Part of the myth of Perseus played out across the northern sky, the tragic figure
of Cassiopeia was the wife of Cepheus and the mother of Andromeda. Her
prominent W-shaped constellation is one of the most immediately recognisable
in the entire sky.
Cassiopeia's constellation straddles the Milky Way in the far northern sky,
neighbouring the constellations dedicated to her husband King Cepheus, her daughter
Andromeda, and Andromeda's rescuer Perseus.
The familiar 'W' of Cassiopeia is made up from five stars, Segin, Ruchbah, Cih, Schedar
and Caph. The central star Cih is the brightest of these (at magnitude +2.2), while Caph
is the closest of the five to Earth's Solar System at 55 light years. By a curious coincidence, all
five of these stars are variables. In particular, Cih can vary by nearly 5% of a magnitude,
and is the prototype of a class of irregular variables, 'Gamma Cassiopeiae' variables, that
periodically cast off rings of matter.
Also notable among the stars of Cassiopeia is Achird, a fairly bright star roughly
half-way between Cih and Schedar. This yellow binary, though not as bright as some
of Cassiopeia's other stars, is much closer to Earth, at roughly nineteen light years'
Cassiopeia includes a number of open clusters, but none of these is visible to the
naked eye. Particularly notable are two Messier objects, M52 and, close to Ruchbah in the
sky, M103. Another cluster, the Owl Cluster, can be found near the distant star Phi Cassiopeiae.
All of these clusters are young (no more than 35 million years old), and their distances
range between five and ten thousand light years, in the direction of the Galaxy's rim.
Cassiopeia has been the site of two supernovae in historical times. The first occurred
in 1572, and reached a maximum magnitude of roughly -4, making it brighter than Venus in
the sky. The second took place nearly a century later, leaving a shattered region that
is still detectable today, especially by radio telescopes, and is designated Cassiopeia A.
These stellar explosions took place about 10,000 light years away.