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.[4] 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.[5] In 1953 Roger Jennison and M K Das Gupta showed it to be a double source.[6] 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☉.[3]

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