Cassiopeia is a northern constellation which greek mythology considered to represent a vain queen. It is one of the 88 modern constellations, and was also one of the 48 listed by Ptolemy.

Notable features

Five of the stars of Cassiopeia form a W/Σ/M shape (depending on its position in the sky), which is one of the most distinctive patterns in the northern sky. Since it is close to the north celestial pole, it remains in the sky all night long in most northern countries.

It is frequently used as a rough indicator of sidereal time; the leading bright star of the W, β Cassiopeiae, called Caph, lies almost at zero hours right ascension and hence a line drawn through Polaris and β Cassiopeiae must pass close to the vernal equinox. The hour angle of this line must be equal to sidereal time. Hence, when β Cassiopeiae is on the meridian directly above the pole the sidereal time is zero, when on the meridian directly below the pole the sidereal time is 12 hours, etc.

γ Cassiopeiae is a peculiar variable star. The brightness varies from 1.6 to 3 magnitudes. γ Cassiopeiae is assumed to be a binary star containing a Be star and a neutron star. It is the brightest X-ray binary on the sky, no other X-ray binaries can be seen with naked eye.

Cassiopeia was the site of Tycho Brahe's supernova of 1572, and is also the location of Cassiopeia A, the strongest radio source in the sky (other than the sun). Cassiopeia A is the remnant of a supernova which apparently occurred in about 1667, although there is no record of it having been observed.

Cassiopeia contains two stars visible to the naked eye that rank among the brightest in the galaxy: ρ Cassiopeiae and V509 Cassiopeiae .

If we were to observe Earth's Sun from Alpha Centauri, it would appear to be in Cassiopeia, at about 0.5 magnitude. Roughly speaking, the \/\/ of Cassiopeia would become a /\/\/, with the Sun at the leftmost end, closest to ε Cassiopeiae .

Mythology

Viewing the fainter stars, visible to the naked eye, renders the constellations a human figure, sitting on a chair. Due to the proximity to the pole star, it is visible the whole year, although sometimes upside down whilst in the chair [1]. The greeks considered that this was an undignified position (being upside down, and also the normal way up, in a chair), and must be a suitable punishment for some crime. Since the punishment was one of losing dignity, vanity is a suitable crime.

Together with other constellations nearby (Andromeda, Perseus, Cepheus, and possibly Pegasus), and the constellation Cetus below Cepheus, this may be the source of the myth of the Boast of Cassiopeia, with which it is usually identified.