Cygnus A

An active elliptical galaxy that is one of the strongest radio sources in the sky. Cygnus A (3C 405) is the strongest radio source in the constellation Cygnus and was detected by the first radio telescopes. The source consists of two similar clouds of radio emission, symmetrically located either side of a fifteenth magnitude galaxy at a redshift of 0.057. Cygnus A is the prototype of all powerful radio galaxies. It is one of the largest physical structures in the universe, its clouds spanning a total extent of about 300,000 light years. The energy associated with the radio clouds is about 1053 joules, about ten million times more than with normal galaxies such as the Andromeda Galaxy (M31). The galaxy has strong emission lines in its optical spectrum, indicating that it contains an active galactic nucleus. It is widely accepted that the energy generated by Cygnus A can be explained only by a central black hole that is releasing very large amounts of energy as matter falls towards it.