PETE LAWRENCE
The Northern Hemisphere’s winter solstice occurs at 04:19 GMT on Dec 22, the instant when the Sun’s southerly movement against the stars slows to a stop and reverses direction. Consequently, during December the Sun never climbs far above the southern horizon during the day. In contrast, it dips very far below the northern horizon during the middle of the night, giving the UK its darkest skies of the year.
This is well timed, because the winter sky contains big, bright and bold constellations fronted by that most recognisable of patterns, Orion the Hunter. The most identifiable part of Orion is his belt, formed from three similar brightness stars called Alnitak, Alnilam and Mintaka. The belt stars are part of an open cluster called Collinder 70; stars which formed out of the same nebula material.
Below the eastern belt star, Alnitak, it is possible to see dimmer Sigma Orionis. The region slightly east of this star contains IC 434, a curtain of glowing nebulosity best revealed by long exposure colour photography. This brings out the beautiful red hues, colours given off by excited hydrogen gas at the hydrogen alpha wavelength (656.28nm). West of the brightest part of the curtain, many faint stars are seen. East, the number is significantly lower. This is due to a huge, dark cloud of foreground material to the east of IC 434 blocking visible light from more distant background stars.
A finger of dark material protrudes in front of the glowing curtain. This is the famous Horsehead nebula, Barnard 33.
Imagining Orion’s Belt as a physical entity, rotate its line about Alnitak by 90 degrees anticlockwise. The rotated end points directly at another glowing nebula called Messier 78 (M78). Unlike IC 434 or the Horsehead, M78 is a reflection nebula, a cloud of dust shining by virtue of reflected starlight, hinting at a number of embedded hot young stars in the process of formation.
The stars marking the four corners of the main pattern of Orion are bright. The dimmest is Saiph in the south-east corner. This star’s name means “sword”, somewhat misleading because Orion already has a sword, containing several clusters and distinct patches of nebulosity.
The south-west corner is marked by the blue-supergiant Rigel. This is the seventh brightest star in the night sky and a massive object, 79 times larger than the Sun and 120,000 times more luminous. In the north-west corner we have Bellatrix (“female warrior”), the apparent diameter of which is 1/2,500,000th the apparent diameter of the full Moon!
The red-supergiant Betelgeuse marks Orion’s north-east corner. This is frequently quoted as the most likely bright star to explode as a supernova; an event caused when the star’s internal fuel no longer produces the energy required to keep itself “inflated”. Under the influence of gravity, the outer layers collapse into the core and a tremendous explosion ensues, ripping the star apart. Betelgeuse is likely to go supernova sometime in the next 100,000 years. When it does, its brilliance is likely to be similar to a full Moon. After the supernova fades, Orion will be irrevocably ruined!
It is well worth taking your time to view Orion by whatever means you have to hand. Even to just the naked eye, it’s a magnificent constellation.