Cygnus A (3C 405) is a radio galaxy, one of the strongest radio sources in the sky. A concentrated radio source in Cygnus was discovered by Grote Reber in 1939. In 1946 Stanley Hey and his colleague James Phillips identified that the source scintillated rapidly, and must therefore be a compact object. In 1951, Cygnus A, along with Cassiopeia A, and Puppis A were the first "radio stars" identified with an optical source. Of these, Cygnus A became the first radio galaxy, the other two being nebulae inside the Milky Way. In 1953 Roger Jennison and M K Das Gupta showed it to be a double source. Like all radio galaxies, it contains an active galactic nucleus. The supermassive black hole at the core has a mass of (2.5±0.7)×109 M☉.

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.