Any cosmic source of radio waves. Major celestial radio sources include the Sun, Jupiter, flare stars, novae, pulsars, supernova remnants, H II regions, the Milky Way, the galactic centre, radio galaxies, quasars and other active galaxies, and the cosmic microwave background. Radio sources are designated by a number of methods. The first sources were named after the constellation in which they lay (e.g. Sagittarius A, Virgo A), but later they were labelled with a serial number and a prefix denoting the survey in which they were catalogued (e.g. 3C for the Third Cambridge Survey). Modern practice is to identify sources by their coordinates, either right ascension and declination, or galactic coordinates. Radio source counts can yield important information about the early Universe.
Subjects: Astronomy and Astrophysics.