The Orion Cloud and Association

This giant cloud, or complex of clouds, of interstellar matter and young stars contains, besides M42 and M43 and the nebulosity associated with them (NGC 1973-5-7), a number of famous objects: Barnard's Loop, the Horsehead Nebula region (also containing NGC 2024 = Orion B), and the reflection nebulae around M78.

Within this cloud, stars have formed recently, and are still in process of formation. These young stars make up the so-called Orion OB1 Association; OB because the most massive, most luminous, and simultaneously hottest of these stars belong to spectral types O and B. Because they are so luminous, they use up their nuclear fuel quickly and have only a short time to live. The association can be divided in subgoups, usually called 1a, 1b, and 1c, where the subgroup 1b includes and surrounds the stars of Orion's Belt, the subgroup 1a lies north-west (preceding) of the belt stars, and the subgroup 1c contains Orion's Sword. The stars of the Orion Nebula, M42 and M43, form a subset of this group, and are sometimes separately counted as subgroup 1d, the very youngest stars of the Orion OB1 association.

Our image on the right shows the whole constellation Orion with bright reddish Betelgeuse slightly left and above the middle, and bright blueish-white Rigel at the lower right. The brightest nebula, in the "sword" of Orion and below his belt stars, is the Great Nebula M42 with M43, with the faint extension to North (up), NGC 1973-5-7. At the left-most belt star, Zeta Orionis, the Horsehead Nebula region with Orion B is conspicuous. The huge, delicate bow around this region, spanning the southern (lower) half of the constellation, is Barnard's Loop. Very north in Orion, around the star Lambda, is another very faint and round, huge nebulous cloud.

Our image was obtained by Sven Kohle and Till Credner of Bonn, Germany on October 29, 1995 at 3:25 UT from Calar Alto with a f=55 mm 1/3.5 photo lens on Kodak Ektachrome 400 Elite film, exposed 60 minutes (without filter). The image is copyrighted by the observers.

UKS image of a great portion of the Orion cloud.

The bright nebula near the bottom is the Great Orion nebula M42 with its northern part M43 and northern extension NGC 1973-5-7. At the upper left is the nebula complex around the star Zeta Orionis, consisting of bright Orion B (NGC 2024) left of the star, and IC 434 with the conspicuous dark Horsehead Nebula, plus various small nebulae. The whole region is filled with gas and dust, but only parts have yet formed stars (i.e., clusters) and are thus illuminated. Barnard's Loop extends far beyond this image and cannot be traced here very well, as it is brightest at its edges.

This image is copyrighted and may be used for private purpose only. For any other kind of use, including internet mirroring and storing on CD-ROM, please contact the Photo Permissions Department (photo at aaoepp.aao.gov.au) of the Australian Astronomical Observatory. See AAO copyright policies.

At least three remarkable stars are observed to escape from the Orion complex with high space velocities:

• AE Aurigae, a variable of spectral type O9.5 and about 6th magnitude (corresponding to an absolute magnitude of - 2.5 at its 1,600 light year distance, variations being erratic between 5.4 and 6.1 magnitude, according to Burnham), which illuminates the diffuse nebula IC 405, also called the "Flaming Star Nebula", escaping at about 130 km/s,
• 53 Arietis (apparent mag 6.09, spectrum B2 V) escaping with 60 km/s,
• Mu Columbae (5.16 mag, spectrum O9.5 V) running away at about 120 km/s.

IRAS investigated the Orion Cloud in the Infrared

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Hartmut Frommert
Christine Kronberg
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Last Modification: June 27, 1998