ASTRONOMY: H. SHAPLEY 
27 
illustrates the combined results. The most important features are the 
great diversity of color index and the general resemblance of this fre- 
quency curve to that for the brighter stars in the immediate neighbor- 
hood of the sun.^ 
The full presentation of the material will appear in two forthcoming 
Contributions from the Mount Wilson Solar Observatory. Some of the 
conclusions to be drawn from the results may be summarized as follows : 
1. Messier 11, in common with many other open clusters, does not 
differ greatly in the brightness and color of its stars from the non-con- 
densed fields which surround it. Though its members without doubt 
form a distinct physical system, it is probably at approximately the 
same distance from the sun as the stars of corresponding color and 
apparent magnitude in the galactic cloud. In fact, the study of colors 
and magnitudes tends to confirm Barnard's inference, based on general 
appearances, that the cluster may be nothing more than a nucleus of 
the extensive surrounding stellar masses.^ In this similarity to its 
environment, therefore, Messier 11 is vastly different from a globular 
cluster, such as Messier 13, which is apparently much more distant 
than the non-cluster stars in the same part of the sky.^ 
2. The distance of all the stars of Messier 11 is sensibly the same; 
hence apparent magnitudes represent relative absolute luminosities. 
Plotting the average color index against magnitude, as in figure 2, a 
striking progression of color with decreasing brightness is revealed. The 
neighboring stars, though showing much the same frequency of color 
class, are probably more scattered in space, and each interval of magni- 
tude includes stars varying greatly in distance and of most diverse 
spectral t3^es. The indefinite relation between magnitude and color 
index here observed is, therefore, not unexpected. 
3. Accepting the present results as dependable and taking color class 
and spectral type as closely analogous, the presence of the small and 
negative color indices in the galactic clouds indicates either that the 
stars are at a great distance or that they are not similar in luminosity 
to the brighter stars near the sun. In the light of recent researches on 
the dispersion of absolute magnitudes of B and A-type stars, the first 
alternative is decidedly preferable. The parallax of a typical B-type 
star of apparent magnitude 13.5 would be about 0^0002, a value that 
may be taken as a first estimate of the distance of Messier 11. The 
Z>-class stars in the surrounding clouds are between magnitudes 13 and 
15 and some are possibly fainter — suggesting still greater distances for 
parts of the stellar background. 
4. Just as a wide range of color class and the presence of negative 
