Barnard's Loop (catalogue designation Sh 2-276) is an emission nebula in the constellation of Orion. It is part of a giant molecular cloud which also contains the bright Horsehead and Orion nebulae. The loop takes the form of a large arc centred approximately on the Orion Nebula. The stars within the Orion Nebula are believed to be responsible for ionizing the loop.

The loop extends over about 600 arcminutes as seen from earth, covering much of Orion. It is well seen in long-exposure photographs (eg here), although observers under very dark skies may be able to see it with the naked eye

It is estimated to lie at a distance of approximately 1600 light years, giving it actual dimensions of about 300 light years across. It is thought to have originated in a supernova explosion about 2 million years ago - several known runaway stars may have resulted from the same explosion - AE Aurigae , Mu Columbae and 53 Arietis are thought to have been part of a multiple star system in which one component exploded as a supernova about 2 million years ago.

References

1. Gaylard M.J. (1984), Detection of the H 142-alpha line from the Barnard Loop, Monthly Notices of the Royal Astronomical Society, v. 211, p.149
2. Reynolds R.J., Ogden P.M. (1979), Optical evidence for a very large, expanding shell associated with the I Orion OB association, Barnard's loop, and the high galactic latitude H-alpha filaments in Eridanus, Astrophysical Journal, v. 229, p.942

External links

• Photograph identifying several nebulae in Orion
• Encyclopaedia of Astrobiology, Astronomy, and Spaceflight entry