Editing Barnard's Loop

{{short description|Emission nebula in the constellation of Orion}}
{{Use mdy dates|date=May 2025}}
{{Infobox nebula
| name = Barnard's Loop
| image = BarnardLoopHunterWilson.jpg
| caption = Barnard's Loop can be seen on this image as a diffuse red semicircle.
| type = H II region
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| ra = {{RA|05|27.5}}
| dec = {{DEC|-03|58}}
| dist_ly = either 518 or 1434
| appmag_v = 5| size_v = 10[[Degree (angle)|°]]
| constellation  = [[Orion (constellation)|Orion]]
| radius_ly = either 50 or 150
| absmag_v = 
| notes = 
| names = Sh 2-276
}}
'''Barnard's Loop''' (catalogue designation '''[[Sharpless catalog|Sh 2]]-276''') is an [[emission nebula]] in the [[constellation]] of [[Orion (constellation)|Orion]].  It is part of the [[Orion molecular cloud complex]] which also contains the dark [[Horsehead Nebula|Horsehead]] and bright [[Orion Nebula|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 [[arcminute]]s 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 [[parsec]]s (518 [[light-year]]s) or 440 pc (1434 ly), giving it dimensions of either about 100 or 300 ly across, respectively.<ref>{{cite journal |last1=Wilson |first1=B.A. |last2=Dame |first2=T.M. |last3=Masheder |first3=M.R.W. |last4=Thaddeus |first4=P. |title=A uniform CO survey of the molecular clouds in Orion and Monoceros |url=http://www.aanda.org/index.php?option=com_article&access=bibcode&Itemid=129&bibcode=2005A%2526A...430..523WFUL |journal=Astronomy and Astrophysics |volume=430 |issue=2 |date=2005 |pages=523–539 |doi=10.1051/0004-6361:20035943| bibcode=2005A&A...430..523W |arxiv=astro-ph/0411089v1 |s2cid=13916583 }}</ref><ref>{{cite journal |last1=O'Dell |first1=C.R. |last2=Ferland |first2=G.J. |last3=Porter |first3=R.L. |last4=van Hoof |first4=P.A.M. |title=Physical Conditions in Barnard's Loop, Components of the Orion-eridanus Bubble, and Implications for the Warm Ionized Medium Component of the Interstellar Medium |journal=The Astrophysical Journal |volume=733 |date=2011| issue=1 |page=9 |doi=10.1088/0004-637X/733/1/9| bibcode=2011ApJ...733....9O |arxiv=1103.2789|s2cid=5019523 }}</ref> It is thought to have originated in a [[supernova]] explosion about 2 million years ago, which may have also created several known [[runaway star]]s, 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.<ref>{{cite web|url=http://www.daviddarling.info/encyclopedia/B/Barnards_Loop.html|publisher=David Darling|title=The Internet Encyclopedia of Science: Barnard's Loop (Sh 2-276)|access-date=April 22, 2008| archive-url= https://web.archive.org/web/20080312105026/http://www.daviddarling.info/encyclopedia/B/Barnards_Loop.html| archive-date= March 12, 2008 | url-status= live}}</ref>

Although this faint nebula was certainly observed by earlier astronomers, it is named after the pioneering [[Astrophotography|astrophotographer]] [[Edward Emerson Barnard|E. E. Barnard]] who photographed it and published a description in 1894.<ref name="EEBarnard">{{cite journal|title=The great photographic nebula of Orion, encircling the belt and theta nebula|author=Barnard, E. E.|journal=Popular Astronomy|volume=2|pages=151–154|year=1894|bibcode=1894PA......2..151B}}</ref>

<gallery>
Image:Nebula-Barnard's-Loop.jpeg|Long exposure of Orion with red clouds of ionized hydrogen (H-alpha). The big bow on the left is Barnard's Loop.
Image:Nebula-Barnard's-Loop-bw-inverse.jpeg|Previous photo of Barnard's Loop nebula in inverted black and white of the red channel
Image:Orion Head to Toe.jpg|Barnard's Loop seen against the major stars and nebula of Orion
</gallery>

==References==
{{Reflist}}
{{refbegin}}
*{{citation |last=Gaylard |first=M. J. |year=1984 |title=Detection of the H 142-alpha line from the Barnard Loop |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=211 |pages=149–153 |doi=10.1093/mnras/211.1.149 |doi-access=free }}
*{{citation |last1=Reynolds |first1=R. J. |last2=Ogden |first2=P. M. |year=1979 |title=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 |journal=Astrophysical Journal |volume=229 |pages=942–953 |doi=10.1086/157028 |bibcode=1979ApJ...229..942R }}
{{refend}}

==External links==
{{commons category}}
*[http://www.astropix.com/HTML/B_WINTER/ORION_O.HTM Photograph identifying several nebulae in Orion]
*[http://www.daviddarling.info/encyclopedia/B/Barnards_Loop.html ''Encyclopaedia of Astrobiology, Astronomy, and Spaceflight'' entry]
*[http://apod.nasa.gov/apod/ap120312.html The Scale of the Universe] ([[Astronomy Picture of the Day]] March 12, 2012)
{{Supernovae}}
{{Sh2 objects}}

[[Category:H II regions]]
[[Category:Orion molecular cloud complex]]
[[Category:Sharpless objects]]
[[Category:Discoveries by Edward Emerson Barnard]]