galaxy

galaxy Huge, gravitationally bound, assemblage of stars, dust, gas, and dark matter. Current theories suggest all galaxies were formed from immense clouds of gas soon after the Big Bang. There are three main types, as classified by Edwin Hubble in 1925. Elliptical galaxies (E) are round or elliptical systems, showing a gradual decrease in brightness from the centre outwards. Spiral galaxies (S) are flattened, disc-shaped systems in which young stars, dust and gas are concentrated in spiral arms coiling out from a central bulge, the nucleus. Barred spiral galaxies (SB) have a bright, central bar from which the spiral arms emerge. In addition to these three main classes are transitional lenticular galaxies, systems with a disc and nucleus but with no apparent spiral arms. Irregular galaxies, such as the Magellanic Clouds, are systems with no symmetry. Galaxies can exist singly or in clusters. Interactions between gas-rich galaxies can produce bursts of star formation and may act as a trigger for generating active galactic nuclei such as quasars. About one galaxy in a million is a radio galaxy, emitting strong electromagnetic radiation. A Seyfert galaxy has a bright, compact nucleus and is a strong emitter of infrared waves. A general property of galaxies seems to be the presence of much more mass than can be accounted for by all the material that can currently be detected. This ‘dark matter’ makes up 90% of the mass in typical galaxies. It seems that supermassive black holes lie at the centre of most galaxies, including our own. Our own Galaxy, the Milky Way, is spiral and c.100,000 light years in diameter. Earth is 26,500 light years from the centre. The Solar System lies at the edge of one of the spiral arms, c.30,000 light years from the centre.