- Barnard 33 The Horsehead Nebula
Barnard 33 is the famous Horeshead Nebula, perhaps the best-known dark nebula, having been imaged innumerable times. It was first observed in 1888 on a photographic plate taken by the Scottish astronomer Williamina Fleming while working at the Harvard Observatory. The Horeshead Nebula is located around 1500 LY away. While it lies just below the bright star Alnitak in Orion's Belt, the illumination causing the bright background glow (which makes the dark nebula visible) actually comes from σ Orionis, the bright blue-white star on the right side of my image. This star is physically located a bit beyond the Horeshead Nebula at a distance of 1165 LY, and its strong UV radiation ionizes the background hydrogen gas to produce the characteristic red Hα emissions. In my image, Hα emissions dominate the Hβ emissions, so the background nebula glows more red than pink. Strong magnetic fields in the region organize the ionized gas into streamers flowing out from behind the dark wall of gas and dust from which the Horsehead arises.
Sigma Orionis is itself a multiple star system with at least five components. The brightest, Sigma A, is a Class O star with an apparent magnituide of 4.2, while Sigma B is a Class B star with an apparent magnitude of 5.1. These two stars form a physical pair, orbiting each other with a period of 170 years. The other stars in the system are drawf stars with spectral Classes B and A.
Lying in the upper left corner of my image is the bright reflection nebula NGC 2023. It represents a cavity in the large dark molecular cloud illuminated by the Class B star HD 38087.
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