Barnard's Loop (catalogue designation Sh 2-276) is an

emission nebula An emission nebula is a nebula formed of ionized gases that emit light of various wavelengths. The most common source of ionization is high-energy ultraviolet photons emitted from a nearby hot star. Among the several different types of emission n ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

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

arcminute A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

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 The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s (518

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s) or 440 pc (1434 ly), giving it dimensions of either about 100 or 300 ly across, respectively. It is thought to have originated in a

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

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. 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. 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

References

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External links

Photograph identifying several nebulae in Orion
* ttp://apod.nasa.gov/apod/ap120312.html The Scale of the Universe(

Astronomy Picture of the Day Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest include ...

March 12, 2012) {{Sh2 objects H II regions Orion molecular cloud complex Sharpless objects Discoveries by Edward Emerson Barnard