A supergiant star

Ken Kennedy, of Dundee Astronomical Society, tells us what we can expect to see in the December night sky. “The constellation Perseus follows Andromeda to its highest point in the evening skies of December,” he says. “Perseus was the hero of the Greek tale of the rescue of Andromeda from the sea monster, Cetus, and her return to her parents Cassiopeia and Cepheus. These constellations are grouped quite closely together in the sky with Perseus’ steed, Pegasus, in the south-west during December evenings.

“Perseus is one of the classic 48 constellations listed by Ptolemy in the Second Century. It does not contain any particularly bright stars but is easy to spot as an inverted ‘Y’ between Andromeda and Auriga.

“The brightest star of Perseus is Mirfak, a supergiant star which is bigger and hotter than our sun and is at a distance of 510 light years. Better known, but a bit less bright than Mirfak, is Algol, the Demon Star, given this name because of its variable brightness. The brightness of Algol dips regularly every 2.86 days for about 10 hours then returns to its previous brightness.

“The mechanism for this very precise change was proposed by John Goodricke in 1783. He suggested that a dark body was passing in front of the star and it was proven to be correct in 1881.

“Perseus is also the location of a fine double star cluster, simply known as the Double Cluster, which are open clusters within our Milky Way galaxy at a distance of 7,600 light years. Both clusters contain a few hundred bright, hot stars and are believed to be young with an age of only about 13 million years. They make a fine sight in a pair of binoculars and can be seen by the unaided eye from a dark location.