An Etymological Dictionary of Astronomy and Astrophysics

A widely used scheme for classifying galaxies which is an extension to the → Hubble sequence Hubble sequence. De Vaucouleurs introduced a more elaborate classification system for spiral galaxies (→ spiral galaxy) based on three morphological characteristics:

1. Bars. Galaxies are divided on the basis of the presence or absence of a nuclear → stellar bar. SA denotes spiral galaxies without bars, complementing Hubble’s use of SB for → barred spirals. De Vaucouleurs also defined an intermediate class, SAB, containing weakly barred spirals. → Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not.
   

2. Rings. Galaxies possessing ring-like structures are denoted (r), those without rings (s), and transition galaxies (rs).
   

3. Spiral arms. These are divided into the following subclasses: Sd (SBd), that is diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central → bulge;

Sm (SBm): irregular in appearance; no bulge component; and Im, that is highly irregular galaxy.

Galaxies in the classes Sm and Im are termed the → Magellanic spirals and irregulars, respectively. The → Large Magellanic Cloud is of type SBm, while the → Small Magellanic Cloud is an irregular (Im).

See also: → de Vaucouleurs radius; → system.