344 SHAPLEY— STAR CLUSTERS. 



to the regions where scattered galactic stars are numerous and where 

 all the open clusters are found. 



* The foregoing analysis of a single typical globular system indi- 

 cates the relevancy of the investigation of clusters to general stellar 

 problems. The factor that contributes most to knowledge of the 

 sidereal universe naturally is that of distance, for distance is the 

 key to the actual dimensions of a cluster and to the real luminosity 

 of stars. We directly approach problems of even greater interest, 

 however, when the distance is determined not only for Messier 13 

 but also for the 85 other globular systems that are now recorded in 

 our catalogues. Those problems are the size of the known stellar 

 universe, and the arrangement and relationships of its various parts. 



Probably the most interesting of many results deduced from the 

 study of the aggregate of open and globular clusters is the evidence 

 of the insignificance of the earth, sun, and brighter stars in the gen- 

 eral galactic system. We are impressed with the shortness of the 

 sidereal distances commonly measured as compared with those of 

 clusters, Cepheid variables, and star clouds. The sun, it appears, is 

 only a yellow, dwarfish, very old star, eccentrically situated in a 

 large moving star cluster which is itself situated still more eccen- 

 trically in an immensely larger stellar organization — that is, in the 

 general galactic system, which appears to include practically all 

 known objects of the stellar universe. 



The details and general results of the investigation of the dis- 

 tances and distribution of all globular clusters cannot be mentioned 

 here ; but we may conclude and briefly summarize by stating that 

 the study indicates that in volume the galactic system is more than 

 a hundred thousand times as large as we formerly believed it to be. 

 The center of the great ellipsoidal system appears to lie in the di- 

 rection of the rich star clouds of Sagittarius, at a distance of at least 

 sixty thousand light-years. Its most striking feature, besides its 

 dimensions and probable mass, is the extensive, much flattened, mid- 

 galactic segment, which contains open clusters, isolated stars, and 

 nebulae in abundance, but appears to be empty of globular clusters. 

 Apparently the globular clusters are approaching this segment from 

 without — their radial motions, their distribution in space, and their 



