18 
The Astronomical Survey of the Universe [ch. i 
substantially more than 1000 parsecs from the sun. Seares considers that its 
diameter may be from 60,000 to 90,000 parsecs, and determines the direction 
of its centre as galactic latitude 319°. 
This is in substantial agreement with Shapley’s determination of the 
system defined by the “globular clusters” (§ 27). He finds that this system 
lies approximately in the galactic plane, its maximum diameter being about 
75,000 parsecs, and its centre lying about 20,000 parsecs away in galactic 
longitude 325°. Oort* finds that the radial velocities of faint stars suggest 
orbital motion about a centre some 6000 parsecs distant in galactic longitude 
324°, while from a similar study of 0 and B type stars, J. S. Plaskettj* finds 
evidence of orbital motion about a centre in galactic longitude 3245°. 
The Great Nebulae. 
16 . Within the limits of distance set by the Milky Way lie not only the 
stars we have just discussed, but also an abundance of non-stellar objects. 
For instance, the irregular nebulae, such as the great nebula in Orion (see 
Plate V) are found to be comparatively near objects, most if not all of which 
lie within the confines set by the galaxy. 
In addition to these the telescope reveals a great number of nebulae of 
regular shapes—circular, elliptical, spindle-shaped and spiral. Two of these, 
the great nebula M 31 (N.G.C. 224) J in Andromeda and the nebula M 33 
(N.G.C. 598) in Triangulum, are of outstanding brightness and apparent size, 
both being visible to the naked eye. 
The former of these nebulae is shewn in Plate I, enlargements of its central 
portion and of an outlying region (top left-hand corner) being shewn in 
Plates VII and VIII below. The nebula in Triangulum is shewn in Plate XI. 
These nebulae appear to be thin discs in shape; when seen edge-on they 
present the characteristic appearance exemplified by nebula N.G.C. 981 
shewn in Plate II (see also other examples in Plate XIII). 
The distances of the two nebulae M 31 and M 33 are revealed by the 
circumstance that they are found to contain Cepheid variables. Calculating 
their distances in the manner already explained, Hubble has estimated 
the distance of M 31 § as 285,000 parsecs, and that of M 331| as 263,000 
parsecs. By the same method he estimates the distance of the star-cloud 
N.G.C. 682211 to be 214,000 parsecs. 
These figures amply shew that these nebulae and star-clouds are quite 
outside our system of stars; they constitute what Herschel described as 
* Bulletin of the Astronomical Institute of the Netherlands, No. 120 (1926). 
t Monthly Notices of the R.A.S. lxxxviii. (1928), p. 395. 
t The prefix M to a nebula precedes its number in Messier’s Catalogue of Nebulae; the prefix 
N.G.C. similarly refers to Dreyer’s New General Catalogue (Mem. lloyal Ast. Soc. 49 (1888), 
Part I). 
§ Popular Astronomy, xxxm. (1925), No. 4 or The Observatory, xnvrn. (1925), p. 139. 
|| Astrophys. Journal, lxiii. (1926), p. 236. H Ibid. lxii. (1925), p. 409.