SHAPLEY— STAR CLUSTERS. 343 



Although these cluster stars are gigantic when compared with 

 the sun, and are highly concentrated into a compact and symmetrical 

 organization, they do not differ in physical properties, so far as we 

 now can tell, from hundreds of isolated stars of the Galaxy. Many 

 of the naked eye stars equal them in light power, in color, in volume. 

 The Cepheid variables in the cluster have light-curves, color varia- 

 tions, and spectra like those of the variables near the sun. In other 

 words, the members of clusters are normal stars. 



Until recently the globular clusters have been accepted as spheri- 

 cal in shape. When projected on the sky we should therefore ex- 

 pect them to be circular in outline, and, except for accidental errors 

 of distribution, the number of stars should be the same whatever the 

 direction from the center. A systematic study of the photographs 

 at Mount Wilson has shown, however, that a majority of globular 

 clusters, as seen in the sky, are slightly but symmetrically elongated. 

 This condition has been interpreted as a flattening of the cluster 

 system — an indication that the clusters are not spheres but rather 

 are oblate spheroids or ellipsoids. Messier 13 is one of the most 

 flattened ; and though the elongation, in the direction indicated in 

 the picture by inclined white lines, can be micertainly seen, either 

 visually or on photographs of the cluster, it is very readily shown 

 by counts of the individual stars. There are about thirty per cent 

 more stars in the direction of elongation than at right angles thereto. 



The flattening suggests that this great stellar system may be in 

 rotation about its shorter axis. Observations have not as yet deter- 

 mined whether or not such a motion exists. It is known, however, 

 from spectroscopic work at the Lowell Observatory and at Mount 

 Wilson, that the cluster as a whole is moxing with the high velocity 

 of two or three hundred kilometers a second. Noting that the mass 

 of the whole cluster is probably in excess of 100,000 suns, we appre- 

 ciate that the momentum of this moving cosmic unit must be exceed- 

 ingly great. 



The only component of the motion of Messier 13 now known is 

 directed toward the earth and toward the greatly extended strata of 

 stars that constitute the galactic system. If the cluster moves con- 

 stantly with this velocity, it will have reached the galactic plane 

 within fifty million years, coming from its present isolation in space 



