Cassiopeia

The constellation of Cassiopeia is very easy to find. From the Plough, follow the line of the “pointers” through the Pole Star. The unmistakeable “W” shape of Cassiopeia’s five main stars is at an equal distance on the other side. Cassiopeia is circumpolar from the UK, meaning that it never sets, and so can be well observed throughout most of the year. On November evenings however it is right overhead.

Cassiopeia is well endowed with most types of deep sky object. For instance there is plenty for variable star observers to keep their eyes on. Gamma, the central star of the “W” is a highly luminous star that is slowly losing material into a surrounding disk or shell, which is probably what makes it an unpredictable variable. It normally shines at magnitude two, but in 1937 it brightened almost to the first magnitude and at other times it has been as faint as the third. Fourth magnitude Rho Cassiopeiae is a huge star that has shown recent interesting fluctuations in its brightness and spectrum. Rho is thought to have a luminosity of as much as a million times that of the Sun and is probably a prime candidate to go supernova some time in the next few thousand years.

If Rho were to go supernova this would be a remarkable coincidence because Cassiopeia has already played host to two of these exceedingly rare events in the last few centuries. One of these, observed famously by Tycho Brahe, occurred in 1572. Tycho’s star outshone Venus and was even visible in daylight for a period of about two weeks. Only a few very faint remnants are left of it now though. Cassiopeia’s second supernova is thought to have occurred about 100 years later but was not observed visually at the time. It is thought that this was because it was obscured by intervening clouds of gas and dust in the Milky Way. The remnant, known as Cassiopeia A, is however one of the strongest radio sources in the sky.

Lying as it does directly on the plane of the Milky Way, Cassiopeia contains many bright galactic clusters. The most prominent of these are Messier 52 &103 and NGC 663, 457 & 7789. For something a little more challenging, try tracking down the 12th magnitude galaxies NGC 185 & 147 – distant dwarf companions of the Great Andromeda Galaxy. Finally, Eta is a fine double star for small telescopes and Iota an outstanding triple.

POWERPOINT SLIDE SHOW (489KB)