Cepheus is a
constellation in the northern sky. It is named after
Cepheus, King of Aethiopia in
Greek mythology. It was one of the 48 constellations listed by the 2nd century astronomer
Ptolemy, and remains one of the 88 modern constellations. Its brightest star is
Alpha Cephei with an apparent magnitude of 3.5.
Delta Cephei is the prototype of an important class of star known as a Cepheid variable.
RW Cephei, an orange hypergiant, together with the
red supergiants Mu Cephei,
VV Cephei and
V354 Cephei are among the
largest stars known. In addition, Cepheus also has the hyperluminous
quasar S5 0014+81, hosting an ultramassive
black hole in its core at 40 billion solar masses, about 10,000 times more massive than the
central black hole of the Milky Way, making it the
most massive black hole known in the universe.