Cassiopeia

Constellation of the northern sky

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GenitiveCassiopeiae
AbbreviationCas
Right Ascension22h56 to 03h06
Declination+46° 30' to +77° 30'
Area (sq deg)598
Brightest StarCih
Optimum VisibilityOctober
Map of Cassiopeia
Relative Galactic Position of Cassiopeia

The dense band of the Milky Way passing through Cassiopeia shows that it lies on the plane of our Galaxy. The familiar W-shaped constellation looks out towards the Galaxy's rim.

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.

Stars

The familiar 'W' of Cassiopeia is made up from five stars, Segin, Ksora, Cih, Schedir 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 Schedir. This yellow binary, though not as bright as some of Cassiopeia's other stars, is much closer to Earth, at roughly nineteen light years' distance.

Star Clusters

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 Ksora in the sky, M103. Another cluster, NGC 457, 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.

Supernovae

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.

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