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.

“Last month I mentioned the favourable but rather sparse meteor shower, the Leonids. December brings us one of the best meteor showers of the year which will be very favourable because of the 26-day-old moon at the time of the maximum of this shower.

“The Geminid meteor shower can be seen between December 8 and 17, but maximum occurs at 2am on the 14th. Around that time there could be more than 100 meteors per hour and these tend to be quite slow with a high proportion of bright meteors.

“The source of the particles which produce the meteors appears to be an asteroid, 3200 Phaethon, rather than a comet. It has an orbit of the sun which takes 1.4 years and has been called a ‘rock comet’.

“Mercury will be low in the south-east from around December 20, rising one and a half hours before the sun by the end of the month. Venus is not visible throughout December. Mars rises at 3.30am, about four hours before the sun and can be seen towards the south at around 7am, to the right of Jupiter which is brighter and more noticeable. Saturn is not visible this month.

“The moon is full on the 3rd, at last quarter on the 10th, new on the 18th and at first quarter on December 26.”