ASTRONOMY: H. SHAPLEY 
25 
STUDIES OF THE MAGNITUDES IN STAR CLUSTERS, IV. ON THE 
COLOR OF STARS IN THE GALACTIC CLOUDS SURROUNDING 
MESSIER 11 
By Harlow Shapley 
MOUNT WILSON SOLAR OBSERVATORY. CARNEGIE INSTITUTION OF WASHINGTON 
Communicated by G. E. Hale, December 18, 1 91 6 
The investigation of phenomena relating to brightness and color in 
the clusters that are located in rich regions of the sky necessitates a 
knowledge of certain properties of the neighboring stars. A catalog 
of the members of any cluster must inevitably include some stars ex- 
traneous to the group, and to a greater or less extent the colors and 
magnitudes of such objects will influence the interpretation of the 
results. For the globular clusters, particularly in high galactic latitude, 
the preponderance of cluster stars over those that chance to be included 
with them is such that no serious effect is to be feared. For the open 
clusters, however, and especially for those in the Milky Way, the number 
of background or foreground stars may easily exceed the number of 
physical members of the cluster, even at the center of the group. In 
these cases it becomes necessary to make a direct and special study of 
the extra-cluster stars in fields far enough removed from the cluster to 
be practically free of its outlying members, but not so distant that the 
field stars can not be considered representative of those that in pro- 
jection appear commingled with the cluster. 
A study of the colors in the open galactic system Messier 11 (N. G. C. 
6705) has been completed with the compilation of a catalog of the 
magnitudes and colors of nearly five hundred stars. The cluster is 
situated in the constellation Scutum Sobieski, in one of the densest of 
the great star clouds of the Milky Way. Unlike the stars in Messier 
67, in which all the fainter objects are yellow and red (and similar in 
most respects to the neighboring non-cluster stars), those in Messier 
1 1 fall into all color classes, but with a distinct preference for the bluer 
types. The group is but four degrees from the mid-line of the Galaxy; 
Messier 67 is in galactic latitude -f 34°. To find whether the presence 
of blue faint stars in Messier 1 1 is a peculiarity of the cluster or is only 
a result of its low galactic latitude and its association with the star 
clouds, a special study in several neighboring fields was undertaken. 
This investigation bears directly on the relation of the cluster to the 
surrounding stellar masses, and also yields information regarding the 
nature of the galactic clouds themselves and the spectra and intrinsic 
luminosities of the numberless stars that compose them. 
