About 3 per cent of the extra-galactic nebulae lack both dominating
nuclei and rotational symmetry. These form a distinct class which can
be termed “irregular.” The Magellanic Clouds are the most conspicuous
examples, and, indeed, are the nearest of all the extra-galactic
nebulae. N.G.C. 6822, a curiously faithful miniature of the Clouds,
serves to bridge the gap between them and the smaller objects, such as
N.G.C. 4214 and 4449. In these latter, a few individual stars emerge
from an unresolved background, and occasional isolated spots give the
emission spectrum characteristic of diffuse nebulosity in the galactic
system, in the Clouds, and in N.G.C. 6822[12] These features are found
in other irregular nebulae as well, notably in N.G.C. 1156 and 4656,
and are just those to be expected in systems similar to the Clouds but
situated at increasingly greater distances.
The system outlined above is primarily for the formal classification of
photographic images obtained with large reflectors and portrait lenses.
For each instrument, however, there is a limiting size and luminosity
below which it is impossible to classify with any confidence. Except
in rare instances, these small nebulae are extra-galactic, and their
numbers, brightness, dimensions, and distribution are amenable to
statistical investigation. For cataloguing purposes, they require a
designating symbol, and the letter “Q” is suggested as convenient and
not too widely used with other significations.
[Illustration: PLATE XIV Irregular Nebula N.G.C. 4214
Direct photograph with 100-inch reflector March 18, 1925.
Scale 1 mm = 3″.3
Slitless spectrogram at primary focus of 100-inch reflector,
March 19, 1925]