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spiral galaxy  A NASA image of a spiral galaxy, one of the main classes of galaxy, of which Andromeda and our own Milky Way are examples. There may be billions of stars in such a system, held together by gravity. With the advanced technology now available to astronomers, especially the Hubble Space Telescope, the mysteries of galactic structure are being uncovered. | One of the main classes of galaxy in the Hubble classification comprising up to 30% of known galaxies. Spiral galaxies are characterized by a central bulge surrounded by a flattened disc containing (normally) two spiral arms composed of hot young stars and clouds of dust and gas. In about half of spiral galaxies (barred spirals) the arms originate at the ends of a bar across the central bulge. The bar is not a rigid object but consists of stars in motion about the centre of the galaxy. |
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