4.4 Graphical Representation of the Classification

Visualization of the classification system has always been useful. The 1936 Hubble system is illustrated in the well-known tuning-fork diagram shown in Figure 2, taken from The Realm of the Nebulae. ⁽⁴⁾

A more complicated diagram is needed to illustrate the separation of each branch of the tuning fork into the r and s varieties. A useful representation proved to be a three-dimensional figure such as Figure 3, which is taken from Hodge's (1966) version of a less complete diagram given in the Hubble Atlas (p. 26). Mixed varieties rs and transition types SAB are not readily accommodated in this representation, but could form the other sides of the figure for the rs and sr varieties, and would fill the interior of the volume for the mixed SAB types if this representation were carried further.

Figure 2. Hubble's original tuning-fork diagram as published in 1936 in his Realm of the Nebulae.

The extension was made by de Vaucouleurs (1959a) in a remarkable generalization of the entire scheme by filling the interior of the ``classification volume'' with both the r and s varieties and the A, B, and AB families. The basic linear sequence of classes is then separated along the axis of the volume. Figures 4 and 5 illustrate the revision, and are taken from de Vaucouleurs (1959a).

Figure 3. First stage of development of the concept of the classification volume. Here, the ordinary and barred families are separated onto opposite sides of a box. Within each family, a separation is made into the r and s strains, depending on whether the arms start from a ring or from the nucleus. Transition cases between the barred and ordinary families are not formally recognized in this visualization, but would fit in the interior of the box, a concept that leads into de Vaucouleurs's generalization shown in figs. 4 and 5. (Rendering of a diagram from Hodge 1966.)

Figure 4. The classification volume of de Vaucouleurs. The division into gross types is made along the axis of the figure from left (E) to right (Sm),the division into the ordinary and the barred family is by position on the surface (from de Vaucouleurs 1959a).

Figure 5. A cross-section of fig. 4 near the region of the Sb and SBb spirals, showing the manner in which the transition cases between ordinary (A) and barred (B) families, and the (r) and (s) strains can be accommodated (from the Reference Catalogue of Bright Galaxies, G. and A. de Vaucouleurs 1964).

⁴ Some history can be read from Figure 2. The revision of criteria and the redistribution of the S0, Sa, and SBa types (Section 3.3) had not yet been made by Hubble when this diagram was prepared. This is shown by the nature of the galaxy labeled SBa, which has no spiral arms and would be a modified S0 type in modern notation. Consequently, Hubble isolated the S0 class only after 1936. Back.