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Hubble classification
(hub -'l) A classification scheme for galaxies introduced by Edwin Hubble in 1925. Although more complex and refined schemes have since been devised the simple but slightly revised Hubble scheme is still widely used. The scheme recognizes three main types of galaxy: elliptical, spiral, and barred spiral. Each is divided into subtypes according to gross observable characteristics, the principle one being shape. These subtypes are arranged into a morphological sequence (see illustration).Elliptical galaxies are denoted by the letter E followed by a number from 0 to 7 that indicates the apparent degree of flattening; this number is the nearest integer to 10(1 – b /a ), where a and b are, respectively, the semimajor and semiminor axes of the observed profile. Thus E0 galaxies are almost circular in outline and E7, the most elliptical, have an a :b ratio of about 3:1.
The spirals (type S) and barred spirals (type SB) are separated into subtypes a, b, and c along a sequence that follows a progressive decrease in central bulge luminosity compared to the disk and an increase in the openness and prominence of the spiral arms. An additional subtype, d, has been introduced. Galaxies with intermediate characteristics can be classified as Sab, SBab, Sbc, SBbc, etc. The S0 galaxies, placed at the junction of the elliptical and spiral sequences, resemble the spirals in general shape but lack spiral arms. This type was predicted by Hubble in 1925 but identified only later.
The less commonly observed irregular galaxies were not included in Hubble's original diagram, but are now placed to the right of the S and SB sequences. They were originally divided into two types. Irr I galaxies lack both symmetry and a nucleus and are mainly comprised of blue population I stars and H II regions. Irr II galaxies were objects that could not otherwise be classified, possessing either a highly perturbed morphology or some prominent feature unusual for their class. The term irregular galaxy is now applied only to Hubble type Irr I, and Irr II systems are now usually reclassified as interacting or starburst galaxies as appropriate.