• noun a massive assembly of stars, dust, gas and other components held together by gravity
  • Galaxies are massive assemblies of stars, dust, gas and other components, and appear to be the main building blocks of the universe. Galaxies gather in clusters of dozens of galaxies and superclusters with hundreds of members, and interact with each other by colliding and in other ways, but are usually autonomous groupings of stars and other material. The main types (Hubble Classification) are spiral galaxies, barred spirals, elliptical, irregular and peculiar, a portmanteau term whose members account for only a few percent of the total. It appears from the velocities at which stars orbit the galactic centre that astronomers are observing only a small fraction – perhaps about 10 per cent – of the mass of galaxies in the form of visible matter, and that they contain a much larger amount of dark matter in and around the visible disc of an average galaxy. However, it was also suggested in 1995 that part of the missing mass might be present in the form of small galaxies which have been missed by previous sky surveys. Little is known about the relationship between the different types of galaxy, or about the mechanism whereby contrasting types can appear near each other in a single cluster. The rare distorted types seem to be produced by the gravitational effects of near-collisions between galaxies.

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