High in the sky, circling the north celestial pole is the distinctive W of Cassiopeia – or sometimes M, depending on the time of year. It's an asterism, a pattern of stars that isn't a constellation. Here is the whole constellation Cassiopeia.
In Greek mythology, Cassiopeia was the queen of ancient Ethiopia, wife of Cepheus and mother of Andromeda. Gorgeous, but boastful, she claimed to be more beautiful than the sea nymphs. (In some versions of the story she also made such a claim for her daughter.) The nymphs were outraged and on behalf of her sisters Amphitrite appealed to her husband, the sea god Poseidon. As a punishment, he sent a sea monster to terrorize the coast of the kingdom.
An oracle told the king and queen that to save the country they had to sacrifice Andromeda to the monster. So they chained the princess to a rock and left her to Cetus. Fortunately, the hero Perseus came on the scene while headed home after killing Medusa the Gorgon. He was smitten by Andromeda and offered to rescue her if he could marry her. So the hero slew the monster and got the girl.
When Cepheus and Cassiopeia died they were put into the sky. Andromeda and Perseus are also nearby. Ever unforgiving, Poseidon put Cassiopeia in a chair so that as she rounds the pole, half the time she's upside down, clinging to save herself from falling. Yet being upside down hasn't stopped Cassiopeia from fussing with her hair in this 19th century representation.
Four of the five stars of the W are second-magnitude stars. This makes it easy to find, even though the fifth star is a magnitude dimmer and is often not visible in urban areas. (Magnitude is a measure of the seeming brightness of a star. Higher magnitudes indicate dimmer stars, with sixth magnitude stars at the limit of our unaided vision.)
Currently, the brightest star in the constellation is Gamma Cassiopeiae in the center of the W. It's an eruptive variable, a star whose luminosity changes unpredictably. Gamma Cassiopeiae doesn't have a traditional name, but it got a nickname when, as a joke, astronaut Gus Grissom (1926-1967) renamed three of the Apollo navigation reference stars. It became Navi, which is Grissom's middle name spelled backwards.
A particularly interesting star in the constellation is Rho Cassiopeiae, which is one of only seven known hypergiants in the Milky Way. It's BIG! Even at ten thousand light years away, it's still visible to the unaided eye. It may well have already exploded as a supernova, but, of course, the evidence takes ten thousand years to get here.
Deep sky objects
The Milky Way runs through Cassiopeia, so it's rich in star clusters and other deep sky objects. Here are some of them.
Eighteenth-century astronomer Charles Messier compiled a catalog of around a hundred nebulous objects that might be mistaken for comets. The catalog numbers are still in use alongside those of the New General Catalogue (NGC) and some later catalogs.
Two Messier objects in Cassiopeia, M52 and M103, are open star clusters. Open clusters are made up of stars that formed together and remain loosely held together by gravity. NGC 457, a third open cluster, is known as the Owl Cluster – or the ET cluster – because it looks like it has two bright eyes. Here is a picture of NGC 457 from Ashley Murphy.
One of the oldest known clusters, now cataloged as NGC 7789, was discovered by Caroline Herschel (1750-1848). It's commonly known as the White Rose Cluster or Caroline's Rose Cluster.
Galaxies rarely appear alone. They're usually found in groups. The Milky Way is part of the Local Group. Two of the Group's dwarf galaxies in Cassiopeia are satellites of the Andromeda galaxy. However the most interesting dwarf galaxy is IC10, the Local Group's only known starburst galaxy. A starburst galaxy is highly luminous because it contains a large number of young hot stars.
Nebulae of birth and death
One of the best known nebulae in Cassiopeia is NGC 281, also known as the Pacman Nebula because it resembles an old video game character of that name. It's not only a region of star formation, but also contains a star cluster and a quintuple star system.
At the end of their lives massive stars explode as supernovae, leaving behind nebulae called supernova remnants. Cassiopeia has two of them.
Cassiopeia A is an extremely strong radio source. It's a shell of hot gas that's now ten light years across and expanding at the rate of 16 million kilometers per hour (over ten million mph). The supernova that created it occurred about 300 years ago, though there is no historical record of it. By contrast, our last object is the remnant of a supernova that was extensively observed in 1572. First seen early in November, it was still visible into 1574, and for two weeks was even visible during the day.