The Hubble tuning fork diagram

The Hubble classification system recognizes three form families: ellipticals (E), spirals (S) and irregulars (Ir). The ellipticals are assigned an ellipticity ∈ defined as ∈=10(a –b)/a, in which a and b are the major and minor image diameters, respectively. Classification types for ellipticals range from E0, for objects that appear circular in projection, to E7 for the most highly flattened ellipticals. Spiral galaxies occur in two flavors – normal spirals (S), and barred spirals (SB). Within each of these there are three stages: Early-type galaxies of stage Sa/SBa have large nuclei and tightly coiled (and usually rather smooth) arms, objects in stage Sb/SBb have a more open spiral structure, and smaller central bulges. Late-type galaxies in stage Sc/SBc have small nuclear bulges and exhibit wide-open and rather patchy spiral arms. Finally, irregular galaxies have a patchy structure and exhibit no spiral arms. The original Hubble (1926) classification scheme was modified by Hubble (1936) who introduced a class of lenticular (S0) galaxies to span the chasm between spiral and elliptical galaxies (see Figure 2). The Hubble classification system is described and richly illustrated in The Hubble Atlas of Galaxies (Sandage 1961). Classifications for 1246 bright galaxies are given in A Revised Shapley–Ames Catalog of Bright Galaxies by Sandage & Tammann (1981). The Hubble/Sandage classification system reaches its ultimate form in The Carnegie Atlas of Galaxies (Sandage & Bedke 1994).