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 = J2000

| ra = {{RA|05|27.5}}

| dec = {{DEC|-03|58}}

| dist_ly = either 518 or 1434

| appmag_v = 5| size_v = 10°

| constellation = Orion

| radius_ly = either 50 or 150

| absmag_v =

| notes =

| names = Sh 2-276

}}

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.{{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 }}{{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 }} 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.{{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}}

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.{{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}}

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