The classification used in the present investigation is essentially the detailed formulation of a preliminary classification published in a previous paper.[6] It was developed in 1923, from a study of photographs of several thousand nebulae, including practically all the brighter objects and a thoroughly representative collection of the fainter ones.[7] It is based primarily on the structural forms of photographic images, although the forms divide themselves naturally into two groups: those found in or near the Milky Way and those in moderate or high galactic latitudes. In so far as possible, the system is independent of the orientation of the objects in space. With minor changes in the original notation, the complete classification is as follows, although only the extra-galactic division is here discussed in detail:
CLASSIFICATION OF NEBULAE
| Symbol | Example | ||||||
|---|---|---|---|---|---|---|---|
| I. | Galactic nebulae: | ||||||
| A. | Planetaries | P | N.G.C. 7662 | ||||
| B. | Diffuse | D | |||||
| 1. | Predominantly luminous | DL | N.G.C. 6618 | ||||
| 2. | Predominantly obscure | DO | Barnard 92 | ||||
| 3. | Conspicuously mixed | DLO | N.G.C. 7023 | ||||
| II. | Extra-galactic nebulae: | ||||||
| A. | Regular: | ||||||
| 1. | Elliptical (n=1, 2, ..., 7 indicates the ellipticity of the image without the decimal point) | En | N.G.C. 3379 E0 N.G.C. 221 E2 N.G.C. 4621 E5 N.G.C. 2117 E7 | ||||
| 2. | Spirals: | ||||||
| a) | Normal spirals | S | |||||
| (1) Early | Sa | N.G.C. 4594 | |||||
| (2) Intermediate | Sb | N.G.C. 2841 | |||||
| (3) Late | Sc | N.G.C. 5457 | |||||
| b) | Barred spirals | SB | |||||
| (1) Early | SBa | N.G.C. 2859 | |||||
| (2) Intermediate | SBb | N.G.C. 3351 | |||||
| (3) Late | SBc | N.G.C. 7479 | |||||
| B. | Irregular | Irr | N.G.C. 4449 | ||||
Extra-galactic nebulae too faint to be classified are designated by the symbol “Q.”
REGULAR NEBULAE
The characteristic feature of extra-galactic nebulae is rotational symmetry about dominating non-stellar nuclei. About 97 per cent of these nebulae are regular in the sense that they show this feature conspicuously. The regular nebulae fall into a progressive sequence ranging from globular masses of unresolved nebulosity to widely open spirals whose arms are swarming with stars. The sequence comprises two sections, elliptical nebulae and spirals, which merge into each other.
Although deliberate effort was made to find a descriptive classification which should be entirely independent of theoretical considerations, the results are almost identical with the path of development derived by Jeans[8] from purely theoretical investigations. The agreement is very suggestive in view of the wide field covered by the data, and Jeans’s theory might have been used both to interpret the observations and to guide research. It should be borne in mind, however, that the basis of the classification is descriptive and entirely independent of any theory.
Elliptical nebulae.—These give images ranging from circular through flattening ellipses to a limiting lenticular figure in which the ratio of the axes is about 1 to 3 or 4. They show no evidence of resolution,[9] and the only claim to structure is that the luminosity fades smoothly from bright nuclei to indefinite edges. Diameters are functions of the nuclear brightness and the exposure times.
PLATE XII
