MOUNT WILSON SOLAR OBSERVATORY. 219 



Investigations op Star Clusters. 



The study of magnitudes in clusters by Mr. Shapley has led to a 

 number of results that indicate the advantage, for certain astrophysical 

 problems, of dealing with large stellar systems from a point outside 

 the sj^stem. Thus we may hope eventually to obtain fairly precise 

 information concerning the size and form of the typical clusters and the 

 distribution of stars within them. We may determine the relative 

 frequency of stars of different absolute luminosities and colors, as well 

 as the relationship of color classes to magnitude and to distance from 

 a center of attraction. Such results for clusters will be directly appli- 

 cable, at least in part, to similar problems in the general galactic system. 



The investigation of magnitudes and colors in the globular clusters 

 that contain typical short-period variable stars has suggested a method 

 of estimating, apparently with considerable accuracy, the relative dis- 

 tances of globular clusters. Since these objects are widely distributed 

 over one hemisphere and relatively are very remote, the method affords 

 an indication of the distance to which the visible stellar universe 

 extends. Although the periods, ranges, and maximum magnitudes of 

 the variables differ, their median magnitudes are constant for each 

 cluster, though they vary from one system to another. Moreover, the 

 median magnitude appears to be definitely related to the magnitudes 

 of the brightest stars in the cluster. If the obvious assumption that 

 differences in the median magnitudes are due entirely to distance is 

 correct, then not only can we derive relative parallaxes as soon as the 

 apparent magnitudes have been measured, but, with the derivation 

 of a value for the actual luminosity of such variables, we can also 

 obtain very accurate absolute distances for all clusters containing 

 typical variables, as well as for the isolated variables 

 of this class. 



In collaboration with Mr. Pease, an investigation has 

 been made of the distribution of stars in 12 globular 

 clusters. The main outcome is the discovery that an 

 elliptical arrangement of stars is a general characteristic 

 of photographs of such systems. Though rarely ap- 

 parent among the brighter stars, the elongation is 

 revealed when the star counts are extended to the 

 fainter objects and examined with respect to direction 

 from the center. For example, from one plate of 

 Messier 13 the number of stars for successive intervals 

 of position angle is as shown herewith. 



The two maxima in the series of numbers define the 

 axis of elongation where the stars are 30 per cent more numerous than 

 along the minor axis. A simple interpretation of the phenomenon is 

 that the stars in the so-called globular clusters are distributed through- 

 out a more or less flattened spheroid, whose projection on the celestial 



