Barnard's loop

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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.

Properties and Structure

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 visisible 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.

References

  1. 1.0 1.1 Barnard's Loop from the SIMBAD Astronomical Database
  2. 2.0 2.1 2.2 2.3 2.4 Barnard's loop from messier.seds.org
  3. Barnard's Loop Around Orion from apod.nasa.gov
  4. By direct calculation using values given here.
  5. 5.0 5.1 O'Dell, C., Ferland, G., Porter, R. and van Hoof, P. (2011). Physical Conditions in Barnard’S Loop, Components of the Orion-Eridanus Bubble, and Implications for the Warm Ionized Medium Component of the Interstellar Medium. The Astrophysical Journal, 733(1), p.9. arXiv:1103.2789
  6. Barnard's Loop from daviddarling.info