Holmberg [51] first divided irregular galaxies into two types: Type I Irregulars are highly resolved systems similar to the Magellanic Clouds, while Type II Irregulars show a smoothed ``amorphous'' distribution of light, often broken by irregular patches of obscuration. Type I Irregulars have subsequently found a natural home as Im galaxies following the late spirals of class Sd and Sm [27 ]. Their star-formation history is summarized in [54]; while global radio, optical and spectral properties are reviewed in [42, 55]. On the other hand, Sandage and Brucato [159] have suggested that the original Irr II galaxies can be sub-divided, and that a single class should be abandoned. The properties of the Irr II class galaxies (also called I0 in [31]) have been summarized in [85]. Those few objects which show no spiral structure, but have unresolved disks, are now termed amorphous galaxies [159]. Examples are NGC 3077, NGC 5253, and M82.
| Morphological Type | (Hubble Revised) | Code t |
|---|---|---|
| Compact (high density) Ellipticals | cE | -6 |
| Dwarf (low density) Ellipticals | dE | -5 |
| Normal Elliptical Systems | E | -7 |
| Giant Ellipticals with extended optical coronae (in particular the Morgan type cD) | E+ | -4 |
| Lenticular Systems | L- L L+ | -3 -2 -1 |
| Irregular Systems of Type II | I0 | |
| Lenticular-Spiral Systems | S0/a | 0 |
| Spiral Systems | Sa Sab Sb Sbc Sc Scd Sd Sdm | 1 2 3 4 5 6 7 8 |
| Spirals of Magellanic Type | Sm | 9 |
| Irregular of Magellanic Type (=Irr of Type I) | Im | 10 |
| Compact Blue Irregulars (Isolated Extragalactic H II Regions) | cI | 11 |
Heidmann et al. [50] (above table) introduced a numerical code of the morphological types in the revised Hubble system. This code was extended and used in the Second Reference Catalogue of Bright Galaxies, RC2 ([8] in Secondary Galaxy Catalogs).
More detailed tables for coding of Hubble types, revised morphological types (de Vaucouleurs [27]), DDO (van den Bergh) types and Yerkes (Morgan) types etc. are given in the RC2.