342 SHAPLEY— STAR CLUSTERS. 



and because of the great numbers of stars available, we can make 

 these investigations with more ease and accuracy in a globular clus- 

 ter than with the bright stars around the sun. 



A conception of the great size of a globular cluster may be gained 

 by indicating on the picture of Messier 13 some of the familiar dis- 

 tances of the solar neighborhood. The distance separating the near- 

 est known star (Alpha Centauri) from the sun is indicated by the 

 short black line near the center. In the cluster, within the corre- 

 sponding distance from the center, there appear to be thousands of 

 stars, illustrating how greatly different from the conditions of our 

 own immediate part of the universe is the concentration of mass 

 and luminosity in a globular system. The distance to the Hyades, 

 the well-known group of bright stars in Taurus, is represented on 

 the photograph by the distance from the center of the cluster to the 

 star marked H. The diameter of the large circle is about fifteen 

 minutes of arc on the surface of the sky, corresponding at the dis- 

 tance of the cluster to ten million astronomical units ; a sphere of 

 that diameter with the sun at the center would include all the stars 

 within eighty light-years. 



The total angular diameter of the cluster is about thirty-five 

 minutes of arc, corresponding to twenty-three million astronomical 

 units, or more than three hundred and fifty light-years. 



All the cluster stars shown on the photograph are giants in 

 actual luminosity. At the cluster's distance of 36,000 light-years, 

 a star as bright as our sun would be considerably fainter than the 

 twentieth magnitude and would not appear on this reproduction. 

 The three stars whose images are enclosed in small circles are pho- 

 tographically almost exactly one hundred times as bright as the 

 sun, and the most luminous giants in the cluster exceed a thousand 

 suns in light emission. 



It is a remarkable fact that the brightest of these cluster stars are 

 red and the fainter ones are blue. The red stars, because of low 

 surface temperature, emit much less light for a given amount of sur- 

 face area. To excel in brightness, therefore, their volumes must 

 be enormously large — in extreme cases more than a hundred thou- 

 sand times that of the sun, and, since stellar masses in general are 

 probably not very unequal, the mean densities of these red giants 

 are correspondingly small. 



