Top Deep-Sky Objects in Orion

Deep Sky Objects in Orion
Image Credit & Copyright: Rogelio Bernal Andreo

The constellation Orion counts among the oldest of the recognized constellations, and since it is visible from both hemispheres, it has featured prominently in many ancient cultures, many of which predate the old Roman and Greek mythologies. Orion also contains Rigel and Betelgeuse, two of the brightest stars in the sky both of which form part of two major asterisms, namely, the Winter Triangle, and the Winter Hexagon.

In terms of deep sky objects, Orion has much to offer amateur observers with modest equipment, being embedded, in a manner of speaking, within the giant Orion Molecular Cloud Complex. This vast collection of different types of nebulae, star clusters, and H II (star forming) regions is located around 1,600 light-years away, and spans a distance of 400 light years from north to south, and 1,000 light years from east to west, with the structure encompassing both Orion and the constellations of Eridanus. Below are some details of some of the most prominent features of the Orion Molecular Cloud Complex (OMC) that can be observed with small telescopes:

Orion Trapezium Cluster

Orion Trapezium Cluster
Image Credit: Theofanis N. Matsopoulos

• Coordinates: RA 05h 35.4m |Dec. -05° 27″
• Apparent magnitude: +4.0
• Distance: 1,344 light years
• Apparent dimensions: 47 seconds of arc
• Diameter: 20 light years

Under dark skies and good seeing conditions, the central six stars in this cluster can be resolved with 5-inch telescopes. As shown in the image above, the central grouping of 4 stars are arranged in the form of a trapezoid, hence the name Trapezium Cluster.

It is thought that the Trapezium may be a part of the larger Orion Nebula cluster, a collection of around 2,000 stars that span an area of about 20 light years, and all of which formed directly out of the material contained in the surrounding nebula. However, the Trapezium stars are all within 1.5 light years of one another, and they are all between 15 and 30 times as massive as the Sun. These four stars are also very luminous in x-ray frequencies, and they supply most of the light that illuminates the nebula. One of the 4 stars, designated Theta-1 Orionis C, is the hottest known naked-eye star with an effective temperature of 45,500K.

Visibility

Look for the very bright Trapezium Cluster in the heart of the Orion Nebula, which is the central “star” in Orion’s sword.

Orion Nebula (M42, NGC 1976)

Orion Nebula• Coordinates: RA 05h 35m 17.3s |Dec. -05° 23′ 28″
• Apparent magnitude: +4.0
• Distance: 1,344 light years
• Apparent dimensions: 65 × 60 minutes of arc
• Diameter: 24 light years

Also known as Messier 42, the Orion Nebula can be seen with the naked eye south of Orion’s Belt, which is made up of the stars Mintaka, Alnitak, and Alnilam. Without optical aid, the nebula has a decidedly fuzzy appearance, which hints at it being a nebula. It is also one of the brightest nebulae in the sky, due to it being one of the closest star forming regions to Earth.

Visibility

The Orion Nebula can be seen as the central star in the Hunter’s sword. Note that some observers may discern some green in telescopic views, which is caused by an electron transition in doubly-ionized oxygen, which in atomic physics, in known as the “forbidden transition”.

Horsehead Nebula

Horsehead Nebula• Coordinates: RA 05h 40m 59.0s |Dec. -02° 27′ 30.0″
• Apparent magnitude: +6.8
• Distance: 1,500 light years
• Apparent dimensions: 8 × 6 minutes of arc
• Diameter: 7 light years

Also known as Barnard 33, after the astronomer E. E. Barnard, the Horsehead nebula is a dark nebula that is obscuring light coming from behind it. This nebula is also commonly taken to be a stellar nursery that is estimated to contain more than 100 different organic compounds and molecules.

Visibility

Look for the Horsehead nebula almost due south of the star Alnitak, which is the eastern-most star in Orion’s Belt.

Barnard’s Loop

Barnard's Loop
Image Credit: Jonathan Talbot

• Coordinates: RA 05h 27.5m |Dec. -03° 58″
• Apparent magnitude: +5
• Distance: 518 or 1,434 light years, depending on the method used to determine its distance
• Apparent dimensions: 10 degrees
• Diameter: 100-300 light years, depending on the distance chosen

Barnard’s Loop forms a major component of the Orion Molecular Cloud Complex, and appears to be centred on the Orion Nebula, whose stars are believed to be ionizing the Loop-like structure. Barnard’s Loop is believed by most investigators to be the remains of a supernova event that occurred about 2 million years ago that also turned the stars AE Aurigae, Mu Columbae and 53 Arietis into run-away stars.

Visibility

Since Barnard’s Loop covers almost the entire Orion constellation, and spans an area of about 150 light years, it may be possible to see all, or most of the structure without optical aid under dark, clear skies.

De Mairan’s Nebula (M43, NGC 1982)

De Mairan’s Nebula
Image Credit: F.Espanak

• Coordinates: RA 05h 35.6m |Dec. -05° 16″
• Apparent magnitude: +9.0
• Distance: 1,600 light years
• Apparent dimensions: 20 x 15 minutes of arc
• Diameter: 9 light years

This star-forming region is both an emission and a reflection nebula, and while it is separated from the Orion Nebula by a wide dust lane, it is nevertheless an integral part of the Orion Nebula. Although it was discovered by the French astronomer Jean-Jacques Dortous de Mairan in 1731, Charles Messier “appropriated” the discovery and included it in his famous Messier catalogue as entry # 43.

Visibility

Look for De Mairan’s Nebula around 7 minutes of arc almost due north of the Trapezium Cluster.

Messier 78 (M78, NGC 2068)

Messier 78
Image Credit: ESO/Igor Chekalin

• Coordinates: RA 05h 46m 46.7s |Dec. +00° 00′ 50″
• Apparent magnitude: +8.3
• Distance: 1,600 light years
• Apparent Dimensions: 8 × 6 minutes of arc
• Diameter: 2 light years

M78 is the most luminous of a group of reflection nebula that includes the nebulae NGC 2064, NGC 2067, and NGC 2071 that all form parts of the much bigger Orion Molecular Cloud Complex. Since M78 encloses two 10th magnitude stars designated HD 38563A and HD 38563B, these two stars illuminate the nebula from within, which makes it very easy to spot, even with small telescopes. M78 also contains several T Tauri-type variable stars that are still in the process of forming, as well as 17 Herbig–Haro objects, which are small, transient emission nebulas that can be described as the by-products of the star formation process.

Visibility

Look for M78 just to the left of the line between the stars Betelgeuse and Alnitak, about a quarter of the way up from Alnitak.

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