Morgan [127, 128, 129, 130] has produced a system for classifying the central concentration of light in a galaxy to give population groups which apparently correlate with the stellar content of the inner parts of the galaxy [130] as judged by integrated spectral types. These population groups a, af, f, fg, g, gk, and k imply early and late spectral-type stars, respectively, as contributing most of the light from the nucleus; however, the designation is found by inspecting the central concentration of monochromatic light: a galaxies have little or no central concentration while k galaxies are highly concentrated. In addition, Morgan identifies form families, which are explained in below; these correspond to the most basic classification, see above. A recent application of this classification system to southern galaxies is given in [46].
| Form family | Description |
|---|---|
| B | Barred spirals |
| D | Galaxies with rotational symmetry but showing neither spiral structure nor ellipticity |
| cD | Supergiant D galaxies, predominantly found in clusters [120] and embedded in an extensive halo |
| db | Dumb-bell systems |
| E | Ellipticals |
| Ep | Peculiar ellipticals containing conspicuous absorbtion patches |
| I | Irregulars |
| L | Low-surface-brightness systems |
| N | High-luminosity nucleus superimposed on a considerably fainter outer envelope, see also [129] |
| Q | Quasi-stellar objects |
| S | Ordinary spirals |
According to [120] dumb-bell galaxies are a group of objects allied to the D galaxies, in which two, separated, approximately equal nuclei are observed in a common envelope. They may well be related to galaxies that have one or more fainter components in their envelopes. Dumb-bell galaxies are then the extreme cases of very close multiple galaxies in which there are only two, but equal, components. A catalogue of dumb-bell galaxies can be found in [189].