The Night Sky in December
Orion is well placed during dark December nights. His shape, along with that of the Plough, is one that most recognised in the night sky. The most obvious pattern in Orion is his belt, a distinctive straight line of similar brightness stars called Mintaka, Alnilam and Alnitak. The belt stars are part of the young open cluster Collinder 70.
Orion’s Sword hangs from his belt as a short hazy line of fainter stars. M42, the famous Orion Nebula, sits at the centre of the sword and is visible as a misty patch through binoculars. A telescope reveals a bright kidney-shaped centre to the nebula with swept back regions at either end. The brightest region is the Thrust, with the swept back regions being known as the Sail and, somewhat confusingly, the Sword.
The nebula’s heart is marked by the Trapezium Cluster, so named because of the shape of the four brightest cluster stars. The cluster has formed from the nebula material itself and it is the intense radiation emitted from these hot young stars that ionises the surrounding gas causing it to glow.
Orion’s Sword contains other deep sky objects too such as De Mairan’s Nebula (M43), the Running Man Nebula (NGC 1977) and open clusters NGC 1980 and 1981. M43 sits adjacent to brighter M42, the two regions separated by a dark lane. Where this extends towards the Trapezium, the lane is known as the Fish’s Mouth.
Seven stars form the main pattern of Orion, the two most famous being Betelgeuse and Rigel. Move your gaze back-and-forth between these stars and you should see that Betelgeuse looks distinctly orange compared to blue-white Rigel. Betelgeuse is a cool-red supergiant estimated to have a diameter 1,000 times larger than the Sun. Rigel is a hot-blue supergiant 74 times larger than the Sun and around 120,000 times more luminous. It’s also quite distant, lying 863 light years away compared to Betelgeuse at 640 light years. Betelgeuse marks one vertex of the Winter Triangle along with Sirius and Procyon. Find Sirius by following the line of Orion’s belt southeast (down and left from the UK). Procyon lies northeast of Sirius, the brightest star in an otherwise barren area of sky.
The winter Milky Way flows south through the Winter Triangle but is much harder to see than its brighter summer counterpart.
The main constellation inside the Winter Triangle is Monoceros the Unicorn. This faint and rather indistinct pattern is very rich in clusters and nebulosity. One particular example is the Christmas Tree Cluster, which uses the variable star S Monocerotis as its trunk. It should be visible through binoculars, with a low power view through a telescope making the task easier. It may take a while to pick out the outline of a Christmas tree but it is there.
Finally, December is when the Geminid meteor shower reaches its peak. The presentation of a meteor shower depends on two factors: the state of the Moon and the weather. The Moon is very important because a bright Moon in the sky considerably reduces the number of meteors seen. Geminid activity occurs between Dec 4-17, reaching a peak on the night of Dec 13-14 when a visual rate of one meteor every couple of minutes may be expected. Bear in mind though, that meteors don’t play by the rules, and it’s perfectly possible to watch for extended periods and see nothing. Then, just as you’re about to give up, a flurry of trails may occur.