340 



SHAPLEY— STAR CLUSTERS. 



Fig. I. The Globular Cluster, Messier 13. 



Light, traveling with its hardly conceivable velocity of 300,000 kilo- 

 meters in a second of time, requires eight minutes for the passage 

 from the sun to the earth, but it must travel more than two thou- 

 sand million times as long to reach us from this globular cluster. 

 Hence our knowledge of the position and physical characteristics 

 of Messier 13 does not refer to the system as it now exists. Our 



