GENERAL CLASSIFICATION
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.”