Cassiopeia, in astronomy, prominent northern constellation located almost directly opposite the Big Dipper across the north celestial pole. Five bright stars in the constellation form a rough W (or M ) in the sky. Some see in this formation the shape of a chair known as Cassiopeia's Chair. Tycho's Star, a supernova, appeared in the constellation in 1572 and disappeared in 1574. In this constellation is located Cassiopeia A, a discrete radio source emitting 21-cm radiation with great intensity. Cassiopeia reaches its highest point in the evening sky in November, but because of its location near the pole it is visible throughout the year to most northern observers.