ASTRONOMY: H. SHAPLEY 
17 
Although in the relatively near moving clusters, referred to above, 
there are very few of the giant red stars, their very frequent occurrence 
among the stars of the general galactic system is commonly recognized. 
From different lines of evidence we believe them to be tolerably normal 
stellar objects as regards mass, motions, and surface luminosities, but 
differing from dwarf red stars most conspicuously in density, dimen- 
sions, and total brightness. Admitting this interpretation, there is no 
place at all for the giant stars in the older and more conventional scheme 
of stellar evolution, but they harmonize almost completely with the 
newer evolutionary hypothesis. 
From the best data available two years ago, Russell concluded that the 
giant stars of all spectral types have very similar absolute magnitudes. 
We are now in a position to make a further contribution to this subject, 
basing our conclusions upon results from globular clusters. Although 
later researches may show that our immediate galactic domain is com- 
parable with a globular cluster to a limited extent only, there can be, 
nevertheless, but little doubt that each globular cluster is in itself a 
complete system, distinct from the galaxy, and one in which the evolu- 
tion of the constituent stars probably follows analogous Hues to the 
stellar development in our own system. In many respects it should 
be possible to read the observed features of the cluster directly into 
our interpretations of the local system, and the characteristic discussed 
below is probably such a case. 
Table I shows the common dependence of average magnitude upon 
color class in the different regions of the cluster. The relation is better 
exhibited in Table II, which gives the average color index for succes- 
sively fainter intervals of magnitude in different regions. The number of 
stars included in each average is in parentheses. 
TABLE II 
Average Color Indices in Messier 13 
M^SGNITTJDE INTERVAL 
DISTANCE FROM CENTER OF CLUSTER 
ALL REGIONS 
2'.0 to 3'.0 
3'.0 to 5'.0 
> 5'.0 
ll.Sto 12.6 
+ 1.25 ( 8) 
+1.34 ( 5) 
(0) 
+ 1.28 ( 13) 
12.6 13.2 
+0.81 (14) 
+0.97 ( 6) 
+0.85 ( 4) 
+0.86 ( 24) 
13.2 13.8 
+0.95 (11) 
+0.97 ( 6) 
+0.97 ( 6) 
+0.96 ( 23) 
13.8 14.4 
+0.71 (27) 
+0.72 (36) 
+0.81 (13) 
+0.73 ( 76) 
14.4 15.0 
+0.67 (48) 
+0.62 (37) 
+0.67 (25) 
+0.65 (110) 
15.0 15.6 
+0.31 (90) 
+0.46 (97) 
+0.39 (35) 
+0.39 (222) 
The data of the two tables show conclusively that the brightest stars 
in the cluster are the red and yellow giants. In fact there are 16 g's 
