The de Vaucouleurs Revised Hubble Classification System

De Vaucouleurs' ideas on galaxy morphology were greatly influenced by the exhaustive survey (de Vaucouleurs 1956) he made of southern galaxies at Mount Stromlo Observatory between 1952 and 1955. During this short period, he obtained blue-light photographs of 460 galaxies south of declination -35 $^{\circ }$ , including 210 galaxies from the landmark Shapley and Ames (1932) catalogue. It was de Vaucouleurs' ambition to completely revise the Shapley-Ames catalogue by collecting as much information as possible on types, diameters, magnitudes, colors, etc., and by systematizing this information in ways that were homogeneous and standardized.
De Vaucouleurs' revised Hubble classification is a three-dimensional system (Figure 1) whose long axis has the main classes arranged in the order ellipticals E, lenticulars S0, spirals S, and irregulars I. Each of the first three classes was divided into stages, so that the full arrangement of types along the long axis followed the order and notation E, E⁺, S0^-, S0 $^{\circ }$ , S0⁺, Sa, Sb, Sc, Sd, Sm, and Im, the superscripts + and - denoting ``late'' and ``early'', respectively. The main spiral stages were found to be sufficiently broad that de Vaucouleurs combined notation for transition stages, such as Sab, Sbc, Scd, etc.
Figure 1. De Vaucouleurs Revised Hubble Classification System
The other two dimensions in the classification volume reflect the diversity in the morphology at a given stage. They specify what de Vaucouleurs called family and variety, which relate to the presence or absence, respectively, of a bar and of an inner ring. The volume highlights how bars and rings are not absolute characteristics, but their degree of presence or distinguishability varies in a continuous way between the extremes of true barred or non-barred spirals and true ringed and non-ringed spirals (see Figure 2). In order to properly recognize spirals intermediate between the pure barred and non-barred cases, de Vaucouleurs introduced modified notation of the original Hubble system: true barred spirals would still be denoted SB, but the non-barred objects would be denoted SA in order to allow the composite symbol SAB to be used for objects with intermediate bar characteristics (like M83). De Vaucouleurs (1963) highlighted the continuous nature of the family characteristic further by underlining the dominant characteristic: S $\underline{\rm A}$ B was used for the most weakly-barred galaxies (most like SA systems) while SA $\underline{\rm B}$ was used for near-barred spirals (most like SB systems). The same kind of notation was used for variety: pure ringed systems are denoted (r) while pure spiral (s-shaped) systems are denoted (s), with objects of intermediate characteristics denoted using the compound notation ( $\underline{\rm r}$ s), (rs), and (r $\underline{\rm s}$ ). De Vaucouleurs recognized these same families and varieties among lenticulars.
To avoid making the classification system too complicated, we will not, in general, use the underline categories in the main part of the atlas. Only the broad categories A, AB, B and r, rs, s will be used. Fitting galaxies into these broad categories is straightforward, while the underline categories necessitate a consistency that would be difficult to achieve even though we fundamentally agree with the reality of these categories. However, we plan to devote a few pages to illustrating the underline categories using clearcut cases, to highlight de Vaucouleurs's view of the complete continuity of the family and variety characteristics.
Figure 2. Examples of galaxies having different families and varieties in the de Vaucouleurs revised Hubble classification system.
In addition to inner rings, de Vaucouleurs classifications also recognize features known as outer rings. These large, low surface brightness rings typically have a diameter twice that of the bar and are symbolized by (R) preceding the type symbol. Although relatively common among early-type barred spirals, outer rings are in general rare among the galaxian population. Many galaxies, however, show an outer pseudoring made of outer spiral arms that close on each other. De Vaucouleurs recognized these with the symbol (R $^{\prime}$ ) preceding the type symbol. The domain of outer pseudorings tends to be intermediate to late-type spirals.
De Vaucouleurs' 3D revised classification system is essentially an extension of Hubble's original tuning fork diagram. For various reasons the volume is not cubical or rectangular, but is narrow at the ends because bars and rings are not found among E systems and because rings are practically non-existent at the irregular stage Im. On the other hand, at the latest stages, many galaxies still show a bar. Near the transition stage between spirals and S0s, denoted S0/a, the classification volume tends to be much broader than at the ends because here the full range in recognizable family and variety is possible.

The main implication of de Vaucouleurs' system is continuity in all dimensions so that, in principle, a galaxy could be specified by three coordinates in the system (with other coordinates possible if one allows for outer rings and pseudorings, for example). De Vaucouleurs viewed the use of cells merely as a convenience for the cataloguer and classifier, but favored quantitative techniques over these cells when such techniques became available.