Barnard's loop

Barnard's loop
Observational Data
Designation	Sharpless 276
Right ascension	05h 27m 30s[1]
Declination	-03° 58′ 00″[1]
Constellation	Orion
Type of object	Diffuse nebula
Dimensions	>20°[2]
Astrometry
Distance from Earth	1,600 ly[2]

Barnard's loop (Sharpless 276, part of the Orion-Eridanus Bubble) is a diffuse nebula in the constellation of Orion, the hunter.^[2] The nebula was discovered by E. E. Barnard in 1895 utilising long exposure photographic methods.^[3] However it may have been observed by William Herschel, one of the great nebula discoverers, in February 1786.^[2] Unsurprisingly it is therefore far too faint to be observed with the naked eye. The nebula can be seen as a ring of bright gas surrounding Orion's belt. The Orion nebula along with the famous horsehead nebula are situated within Barnard's loop.

At 1,600 light years away, the nebula's apparent size of over 20° corresponds to a physical size of at least 582 light years.^[4] The segment that is visible is believed to be part of a larger shell of material, with only the easternmost segment being visible in optical light.^[5] The nebula is expanding at around 15-23 km/s.^[5] The nebula contains several O and B type stars.

Even this nebula may be the inner surface of an even larger nebula that is at least 30 degrees in diameter.^[6] As only the inner suface is ionized, only this is visible. The enveloping nebula would be truly vast and comprise of very hot, low density gas.^[2] Observations of the nebula in ultraviolet light have suggested the existence of this larger nebula.