Dominating the night sky in the winter months, the constellation of Orion is a favourite constellation for many people. It is an easy one to spot – just look for the three stars known as “Orion’s Belt”. Its key features include Betelgeuse and the Orion Nebula.
Image credit : Torsten Bronger CC BY-SA3.0 Published in seasky.org
Betelgeuse
The brightest star in Orion, alpha Orionis or Belelgeuse as it is commonly known, forms Orions left shoulder and appears distinctly reddish to the naked eye. Despite it’s brightness it is a relatively cool star, but it appears bright because it is a red supergiant weighing in at approximately 12 times the mass of our own Sun. It is a variable star, varying in its apparent magnitude from 0.2 and 1.2. It is also a relatively young star that has evolved quickly due to its mass and for the same reason it is likely to go supernova any time ‘soon” (well, soon, on stellar timescales). Recent observations indicate that this giant star has shrunk by more than 15 per cent since 1993. There are a number of possible explanations: it could be due to long-term oscillation in its size or perhaps it signs of the star’s first death throes.
The Orion Nebula
Any guide to observing Orion must begin with the Orion Nebula or M42 as it is also known. It is one of the brightest nebulae in the night sky which makes it a very easy object to find. Located in the middle of the Sword of Orion, it appears to the naked eye as a fuzzy star. In binoculars however the nebula appears as a bright irregular patch of light, with a few tiny but bright stars embedded like jewels. With a telescope of moderate to high power you can pick out four closely spaced hot, young, bright stars – these four stars form the Trapezium. In a really dark sky you will be able to pick out some detail in the nebula – and perhaps even some colour. Captured on a CCD however, the Orion Nebula is an incredibly beautiful and colourful object:
Image credit FAS member Geof Lewis
Other interesting features of Orion
The Orion Nebula is part of the Orion Molecular Cloud Complex, a star-forming region that includes the famous Horsehead Nebula and its neighbour, the Flame Nebula. It is located just to the south of the star Alnitak, one of the stars in Orion’s Belt The Flame Nebula is an emission nebula caused by hydrogen gas being ionized by the nearby bright star Sigma Orionis; the horse-head feature is created a dark gas and dust. The Horsehead Nebula is a great target for CCD imaging. The Flame Nebula and Horsehead nebula are separated by less than 15 arc minutes and together make a superb widefield target.
The flame nebula is caused by hydrogen gas being ionized by the nearby bright star Alnitak. Additional dark gas and dust lies in front of the bright part of the nebula and this is what causes the dark network that appears in the center of the glowing gas.
Orion also harbours the diffuse reflection nebula M78 which is easily found in small telescopes as a hazy patch with two stars of 10th magnitude.