Orion - Constellation Guide

Orion

Orionis

Ori

The Hunter



Introduction

Orion is a prominent constellation that's one of the brightest and most familiar sights in the night sky. Straddling the celestial equator it can be seen from all locations on Earth. Named after a great hunter in Greek mythology, it contains two first magnitude stars, many other bright stars, a famous belt, spectacular nebulae, some impressive multiple stars and fine open clusters. Its most famous inhabitant, the Orion Nebula, is one of the most spectacular deep sky objects in the sky.

The distinctive pattern of Orion has been used historically and in the modern World extensively. The earliest linking is an ivory carving found in a cave in the Ach valley in Germany, which is estimated to be at least 32,000 years old. In Greek mythology, Orion was a handsome strong hunter, born to Euryale the daughter of King Minos of Crete and Poseidon, the god of the sea. Many myths surround the character particularly involving his death. Various versions exist, but generally Orion brought the wrath of goddess Artemis who sent the scorpion to kill him. The resulting outcome is that the hunter and the scorpion are placed on opposite sides of the sky. When Scorpius rises in the east, Orion is setting in the west.

In medieval Muslim astronomy, Orion was known as al-jabbar, the giant. In old Hungarian tradition, Orion was a magic Archer or Reaper and in China the constellation was one of the 28 lunar mansions that reflect the movement of the Moon. The Egyptians associated Orion with Osiris, the god of death, afterlife, resurrection, regeneration and rebirth. They also aligned part of the Great Pyramid of Giza with Alnitak, one of the stars of the belt. For the Aztecs, Orion rising in the east signaled the time to perform the "New Fire" ceremony, a ritual designed to postpone the end of the World. More recently, the film company Orion Pictures used the constellation's main shape as its logo.

Orion is best seen during the months of December, January and February.

Orion Star Chart - pdf format (credit:- freestarcharts)

Interesting Stars

Bright Star, Multiple Star

Rigel (beta Orionis - β Ori) - mag. +0.13 is a blue-white B8 supergiant that's the brightest star in Orion and seventh brightest in the night sky. It has been assigned the second Greek letter even though it's almost always brighter than alpha Orionis, red supergiant Betelgeuse. In terms of sheer power Rigel is immense. The star shines at absolute mag. -7.9 and is therefore 120,000 times more powerful than the Sun. It also easily outstrips most of its night sky rivals.

At a distance of 860 light-years, Rigel is relatively close, but if it were as near as Sirius it would be spectacular, lighting up the night sky as a brilliant beacon of light almost as bright as the full Moon. During the daytime, Rigel would also be easily seen.

The star has a companion that was discovered by German astronomer F.G.W. Struve in 1831. Known as Rigel B (mag. +6.8), it's not particularly faint but is overpowered by the brilliance of its neighbour. Even so the pair can be split with 80mm (3.1-inch) scopes on nights of good seeing. The separation is 9.5 arc seconds and a magnification of between 150x and 200x should suffice. Rigel B itself is a spectroscopic binary consisting of two blue-white B9V main sequence stars with an orbital period of 9.8 days. They are 1.9 and 2.5 times the mass of the Sun.

Bright Stars, Variable Stars

Betelgeuse (alpha Orionis - α Ori) - marks the upper left-hand corner of the ancient hunter and one of the most well-known stars in the sky. It's an irregular red supergiant that generally fluctuates between magnitudes +0.3 and +0.8, though on rare occasions it's been known to peak at magnitude 0.0 and dim down to magnitude +1.2. Currently it hovers around magnitude +0.42.

Betelgeuse is 640 light-years away and has a diameter of about 1,200 times that of the Sun. It's one of the nearest vast red supergiants.

Alnilam (epsilon Orionis - ε Ori) - mag. +1.69 is a blue-white B0 supergiant 2,000 light-years distant that marks the middle belt star and fourth brightest star in the constellation. Alnilam is a powerful star with a diameter of 25 times that of the Sun. It varies in magnitude between +1.64 and +1.74. Surrounding Alnilam is reflection nebula NGC 1990, a molecular cloud illuminated by light emitted from the star.

Bright Star, Multiple Star

Alnitak (zeta Orionis - ζ Ori) - is the eastern star of the belt appearing to the naked eye as a single blue-white star of magnitude +1.8. Telescopes of order of 80mm (3.1-inch) aperture, at high magnifications, reveal a close companion of magnitude +3.7, separated by 2.5 arc seconds. The main star (mag. +2.1) is a very hot luminous type O supergiant star, which belongs to the rarest class of main-sequence stars. When UV radiation is considered, Alnitak A is the brightest O type star in sky.

Alnitak was first reported as a double star by German astronomer Georg Kunowsky in 1819. More recently, in 1998 the Lowell Observatory discovered that the bright primary has a close blue companion, also of 4th magnitude. In addition, there is another magnitude +9.6 unconfirmed member, separated by 58 arc seconds. The system is 1,250 light-years from Earth and is approx. 7 million years old.

The region around Alnitak is quite stunning. Nebulosity is in abundance with bright emission nebula NGC 2024 (The Flame Nebula) located just east of Alnitak and fainter emission nebula IC 434 and the Horsehead Nebula to the south.

Bright Star, Multiple Star, Variable Star

Mintaka (delta Orionis - δ Ori) - is the western and northernmost of the three stars of Orion's belt. It's a complex multiple star that appears to the naked eye as a single blue-white star of magnitude +2.23. Binoculars or small telescopes reveal a wide companion (mag. +6.9), separated by 52 arc seconds. The main star itself is an eclipsing binary that varies by about 0.2 magnitudes over a period of 5.73 days.

Bright Stars

Bellatrix (gamma Orionis - γ Ori) - at mag. +1.64 is the third brightest star in Orion. It's a hot B2 type blue-white giant star positioned five degrees west of Betelgeuse. Bellatrix is located 250 light-years from Earth.

Saiph (kappa Orionis - κ Ori) - is the southeastern of the four bright stars that make up the main Orion quadrangle. This type B blue supergiant is 650 light-years distant with an apparent magnitude of +2.1.

Multiple Stars

Sigma Orionis (σ Ori) - at 1,150 light-years is a stunning multiple system located just southwest of Alnitak. It's arguably the most impressive and colourful of all Orion star treasures. To the naked eye, it appears as a single blue-white star of mag. +3.7.

Binoculars reveal a mag. +4.0 primary with a wide mag. +6.6 companion, separated by 43 arc seconds. Both stars are blue-white in colour. The primary star itself is a double although the two stars are a mere 0.25 arc seconds apart. Small scopes reveal two additional close companions, a red seventh magnitude star and a ninth magnitude star, at 12 and 11 arc seconds respectively. The fainter star is more difficult to detect due to the surrounding glare.

Located in the same field of view and completing this delightful group is triple star Struve 761. It consists of a triangle of 8th and 9th magnitude stars.

Theta1 Orionis (θ1 Ori) - is the Trapezium and one of the finest multiple stars in the sky. Located at the heart of the Orion Nebula, a small 80mm (3.1-inch) telescope scope easily splits the main components. The brightest four stars are of magnitudes +5.1(C), +6.7(D), +6.7->7.7(A) and +8.0->8.7(B). A challenge for observers is to spot the two fainter Trapezium members. Both are 11th magnitude stars, known as E and F, and can be observed with small scopes on nights of good seeing but much easier with scopes of the order of 150mm (6 inches) or greater.

Eta Orionis (η Ori) - is another complex multiple star. It's a challenging object for small scopes but can be split using a 100mm (4-inch) scope on nights of good seeing. Try magnifications between 150x and 200x. The two components visible in telescopes are both hot blue stars at magnitudes +3.6 and +4.9, set 1.6 arc seconds apart. The brighter star is also an eclipsing binary, varying by 0.1 magnitudes every 8 days.

Eta Orionis is positioned 2 degrees southwest of Mintaka and is estimated to be 1,000 light-years away.

Double Stars

Iota Orionis (ι Ori) - mag. +2.8, is positioned just south of M42 and is the brightest star in Orion's sword. It's an unequal double resolvable in small scopes that consists of magnitude +2.9 and +6.9 components, separated by 11 arc seconds. The primary star itself is a massive spectroscopic binary made up of class O9 and B1 blue giants. Also visible in the same field is a wide binocular pair, Struve 747, magnitudes +4.8 and +5.7, separation 36 arc seconds.

Iota Orionis has traditional names Hatsya and in Arabic Na'ir al Saif, which means the bright one of the sword.

Meissa (lambda Orionis - λ Ori) - located at the head of the Hunter, Meissa (mag. +3.5) forms a nice binocular right-angled triangle with stars phi1 (φ1 Ori - mag. +4.4) and phi2 (φ2 Ori - mag. +4.1). A small telescope at about 90x magnification reveals Meissa as a double star, consisting of magnitude +3.6 and +5.5 components. Various observers over the years have reported seeing colours such as yellow and purple but this purely an optical illusion as both stars are blue-white. The separation of 4.4 arc seconds is virtually unchanged since records began.

Theta2 Orionis (θ2 Ori) - is a wide binocular pair towards the edge of M42 consisting of magnitude +5.2 and +6.5 stars, separated by 52 arc seconds. It's located 3 arc minutes southeast of the Trapezium.

Rho Orionis (ρ Ori) - is an orange giant of magnitude +4.5 with a magnitude +8.3 companion. Separation is 7 arc seconds but the brightness difference between the two stars makes for a difficult pair in small scopes.

32 Orionis - just east of Bellatrix and 300 light-years away is 32 Orionis. It's another close double with a mag. +4.4 yellow primary and mag. +5.7 blue-white secondary. A 100mm (4-inch) scope at high powers will split this colourful pair, separation 1.4 arc seconds.

14 Orionis - is not a spectacular double, but an excellent test of optics and conditions due to its extremely close separation of 0.8 arc seconds. A 150mm (6-inch) size scope at high powers and good seeing conditions will split the pair into magnitude +5.8 and +6.5 components.

Struve 817 - a less well known double consisting of almost identical magnitude +8.7 and +8.9 stars, separated by 19 arc seconds. What makes this pair interesting is location; they are positioned only half a degree south of brilliant Betelgeuse. A small 80mm (3.1-inch) scope at 70x magnification easily splits them. Both stars fit in the same field view as the Betelgeuse, providing a startling brightness contrast.

Variable Stars

U Orionis - is a long period Mira type variable that changes between magnitudes +4.8 and +12.6 over a period of 372.4 days. At its brightest, it's visible to the naked eye and easily seen with binoculars. At the other end of the scale, it requires a medium size scope to be spotted. The star is positioned towards the very northern part of Orion, close to the Taurus and Gemini constellation boundaries.

U Orionis is 1,000 light-years from Earth.

W Orionis - is a semi-regular variable with a magnitude range of +5.7 to +7.0 over a period of 212 days.

BL Orionis - is another semi-regular variable with a magnitude range between +5.9 and +6.6. The period is 153 days.

Deep Sky

Bright Nebulae

M42 (NGC 1976) - mag. +4.0, the Great Orion Nebula or Orion Nebula is the constellation's most celebrated deep sky object. It's an emission / reflection nebula and star forming region that spans more than a degree of sky. M42 is easily visible to the naked eye and a spectacular sight through all forms of optical instrument. It contains multiple star theta1 (θ1) Orionis. This group of hot stars, known as the Trapezium, illuminates the surrounding nebula. The Trapezium name derives from the four main stars that are visible through small telescopes, although apertures of greater than 150mm (6-inch) or so are required to easily reveal another two stars.

M42 is 1,340 light-years from Earth.

M43 (NGC 1982) - is an emission / reflection nebula north of the Great Orion Nebula that's separated from it by a narrow dust lane. It's part of the main nebula although 100x fainter at mag. +9.0. Nevertheless, it's still bright enough to be seen with binoculars appearing as a small, faint fuzzy elongated patch with a prominent centre. At the centre of M43 lies irregular variable star NU Orionis, which changes in brightness between magnitudes +6.5 and +7.6. With a 100mm (4-inch) telescope the nebula looks somewhat comma shape with a bright centre star. It spans 20 x 15 arc minutes.

M43 is sometimes called De Mairan's Nebula after Jean-Jacques Dortous de Mairan who discovered it. The nebula is 1,600 light-years from Earth.

NGC 2024 - also known as the Flame Nebula is a glowing area of gas about 0.5 degrees wide just east of Alnitak (ζ Orionis). It's a bright nebula that despite significant interference from second magnitude Alnitak, shows up surprisingly well in telescopes. Scopes of 150mm (6-inch) or more reveal a circular disk of nebulosity, dissected by a prominent dark band. At high powers, with Alnitak moved out of the field of view, more bands and subtle bright and dark patches are visible. NGC 2024 is a fine emission nebula that would be spectacular if it were not located next to such a bright star. It has an apparent diameter of 30 arc minutes and is about 900 to 1,500 light-years away.

Bright Nebula and Open Cluster

NGC 2174 (Nebula) and NGC 2175 (Open Cluster) - High up in the northern constellation reaches close to the Gemini border is NGC 2174, another star forming region 6,400 light-years distant. Nicknamed the Monkey Head Nebula due to its curious shape, this fairly ruddy emission nebula has been twice spectacularly imaged by the Hubble Space Telescope. When seen through medium size amateur scopes of order of 200mm (8-inch) aperture, it appears fan shaped, slightly brighter towards the centre with faint dust bands visible. It total, it spans 40 arc minutes but only about half of that can be seen visually.

On the southern edge of NGC 2174, embedded in the nebula, is large open cluster NGC 2175. This group of about 60 stars, spread across 18 arc minutes of sky, shines at magnitude +6.8 and is easily visible in binoculars. Small 80mm (3.1-inch) scopes show a grainy condensed cluster of stars. Up to about 25 stars of differing magnitudes are visible in larger scopes, making a nice view.

Spectacular open cluster M35 in Gemini is located 4 degrees north of NGC 2174.

Bright Nebula and Dark Nebula

IC 434 and Barnard 33 (Horsehead Nebula) - Staying in the region surrounding Alnitak, but looking south of the star lies emission nebula IC 434. On a good night with a 150mm (6-inch) scope it appears as a tenuous faint wispy strip of light extending in a north to south direction. Averted vision helps, as does moving Alnitak out of the field of view.

IC 434 covers 60 x 10 arc minutes of apparent sky and normally would be relatively unremarkable except for one major detail, its home to famous dark nebula Barnard 33, otherwise known as the Horsehead Nebula. Positioned 0.5 degrees south of Alnitak, this cloud of dust overlays the emission nebula, cutting out a shape that looks like a horse's head or a black knight from a chess game.

Unfortunately, while the Horsehead Nebula is spectacular when imaged, it's rather difficult to observe visually. The low contrast and small 5 arc minute size contribute to making this a tricky target. Some observers have reported seeing it with 150mm (6-inch) scopes, while others have failed even with 500mm (20-inch) scopes.

Reflection Nebulae

M78 (NGC 2068) - mag. +8.2, is a small elongated reflection nebula, located a few degrees northeast of Orion's belt. It's the brightest reflection nebula in the sky and can be seen with binoculars as a misty patch of light. Medium size amateur scopes reveal a wispy twisty nebula with a brighter northern section, surrounding a pair of 10th magnitude stars.

NGC 1973 / NGC 1975 / NGC 1977 - are a group of three reflection nebulae usually overlooked due to the prominence of nearby M42. This grouping is positioned half a degree northeast of the great nebula. Typical of reflection nebulae, they appear bluish resulting from light reflection from hot young stars by the interstellar dust, with darker regions dividing the three NGC objects. Overall it shines at magnitude +7.0 and covers 40 x 25 arc minutes of sky. Just to the north is open cluster NGC 1981.

The object is listed as number 279 in the Sharpless Catalogue and also is known as the Running Man nebula after Texas Astronomical Society member Jason Ware, who remarked that it looked like a man running through the cosmic dust cloud.

It's best seen in medium to large size scopes.

Open Clusters

NGC 1662 - mag. +6.4, is a loose open cluster visible with binoculars. The brightest components are just about resolvable with binoculars. It contains nearly a dozen 9th magnitude stars, spread across a diameter of 20 arc minutes. Small scopes will show a few fainter stars and through 250mm (10-inch) instruments, up to 30 stars are visible. Although loose, it stands out against the black backdrop.

NGC 1981 - is a large bright scattered cluster of 20 stars located a degree north of the Orion Nebula. At magnitude +4.6, it's visible to the naked eye as a misty unresolved patch of light. Binoculars, such as 10x50s, reveal about 10 stars with the brightest members easily visible with direct vision. In total, this lovely cluster contains about 20 stars spread across 25 arc minutes of sky. Larger scopes hint at faint nebulosity in the background.

NGC 1981 is about 7.5 million years old and 1,250 light-years distant. It marks the northern point of the sword of Orion.

NGC 2169 - at mag. +5.9, is a small cluster visible in binoculars that forms a small triangle in the northeastern part of the constellation with stars nu (ν Ori - mag. +4.4) and xi (ξ Ori - mag. +4.5). Through 10x50 binoculars, this pretty area of sky reveals a somewhat hazy compact looking cluster surrounded by streams of faint stars. The brightest individual component shines at magnitude +6.9 and the brightest few stars are resolvable in binoculars. It's sometimes called the 37 Cluster because it resembles the number 37 when seen through medium size scopes.

NGC 2169 is about seven arc minutes in diameter with up to 30 stars. It's about eight million years old and is located 3,600 light-years away.

NGC 2194 - mag. +8.5, is a rich but faint cluster located a couple of degrees southeast of NGC 2169. Covering 10 arc minutes of sky and containing 80 members, NGC 2194 is well resolved in larger amateur scopes. At high magnifications, of the order of 200x, a 300mm (12-inch) instrument reveals a teardrop shaped patch of light with many dim but resolvable stars. It has a flattened shape.

Orion Data Table

Henry Draper Catalogue (HD) Hipparcos Catalogue (HIP) Bayer Flamsteed Struve Name RA (J2000) DEC (J2000) Visual Mag. Var. Var. Mag. Range Period (days) Double Sep. (arc secs) PA (deg.) Mag. Primary, Sec 39801 27989 Alpha Ori 58 --- Betelgeuse 05h 55m 10s +07d 24m 25s 0.42 Y 0.00 -> 1.20 irregular --- --- --- --- 34085 24436 Beta Ori 19 668 Rigel 05h 14m 32s -08d 12m 06s 0.13 --- --- --- Y 9.5 204 0.1 / 6.8 35468 25336 Gamma Ori 24 --- Bellatrix 05h 25m 08s +06d 20m 59s 1.64 --- --- --- --- --- --- --- 37128 26311 Epsilon Ori 46 --- Alnilam 05h 36m 13s -01d 12m 07s 1.69 Y 1.64 -> 1.74 2 -> 7 --- --- --- --- 37742 26727 Zeta Ori 50 774 Alnitak 05h 40m 46s -01d 56m 33s 1.77 --- --- --- Y AB 2.5 / AC 58 AB 168 / AC 10 A 1.9 / B 3.7 / C 9.6 38771 27366 Kappa Ori 53 --- Saiph 05h 47m 45s -09d 40m 11s 2.09 --- --- --- --- --- --- --- 36486 25930 Delta Ori 34 14 Mintaka 05h 32m 01s -00d 17m 57s 2.23 Y 2.23 -> 2.43 5.73 Y 52 359 2.3 / 6.9 37043 26241 Iota Ori 44 752 Iota Ori 05h 35m 26s -05d 54m 36s 2.77 --- --- --- Y 11 138 2.9 / 6.9 1898 26549 Sigma Ori 48 762 Sigma Ori 05h 38m 45s -02d 36m 00s 3.77 --- --- --- Y AE 43 / AD 12 / AC 11 AE 62 / AD 85 / AC 239 A 4.0 / E 6.3 / D 6.6 / C 8.8 37022 26221 Theta1 Ori 41 748 Trapezium 05h 35m 16s -05d 23m 23s 4.0 --- --- --- Y AB 9 / AC 13 / AD 23 AB 30 / AC 132 / AD 96 A 6.7->7.7 / B 8.0->8.7 / C 5.1 / D 6.7 35411 25281 Eta Ori 28 --- Eta Ori 05h 24m 29s -02d 23m 50s 3.35 Y 3.35 -> 3.45 8 Y 1.6 78 3.6 / 4.9 36861 26207 Lambda Ori 39 738 Meissa 05h 35m 09s +09d 56m 03s 3.47 --- --- --- Y 4.4 50 3.6 / 5.5 37041 26235 Theta2 Ori 43 17 Theta2 Ori 05h 35m 23s -05d 24m 58s 4.98 --- --- --- Y 52 95 5.2 / 6.5 33856 24331 Rho Ori 17 654 Rho Ori 05h 13m 18s +02d 51m 41s 4.46 --- --- --- Y 7 63 4.5 / 8.3 36267 25813 32 Ori 32 728 32 Ori 05h 30m 47s +05d 56m 54s 4.2 --- --- --- Y 1.4 46 4.4 / 5.7 33054 23879 14 Ori 14 98 14 Ori 05h 07m 53s +08d 29m 55s 5.4 --- --- --- Y 0.8 300 5.8 / 6.5 39758 --- Struve 817 --- 817 Struve 817 05h 54m 52s +07d 01m 54s 8.5 --- --- --- Y 19 73 8.7 / 8.9

Orion Deep Sky Data Table