Meteor, in astronomy, small solid body known as a meteoroid that enters a planet's atmosphere from outer space and is raised to incandescence by the friction resulting from its rapid motion. Brilliant meteors, known as fireballs, occur singly and generally consist of a luminous head, followed by a cometlike train of light that may persist for several minutes; some, called bolides, have been seen to explode with a sound like thunder. Fainter meteors, called shooting or falling stars, usually occur singly and sporadically. At intervals, however, hundreds of such meteors occur simultaneously and appear to emanate from a fixed point. These swarms are called meteor showers and are named after the constellation in which they seem to have their point of origin. Some appear annually on the same days of each year and are called periodic showers; others occur infrequently at varying intervals. The periods of meteor showers generally coincide with those of certain comets (see Comet). Most meteors are dissipated in flight and fall to the earth as dust; a meteor that reaches the surface of the earth or another planet is called a meteorite.