332 
The Great Nebulae 
[ch. XIII 
edge on, shew a far greater degree of flattening than the galactic system. 
Moreover, although the outer regions of the nearer spiral nebulae can be 
and submitted to the same telescopic power, obstinately defy resolution, as do- 
all spherical and elliptical nebulae. Plate VIII shews the central regions of the 
Andromeda nebula, and there is no suspicion of resolution into separate stars 
here. Finally, although a study of the superficial appearance of the great nebulae 
298. The great nebulae exhibit an enormous difference of structural 
detail, but Hubble, who has devoted much skill and care to their classification*, 
finds that most of the observed forms can be reduced to law and order. They 
exhibit no spiral arms, and true “ spirals,” consisting of a rather vaguely- 
defined central region from which two spiral arms emerge. Hubble finds that 
the true spiral forms, as regards their main features at least, can be specified 
in terms of three distinct quantities, which he describes as (l)the relative size 
of the unresolved nuclear region, (2) the extent to which the arms are un 
wound (the openness or angle of the spiral), (3) the degree of condensation in 
the arms. When these quantities are estimated or evaluated observationally for 
different nebulae, they are found to vary very approximately in unison, with 
the result that practically all observed nebular configurations can be arranged 
to form linear series. 
Hubble finds that it is not possible to place all observed nebulae in one 
continuous sequence; their proper representation 
demands a Y-shaped diagram such as is shewn in 
nebulae for which b is about 0'3a, f his being the 
Fig. 53. Diagrammatic repre- £ reatest inequality of axes observed in the “ ellip- 
resolved into stars, the central regions, although of course at the same distance 
may suggest that they are star-clouds, and nothing but star-clouds, we shall find 
that a theoretical study gives no support to this view. 
The Classification of the Great Nebulae. 
fall primarily into the two great classes of nebulae of regular shape which 
The lower half of the Y is formed by nebulae of 
approximately elliptical or circular shape. These 
are subdivided into eight classes, designated E 0, 
El, E 2,... E 7, the numerical index being the- 
Fig. 53. 
greatest and least diameters of the nebulae as pro 
jected on the sky. Thus E 0 consists of nearly 
circular nebulae, (6 > 0'95a), while E 7 consists of 
configurations of the Great 
Nebulae. 
sentation of the observed tical ” nebulae. Examples of typical nebulae are 
shewn in the upper half of Plate IX (p, 334). 
Astrophys. Journ. lxiv. (1926), p. 321 and The Observatory , l. (1927), p. 276.