Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Galaxy

Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_4009-3

Definition

A galaxy is a massive, gravitationally bound system of stars, which may also include stellar remnants (white dwarfs, neutron stars, black holes), gas, and dust, as well as nonbaryonic dark matter. The morphological classification of galaxies, established by E. Hubble in the 1930s, includes ellipticals, spirals, and irregulars, each class divided in subclasses and going from “early types” (generally more massive and gas poor, which is the case for ellipticals) to “late types” (less massive and gas rich, as the irregular galaxies). Galaxies are generally found in groups (up to a few tens of members), clusters (a few thousands), and superclusters. Our own Milky Way, often called the Galaxy, is a large spiral with a baryonic mass of ∼5×1010 M, slightly smaller than the largest of the Local Group galaxies (M31 or Andromeda). There is no universally accepted definition of a galaxy, nor a clear lower limit on its stellar mass: some systems characterized as “galaxies” (tidal...

Keywords

Black Hole Dark Matter Bioorganic Chemistry Neutron Star Late Type 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Institut d’Astrophysique de ParisParisFrance