groups of galaxies

Small isolated collections of from three to a hundred galaxies, examples being the Local Group and Stephan's Quintet. Most are gravitationally bound systems but a few may be due to chance line-of-sight projections. If the groups are bound and stable their mass-to-luminosity ratio must be high. The most extreme groups where several galaxies are in close proximity to each other are known as compact groups of galaxies. The members of such groups often show morphological features typical of interacting galaxies.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

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