338 SHAPLEY— STAR CLUSTERS. 



to the Hyades — a more compact and more definitely circumscribed 

 dynamical system — and then to the Pleiades, to Prsesepe, to the 

 double cluster in Perseus and similar faint loose clusters of the 

 Milky Way; thence, by way of Messier ii and Messier 22, we pro- 

 ceed by nearly equal steps to the typical globular systems exemplified 

 in the great Hercules cluster, Messier 13. 



Although we may justly restrict the term " star cluster " to physi- 

 cal systems — that is, to groups which have the characteristics of 

 distinct dynamical organization — it is clear that the svibdivisions of 

 the long sequence of groups from Orion to the Hercules cluster 

 must necessarily be vaguely defined. For convenience we here dis- 

 tinguish only open and globular clusters, and designate all as open 

 except those seventy or eighty highly condensed groups whose stars 

 appear innumerable even with the aid of our biggest telescopes and 

 most sensitive photographic plates. 



Open and globular clusters differ in matters other than richness 

 and apparent circularity. In average distance from the earth the 

 globular clusters much excel, in stellar constituency they are more 

 varied, and we recognize in their wide spatial distribution that from 

 a dynamical point of view the globular clusters are quite distinct 

 from the open groups which closely congregate along the middle line 

 of the Milky Way. 



A few of the nearest globular clusters are visible to the unaided 

 eye as faint hazy objects, and some of them have been in the astro- 

 nomical records for two or three hundred years. To Messier and 

 earlier observers they were known only as starless nebulosities, but 

 Sir William Herschel and his son, with their greater telescopes, par- 

 tially or completely resolved the brighter clusters into myriads of 

 distinct stars. 



The great telescopes of the present time and the powerful modern 

 methods of astrophysical investigation have greatly extended our 

 knowledge of globular clusters, but they have not appreciably added 

 to the total number. The numbers of known stars and nebulas have 

 increased enormously with the increase of optical power, but during 

 the last eighty years less than five new globular clusters have been 

 added to the original lists compiled by the Herschels. In fact, we 

 seem to have passed the era of discovery of such systems; the pres- 



