brightest radio source

The brightest radio source in the sky (other than the Sun) is known as Cassiopeia A. It is a supernova remnant, estimated to be about 300 years old, but it had not been noticed visually until the radio observations drew attention to it. There are no records of visual observations of the explosion, so the star must have been concealed behind dark dust clouds. It also emits gamma-rays and X-rays but does not contain a pulsar. Most of the radio emission comes from the outer part of the expanding shell of gas. The strongest extragalactic source of radio emission is the radio galaxy Cygnus A. Its optical counterpart is a 15th magnitude galaxy. Though some 750 million light years away, it is one of the most powerful radio objects in the sky.