SHAPLEY— STAR CLUSTERS. 339 



ent lists may be considered essentially complete — a condition that 

 does not prevail for any other important type of celestial object. 



It has been shown through the studies made at Mount Wilson 

 that most of the globular clusters are remotely isolated systems, 

 neither intermingled with nor closely surrounded by other stars. 

 They may be treated, indeed, as distinct cosmic units ; and in treat- 

 ing them as such, we may fulfil the purpose of the present commu- 

 nication by discussing briefly a single system. In certain details, 

 there are, to be sure, conspicuous differences from system to system, 

 but in such matters as size, number of stars, and stellar make-up, 

 no greater diversity appears. 



The Great Cluster in Hercules, No. 13 in Messier's well-known 

 compilation of 103 bright nebulae and clusters, is the system chosen 

 for the present illustration. To the unaided eye it is faintly visible 

 as a hazy star of magnitude 5.8, about two degrees south of Eta 

 Herculis. The photograph used for this illustration was made by 

 Professor Ritchey on a plate of medium rapidity with an exposure 

 of eleven hours ; it records something like 30,000 stellar images 

 brighter than the twenty-first magnitude. Nearly all of these are 

 actual members of the cluster and not merely objects of the fore- 

 ground, projected among the cluster stars. 



The distribution, brightness, and colors of many hundreds of 

 the stars in Messier 13 have been specially studied at Mount Wilson. 

 Some attention has also been given to spectral types and radial ve- 

 locities — difficult problems even for large telescopes and powerful 

 spectrographs because of the extreme faintness of the individual 

 stars. Space cannot be given here to describe the methods recently 

 developed for the determination of the distance of Messier 13 and 

 other globular clusters ; we shall only remark that photometry, as- 

 trometry, and spectroscopy are all involved, and that Cepheid vari- 

 able stars play a fundamental role. The adopted value of the par- 

 allax for the Hercules cluster is o". 00009, with an estimated uncer- 

 tainty of less than twenty per cent. 



Even to those who are accustomed to think of the great depths 

 of sidereal space, it is difficult to comprehend clearly the remoteness 

 of the Hercules cluster. Its distance, 3.5 X 10^^ kilometers, is more 

 than eight thousand times that of the nearest star now known. 



