The sky — astronomy

Cassiopeia: The W-Shaped Queen

Cassiopeia is one of the 88 IAU constellations, easy to spot by five bright stars forming a W or M. From mid-northern latitudes its main stars are circumpolar, meaning they never set, so it can be seen on any clear night. It lies across the north celestial pole from the Big Dipper, against the Milky Way.

A W or an M

Cassiopeia (abbreviated Cas) is one of the 88 constellations recognised by the International Astronomical Union (IAU). It is easy to recognise: five fairly bright stars form a lopsided zigzag that looks like a W or an M, depending on how it is turned.

The five stars are Schedar (Alpha), Caph (Beta), Gamma Cassiopeiae, Ruchbah (Delta) and Segin (Epsilon). Gamma Cassiopeiae is a variable star whose brightness changes irregularly, so the shape is not always exactly the same. That zigzag, rather than the official boundary lines, is the practical guide.

Why W or M? The constellation circles the north celestial pole. At different hours and seasons it appears upright, tilted or upside down. You do not need the right letter, just the line of five stars.

Circumpolar constellations

Because Earth rotates, the sky appears to turn around a point called the celestial pole. In the Northern Hemisphere it lies close to Polaris (see Polaris). Stars near enough to this pole trace complete circles without touching the horizon. They are called circumpolar.

The rule is geometric: a star is circumpolar for an observer if its declination (celestial latitude) is greater than 90° minus the observer's latitude. The main stars of Cassiopeia have declinations of roughly +56° to +64°. So for places north of about 34° N they never set. In much of Europe, North America and northern Asia, the W is always above the horizon.

“All year” means it is above the horizon every night, not that it stays at the same height. On autumn evenings it stands high in the northeast, almost overhead. In spring evenings it sinks low in the north, where rooftops and hills can hide it. In summer it rises in the northeast through the night.

In the south the picture changes. Near the equator Cassiopeia is visible only for part of the year and low down, and from far southern latitudes it cannot be seen at all.

How to find Cassiopeia

The easiest method uses two familiar guides. Find the Big Dipper (see the Big Dipper) and the two stars at the end of its bowl that point toward Polaris. Continue the line past Polaris, about the same distance again, and you reach Cassiopeia. The two patterns sit on opposite sides of the pole, so when one is high the other is low.

Practical tips:

• Choose a dark site with a clear northern horizon. Light pollution (see light pollution) hides faint stars, but the five bright ones are visible even from many cities.
• Give your eyes 15 to 20 minutes to adapt to the dark.
• Use a sky-map app to confirm the orientation. More general advice is in how to watch the sky.

Cassiopeia and the Milky Way

Cassiopeia lies on the band of the Milky Way (see the Milky Way), our own galaxy seen from inside. On a dark, moonless night the area around the W is rich with faint stars and hazy patches. Binoculars are rewarding here: the constellation contains open star clusters such as Messier 52 (M52) and NGC 457 (from the New General Catalogue).

Cassiopeia also has a place in the history of science. In 1572 a “new star” appeared in it, and the Danish astronomer Tycho Brahe described it carefully. We now know it was a supernova, the explosion of a star. Another famous object is Cassiopeia A, the remnant of a supernova explosion, studied by space observatories including NASA's.

The myth of the queen

The name comes from Greek mythology. Cassiopeia was queen of Aethiopia, wife of King Cepheus and mother of Andromeda. In the tales she boasted of being more beautiful than the Nereids, the sea nymphs. Poseidon then sent a sea monster (Cetus), and Andromeda was chained to a rock until the hero Perseus rescued her. Cepheus, Andromeda, Perseus and Cetus are all neighbours in the sky, bound together by this story.

In traditional retellings Cassiopeia was placed among the stars sitting on a throne that circles the pole, sometimes upside down. That is cultural tradition, not scientific fact, but it is a handy way to remember the autumn sky. Cassiopeia is among the classical constellations known since antiquity.

In practice, Cassiopeia is a perfect gateway to the northern sky: easy to find, always available, and a pointer to neighbouring constellations, star clusters and the Milky Way itself. Just look north and search for the letter W. For the wider picture of how the sky is divided, see the 88 constellations.

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