The Hunter shines brightly in winter skies. Learn its stars and nebulae.
In early November, rising in mid-evening about four hours after sunset is the magnificent constellation of Orion the Hunter. Arguably the most recognizable constellation in the world, Orion lies on the celestial equator and can be seen from both the North and South hemispheres.
Orion's shape is easy to pick out because of its many bright stars and signature belt consisting of three stars close together and in a nearly straight line. When people see the striking figure of Orion appearing in the night sky, it is a sign of cold weather and approaching winter.
Stars
The brightest star in Orion is the bluish star Rigel, which marks his western knee. Rigel is a blue supergiant 770 light-years from Earth with a magnitude of 0.2 (the lower the number in magnitude, the brighter the star). It is considered the 7th brightest star in the entire sky. The star marking the other knee of Orion is Saiph, a magnitude 2.1 star that is also a blue supergiant and 720 light-years distant.
The second brightest star in Orion is Betelgeuse, which marks the higher shoulder in the constellation of the Hunter. Betelgeuse is the 10th brightest star in the sky at magnitude 0.5. Betelgeuse is a red supergiant 430 light-years away and a whopping 800 times larger than our Sun. If Betelgeuse were placed where our Sun is in our solar system, it would swallow up all the inner planets. The star marking Orion's other shoulder is Bellatrix. Bellatrix, a blue supergiant shining at magnitude 1.6, is the 22nd brightest star in the sky and 245 light-years away.
Extending out from the shoulder star Bellatrix is Orion's arm where where he is holding either a shield or an animal, depending on the drawing you are referring to. The brighter stars marking this object are all 3rd and 4th magnitude.
Extending upward from the shoulder star Betelgeuse is Orion's other arm, which is generally depicted as holding a club. The brightest stars in the arm and club are all 4th magnitude.
The stars that make up Orion's head are a test of your sky's darkness. They range from 3rd magnitude to 6th magnitude. The more stars you can see, the better your skies are.
The three belt stars from left to right (or east to west) are Alnitak (mag. 1.8), Alnilam (mag. 1.7), and Mintaka (mag. 2.2). The stars on the western edge of Orion, from Bellatrix at the shoulder to Mintaka at the belt to Rigel at the knee, are all double stars.
Nebulae
The stars in the sword that hang down from the belt are part of the Orion Nebula (M42). Also known as the Great Nebula in Orion, it includes the M43 designation. The nebula may have appeared as two distinct objects to Messier because of the dark dust lane that bisects it. The nebula itself can be seen without optical aid as a hazy, 4th-magnitude patch. Using a telescope reveals a quadruple star at the center of the nebula. These four newborn stars known as the Trapezium light up their dusty cocoon, making its glow visible to us here on Earth, a vast 1,400 light-years away.
Between Orion's sword and the first belt star, Alnitak, lies the 3rd-magnitude star Sigma Orionis. Sigma Orionis is actually a quadruple star system. Test your eyes and your equipment to see how many members you can spot. Their magnitudes are 3.7, 6.5, 7.5, and 10.3. Just between Sigma Orionis and Alnitak is the famous Horsehead Nebula. This nebula, however, is too faint to be seen in most telescopes; your best bet is to view it in a picture compliments of an astrophotographer.
Along Orion's side between Alnitak and Betelgeuse (but closer to the belt stars) is the 8th magnitude nebula M78. M78 has the awkward title of "brightest diffuse reflection nebula in the sky." With a small telescope you can see the stars which cause the dust of the nebula to reflect their light.
The last notable nebula in Orion is only just running over the edge of the constellation near Rigel. The majority of IC 2118, The Witch Head Nebula, lies in Eridanus the River. It is extremely faint but also extremely large, spanning 3 degrees across. (In comparison, the full moon only spans a half degree in length.) The Witch Head is another wonderful nebula that is best viewed through the work of astrophotographers.
