Description of the system

In an attempt to accommodate the entire range of morphological characteristics of galaxies de Vaucouleurs (1959a) introduced a three-dimensional classification scheme which is illustrated in Figure 5. The main axis of this classification system is the sequence E–S0–Sa–Sb–Sc–Sd–Sm–Im, where the index m refers to magellanic, i.e. resembling the Magellanic clouds. Finer sub-divisions may be provided by distinguishing between E, E+, S0−, S0, and S0+, in which the minus superscript denotes early (= smooth) and the plus superscript indicates late (= patchy). In Figure 5 the second dimension is, as in the Hubble tuning fork diagram, provided by differentiating between galaxies with no bars (SA), those with weak bars (SAB) and those with strong bars (SB). Finally a third dimension is provided by distinguishing between objects that exhibit rings r, intermediate features rs and pure spiral arms s. De Vaucouleurs (1959a) notes that the distinction between his A and B families and between his r and s varieties is most clearly marked at the transition stage S0/a and vanishes between E and S0, and between Sm and Im. The position of a galaxy along the main axis from E to Im in Figure 5 correlates strongly with integrated color, and hence with the mean age of the stellar population. The A and B families do not differ systematically in color and hence, presumably, contain populations of comparable ages.