A prominent radio source in the constellation Cassiopeia, and the brightest in the sky apart from the Sun. It has a ring-like structure, thought to be the expanding shell of a supernova remnant 10 000 l.y. away. Faint traces of optical nebulosity are visible. Measurements of the rate of expansion indicate that the supernova must have occurred around 1660–80, but there are no recorded sightings of it, with the possible exception of a 6th-mag. star recorded near this position by J. Flamsteed in 1680. An envelope of dust ejected by the star before it exploded may have dimmed the light from the supernova by several magnitudes, accounting for the lack of visual observations at the time.

http://www.spitzer.caltech.edu/images/1445-ssc2005-14c-Cassiopeia-A-Death-Becomes-Her