Hubble [53] introduced S0 galaxies as a morphological transition case between ellipticals and early-type spirals. A detailed description of these lenticular galaxies and a possible sub-classification scheme is given in [157]; see the table below. Others [9, 196] have suggested that S0's actually run parallel to the ordinary spiral stages but form one extreme of gas-depleted disk systems (S0a, S0b, S0c) coupled to the gas-rich spirals by yet another intermediate sequence of ``anaemic'' spirals (Aa, Ab, Ac). de Vaucouleurs [27] suggests that lenticulars may be subdivided as well, and his ``early-type'' and ``late-type'' classification is also outlined below.
| Type | Definition | Examples |
|---|---|---|
| (a) de Vaucouleurs [27] | ||
| S0- | Traces of structure can be found in the smooth lens and envelope; a small nucleus may be present. | NGC
5273 NGC 7166 |
| S0° | A weak trace of a ring appears at the edge of the lens; a distinct nucleus and envelope may be present. | NGC 1553 NGC 4459 |
| S0+ | A well defined ring is present separating the inner nuclear bulge from outer incipient spiral structure. | NGC 2855 NGC 7702 |
| (b) Sandage [157] | ||
| S01 | The existence of an outer envelope, flattened to a fundamental plane, defines this sub-type; the ellipticals of the central section of such galaxies are flatter than E7. | NGC 1201 NGC 3245 NGC 4762 |
| S02 | The first appearance of a circular absorption pattern or a true depletion of material in the envelope defines this intermediate sub-type. | NGC 0542 NGC 3065 NGC 4111 |
| S03 | A strong internal circular ring defines this extreme of the S0 class. | NGC 5866 NGC 3032 NGC 4459 |
Note: There are in fact many non-ringed S0° and S0+ galaxies also, see Table 3 (pg.35) in [29].