Various schemes have been devised to bring order to the galactic zoo by pigeonholing galaxies according to one or more properties, including shape, spectrum, and luminosity. A few are listed below but there are many others, including ones that specialize in radio galaxies or unusual systems.
Hubble classification of galaxies
Morgan classification of galaxiesA scheme invented by William Morgan that uses the integrated spectrum of the stars of a galaxy together with its shape (real and apparent) and its degree of central concentration. It specifies the galactic spectral type, a, af, f, fg, g, gk, or k (corresponding to the integrated stellar types); the form, type S (spiral), B (barred spiral), E (elliptical), I (irregular), Ep (elliptical with dust absorption), D (rotational symmetry without pronounced spiral or elliptical structure), L (low surface brightness), or N (small bright nucleus); and the inclination to the line of sight, from 1 (face-on) to 7 (edge-on). For example, the Andromeda Galaxy is classified as kS5.
de Vaucouleurs-Sandage classification of spiral galaxiesSA (ordinary spirals), SB (barred spirals): then in parentheses a lower case s (for S-shaped spirals) or r (for the ringed type). Finally, several transitional stages have been added between the SA or SB spirals and the Magellanic irregulars Im. In this classification, the Andromeda Galaxy is SA(s)b.
DDO (or van den Bergh) classification of galaxiesThis contains two parameters: (1) the galactic type (Sa, Sb, Sc, Ir) and (2) the luminosity class (I, II, III, IV, V), similar to the MKK system of stellar luminosity class. The notations S- and S+ are used to denote subgiant species with low and high resolution, respectively. The notation S(B) has been introduced to denote objects intermediate between true spirals and barred spirals.
Related category GALAXIES
Home • About • Copyright © The Worlds of David Darling • Encyclopedia of Alternative Energy • Contact