25-27] 
Star Clusters 
25 
Globular Clusters. 
27 . In a somewhat different category from the moving clusters are the 
rather mysterious objects known as “globular” clusters. Whereas the moving 
clusters just discussed are of a pronouncedly flattened shape, the globular 
clusters appear at first sight to be strictly of the shape that their name 
implies. Bailey*, however, noticed a departure from actual spherical symmetry, 
and Pease and Shapley j* have found that five out of six clusters they studied 
in detail shewed definite departures from sphericity, being apparently of a 
flattened or spheroidal form. A typical globular cluster, the cluster M 13 in 
Hercules, is shewn in Plate III. 
Whereas the moving clusters are intermingled with the main mass of the 
stars, the globular clusters are so remote as to be either entirely outside our 
system of stars or at least in regions where stars are extraordinarily sparse. 
It has already been mentioned that Shapley has measured their distances 
by the use of the “period-luminosity” law which is obeyed by the Cepheid 
variables contained in them, and finds distances varying from 6500 parsecs for 
&) Centauri to 67,000 parsecs for N.G.C. 7006J; their diameters are all of 
the order of 150 parsecs (490 light-years). Their distribution in the sky is 
peculiar and surprising; Hinks § found that they are practically confined to one 
hemisphere of the sky, while Melotte|| further found that more than half lie 
within 30° of one point in the sky, namely, the point in the galactic plane of 
galactic longitude 325°. Shapley 11 has shewn that they lie within an ellipsoidal 
volume whose centre is at a distance of 20,000 parsecs in this direction and 
whose major axis is about 75,000 parsecs. The sun is near to one end of the 
major axis, which explains why the globular clusters all appear to lie in one- 
half of the sky. 
Slipher has measured the yelocities of the clusters spectroscopically and 
finds that their radial velocities vary from a velocity of approach of 410 kms. 
a second to a velocity of recession of 225 kms. a second. The mean speed of 
these clusters is 150 kms. a second. The average radial velocities of approach 
or recession of the stars in the galactic system are only of the order of 10 kms. 
a second, so that the globular clusters move with far higher velocities than 
individual stars; on the other hand, as we shall shortly see, their velocities are 
substantially inferior to those of the spiral nebulae. After allowing for their 
different distances these globular clusters are found to be remarkably similar in 
structure and size. The law of density of distribution is found to be approxi 
mately the same in all; using counts of stars by Bailey**, Plummerj-f has 
* Harvard, Observatory Annals, lxxvi. No. 4. f Astrophys. Journal xlv. (1917), p. 225. 
X Ibid. xlviii. (1918), p. 154, or Mount Wilson Contribution, No. 151. 
§ Monthly Notices of the R.A.S. lxxi. (1911), p. 693. 
|| Memoirs of the R.A.S. lx. (1915), p. 176. U l.c. p. 169. 
** Harvard Observatory Annals, lxxvi. No. 4. 
ft Monthly Notices of the R.A.S. lxxvi. (1916), p. 107.