitude represents the mean 
value for about 16 stars. 
It should be remembered that the system of Messier 13 is immensely 
remote from the sun and is enormous in its real dimensions, and also 
that it contains in all probabiHty hundreds of thousands of stars. The 
longest exposures at Mount Wilson indicate that there are about 50,000 
stars in the cluster brighter than magnitude 21, and there is no indi- 
cation that the fainter limit has been approached. The same conclu- 
sions are vahd for other condensed globular clusters. Hence, we must 
recognize that in studying stars between magnitudes 12 and 16 in Messier 
13 we are deaHng only with those members of the system that have the 
greatest intrinsic luminosity. Obviously, then, the giant red and yellow 
stars, which in Russell's hypothesis of the order of stellar evolution 
play an important part as the antecedents of the bluer stars, are present 
in great numbers in Messier 13. 
TABLE I 
Average Magnitudes in Messier 13 
LIMITS OF DISTANCE 
COLOR CLASS 
ALL 
FROM CENTER 
b 
a 
f 
g 
k 
COLORS 
2'.0to 2'. 5 
15 
18 
15.12 
14.88 
14.36 
12 
11 
14.76 
2 .5 
3 .0 
15 
22 
14.75 
14.89 
14.03 
12. 
74 
14.58 
3 .0 
4 .0 
15 
16 
15.14 
14.81 
14.18 
12. 
19 
14.67 
4 .0 
5 .0 
15 
25 
15.22 
14.92 
14.70 
13. 
66 
14.91 
5 .0 
6 .0 
15 
22 
14.96 
14.84 
14.15 
14.62 
> 
6 .0 
15 
16 
14.03 
14.74 
14.54 
13. 
69 
14.67 
Total 
15 
20 
15.01 
14.85 
14.28 
12. 
74 
14.71 
