Posted by on Dec 12, 2012 in Constellation Of the Month |

Orion is truly magnificent. Two of its stars, Rigel and Betelgux, are of the first magnitude. Rigel is almost pure white, while Betelgeux is a glorious orange-red, and is a fine sight in binoculars. Betelgeux has a diameter of some 250,000,000 miles, so that it’s vast globe could contain the entire orbit of the Earth round the Sun.

Star Visual Magnitude Spectrum Absolute Magnitude Distance (Lt Years)
Rigel 0.08 B8 -7.1 900
Betelgeux var M2 -5.6 520
Bellatrix 1.64 B2 -4.2 470
Alnilam 1.7 BO -6.8 1,600
Alnitak 1.79 O9.5 -6.6 1,600
Saiph 2.06 Bo.5 -6.9 2,100
Mintaka var O9.5 -6.1 1,500
I 2.76 O9 -6.1 2,000

It is an irregular variable, with a magnitude range of between 0.1 and 1.3. On rare occasions it has been known to equal Rigel, but is more generally of around magnitude 0.5. There is a rough period of about five years, but the fluctuations cannot be predicted with any accuracy. Its changes are due to real alterations in its diameter, and hence it’s output of radiation.

Apart from Betelgeux, all the leading stars of Orion are very hot and white. Particularly notable are the three members of the Hunter’s belt (Alnilam, Alnitak and Mintaka). Mintaka is slightly variable, its average magnitude is 2.5, and it has a seventh magnitude companion. A 3-inch refractor will also show the seventh magnitude attendant of Rigel.

Below the belt lies the Hunters sword. To the naked eye it appears as a faint misty patch. Binoculars or a small telescope reveal the presence of shining gas. The brightest of the galactic nebulae. No 42 in the Messier catalogue. As a direction finder Orion is unrivalled.
M-42. One of the finest sights in the sky, this nebula is easily visible to the naked eye as the “fuzzy” star in the middle of Orion’s sword. It appears distinctly nebulous in binoculars or telescopes, and shows an amazing amount of detail through the telescope. It is fully a degree in extent, with a wealth of fine curling wisps of nebulosity curving out from the brightest region surrounding the four relatively bright stars known as the Trapezium. On good nights with low power, you will be able to see colours in this object. The region around the Trapezium appears as a cold steel blue colour; while the wispy regions further away can appear as a soft ruddy pink.

M-43 Slightly separated from the main nebulosity, is M-43. This nebula is seen as a comma shaped cloud surrounding an eighth magnitude star just north of the Great Nebula. The more time you spend in this area, the more fine detail can be seen.

M-78 This is another fine area of nebulosity. It is about 6′ in diameter, and surrounds two magnitude 10 stars. It is somewhat fan shaped, and appears comet-like at low powers. NGC’s 2064, 2067, and 2071 lie in very close proximity to M-78, and are all nebulous regions as well.

NGC 1973-75-77 Dubbed the “Running Man” nebula by Jason Ware, this is a relatively bright region of nebulosity just north of the M-42 complex. It is large, about 15’x10 and surrounds several relatively bright stars. The “Running Man” is seen as the dark region between the areas of nebulosity, and often shows up in wide angle photographs of the Great Nebula. It would be observed more often if it weren’t located so close to M-42, and greatly deserves more attention.

NGC 2022 This is a small, but relatively bright planetary nebula about 20″ in diameter. It is slightly brighter at the centre, and fades gradually to the edges.