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. The famous Orion Nebula, M42, 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. So distinctive are the shapes here that they even have their own names.
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 cluster’s 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 to blue-white Rigel. Betelgeuse is a cool red-supergiant estimated to have a diameter 1,000 times larger than the Sun. At this size however, its true edge would become difficult to determine, being hazy and diffuse. 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 640 light years for Betelgeuse.
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. In the summer our view along our galaxy’s plane is towards the core. The Milky Way, formed from the integrated light from of billions of stars here, appears bright. In winter we are looking in the opposite direction where there are fewer stars to contribute to the Milky Way’s misty path.
Christmas Tree Cluster
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, outlined by lights of course!
How to watch the Geminid meteor shower
Finally, December is the month when the Geminid meteor shower reaches its peak, arguably the best of the annual showers. The presentation of a meteor shower depends on two factors; the state of the Moon and the weather. The state of 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 and 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 of time and see nothing. Then, just as you’re about to give up, a flurry of trails may occur! Aim to view from the darkest site you can find. Look approximately two thirds up the sky, in any direction and wait. This year the Geminid shower peaks with the Moon in the early morning sky as a waning crescent. Consequently, assuming the weather plays ball, display prospects are good.
The shower gets its name because during the peak period associated meteors appear to emanate from a point - the shower radiant - near the star Castor in Gemini. A good way to locate Castor and its celestial twin Pollux, is to use familiar Orion. Draw an imaginary line from Rigel through Betelgeuse, extending this for 1.75x to reach the vicinity of the twins.
The night sky chart for December
Night sky chart for December 2017
The chart shows how the sky will appear at midnight on 1 December, 11pm on 15 December and 10pm on 30 December. The planets are shown along with the location and phase of the Moon at five-day intervals. The Moon is full on 3 December. This is a perigee-full Moon, the closest of 2017. The stars are shown as circles; the larger the circle the brighter the star. The hazy area represents the Milky Way. Orientate the chart by holding it in front of you rotated so the compass bearing at the bottom matches the direction you’re facing. The bottom of the chart then reflects your horizon with the middle of the chart representing the view directly above your head. The chart is designed to be viewed using a red torch outside. Red light allows you to see the chart detail without ruining your night vision.