Cassiopeia A (3C 461) is the strongest radio source in the sky, apart from the Sun. Cassiopeia A is associated with the second youngest known supernova remnant (after G1.9+0.3) in the Milky Way Galaxy.

Cas A lies about 10,000 light-years away in Cassiopeia (RA 23h 23.4m, Dec +58° 48.9') and coincides with a shell-type remnant of a Type II supernova whose light reached Earth in about 1667. Optically, it is a faint nebula, 10 light-years across, with an expansion velocity of about 800 kilometers per second and a mass of a few solar masses. It is also an extended source of soft X-rays.