Barnard's Loop

Emission nebula
H II region
Barnard's Loop can be seen on this image as a diffuse red semicircle.
Observation data: J2000 epoch
Right ascension	05h 27.5m
Declination	−03° 58′
Distance	either 518 or 1434 ly
Apparent magnitude (V)	5
Apparent dimensions (V)	10°
Constellation	Orion
Physical characteristics
Radius	either 50 or 150 ly
Designations	Sh 2-276
See also: Lists of nebulae

Barnard's Loop (catalogue designation Sh 2-276) is an emission nebula in the constellation of Orion. It is part of the Orion molecular cloud complex which also contains the dark Horsehead and bright Orion nebulae. The loop takes the form of a large arc centered 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 best seen in long-exposure photographs, although observers under very dark skies may be able to see it with the naked eye.

Recent estimates place it at a distance of either 159 parsecs (518 light-years) or 440 pc (1434 ly), giving it dimensions of either about 100 or 300 ly across, respectively.^[1][2] It is thought to have originated in a supernova explosion about 2 million years ago, which may have also created several known runaway stars, including AE Aurigae, Mu Columbae and 53 Arietis, which are believed to have been part of a multiple star system in which one component exploded as a supernova.^[3]

Although this faint nebula was certainly observed by earlier astronomers, it is named after the pioneering astrophotographer E. E. Barnard who photographed it and published a description in 1894.^[4]

• 
  Above photo of Barnard's Loop nebula in inverted black and white of the red channel
• 
  Long exposure of Orion with red clouds of ionized hydrogen (H-alpha). The big bow on the left is Barnard's Loop.
• 
  Barnard's Loop seen against the major stars and nebula of Orion