A widely used system for classifying galaxies according to their visual appearance, illustrated on the tuning-fork diagram. The sequence is based on three criteria: the relative sizes of the central bulge of stars and the flattened disk; the existence and character of spiral arms; and the resolution of the spiral arms and/or disk into stars and H II regions. The system was originated by E. P. Hubble.
The sequence starts with round elliptical galaxies (E0) showing no disks. Increasing flattening of a galaxy is indicated by a number which is calculated from 10 (a − b)/a, where a and b are the major and minor axes as measured on the sky. No elliptical is known that is flatter than E7. Beyond this a clear disk is apparent in the lenticular or S0 galaxies. The classification then splits into two parallel sequences of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral types are subdivided into Sa, Sb, Sc, Sd (SBa, SBb, SBc, SBd for barred spirals). With each successive subdivision the arms become less tightly wound (but more easily resolvable into stars and H II regions), and the central bulge becomes less dominant. Two types of irregular galaxy are defined. Irr I galaxies show rather amorphous, irregular structure with perhaps a hint of a spiral arm or bar, and can be placed at the far end of the spiral sequence. Irr II galaxies are sufficiently unusual to defy assignment to any of the other types, although this category encompasses only about 2% of bright or moderately bright galaxies in the nearby Universe. The original, erroneous idea that the sequence might be an evolutionary one led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies.
Colour and amount of interstellar material vary systematically along the Hubble sequence: ellipticals are red and contain little interstellar gas or dust, whereas late spirals and Irr I galaxies are blue, with significant amounts of interstellar material. The relatively faint dwarf spheroidal galaxies (see dwarf galaxy) were not recognized as a separate type in the Hubble classification. Some variants of the Hubble classification use plus and minus signs to subdivide classes, so that Sa+ is later than Sa, but earlier than Sb−.
Subjects: Astronomy and Astrophysics.