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Figure 1.3. The de Vaucouleurs (1959) revised Hubble–Sandage (VRHS) system of galaxy classification, where the Hubble “tuning fork” has become a classification volume. Top: The VRHS stage sequence. Bottom left: The cross-section of families and varieties. Bottom right: Examples of families and varieties at stage Sbc (from Buta et al. 2007)

The basic outline of the HS system (Sandage 1961) is depicted in Figure 1.2 and that for the VRHS system is depicted in Figure 1.3. In the HS system, Sandage added the (r) and (s) subtypes, which are also used in the VRHS. The HS system still depicts galaxy morphology as a “tuning fork”, but is more complicated than Hubble’s original tuning fork (Figure 1.1). In the VRHS, the tuning fork is replaced with a classification volume whose long axis is the stage (E..S0..Sa, Sb..Im, etc.), and whose short axes are the family (SA, SAB, SB) and the variety [(s), (rs), (r)]. The use of “SAB” to denote galaxies having a bar of intermediate apparent strength and “(rs)” for partial inner rings (pseudorings) are major hallmarks of the VRHS and CVRHS systems. Not depicted in the volume are outer rings, large structures about twice the size of a bar in barred galaxies and nuclear rings, much smaller structures about one-tenth the size of a bar in barred galaxies. The appearance of the volume with a broad middle section and narrower ends is intended to depict how the diversity of families and varieties varies with stage. The greatest diversity is found near stage S0/a, while the diversity diminishes by stages S0 and Im either because bars become difficult to recognize or because rings are simply not present.

The CVRHS is defined by the same kind of classification volume as the VRHS, but with a more finely detailed division into various subtypes. Figure 1.4 shows a cross-section of types through any CVRHS stage from S0° to Sm. The arrangement of families and varieties is the same as in the VRHS, except that de Vaucouleurs’ vision of continuity is carried further with the use of underline notation that he had applied mainly in his 1963 paper, “Revised Classifications of 1500 Bright Galaxies”. The family sequence became SA, SAB, SAB, SAB, SB and the variety sequence became (s), (rs), (rs), (rs), (r). However, in order to recognize outer and nuclear rings in the system, the CVRHS system denotes the presence or absence of an inner feature as the “inner variety”, the presence or absence of an outer feature as the “outer variety”, and the presence or absence of a nuclear feature as the “nuclear variety”. The term “inner feature” now refers to inner rings and lenses, not just inner rings, and the same is true for outer and nuclear features (the latter also including nuclear bars). The recognition of outer pseudorings (R′) is also one of the hallmarks of the VRHS and CVRHS systems. The central subtype in Figure 1.4, (R)SAB(rs,nr), is meant to convey the generalization of VRHS varieties to include inner, outer and nuclear varieties.