TheHorsehead Nebula (also known asBarnard 33 or B33) is a smalldark nebula in the constellationOrion.[2] The nebula is located just to the south ofAlnitak, the easternmost star ofOrion's Belt, and is part of the much largerOrion molecular cloud complex. It appears within the southern region of the dense dust cloud known as Lynds 1630, along the edge of the much larger, active star-formingH II region calledIC 434.[3]
The Horsehead Nebula is approximately 422 parsecs or 1,375light-years from Earth.[1][3] It is one of the most identifiable nebulae because of its resemblance to ahorse's head.[4]
The dark cloud of dust and gas is a region in the Orion molecular cloud complex, where star formation is taking place. It is located in the constellation ofOrion, which is prominent in the winter evening sky in the Northern Hemisphere and the summer evening sky in the Southern Hemisphere.
The Horsehead Nebula and its surroundings. The reflection nebula NGC 2023 in the bottom left corner.
Colour images reveal a red colour that originates from ionisedhydrogen gas (Hα) predominantly behind the nebula, and caused by the nearby bright starSigma Orionis.Magnetic fields channel the gases, leaving the nebula into streams, shown as foreground streaks against the background glow.[8] A glowing strip of hydrogen gas marks the edge of the enormous cloud, and the densities of nearby stars are noticeably different on either side.
Heavy concentrations ofdust in the Horsehead Nebula region and neighbouring Orion Nebula are localized intointerstellar clouds, resulting in alternating sections of nearly complete opacity and transparency.[9] The darkness of the Horsehead is caused mostly by thick dust blocking the light of stars behind it.[10] The lower part of the Horsehead's neck casts a shadow to the left.[11] The visible dark nebula emerging from the gaseous complex is an active site of the formation of "low-mass" stars. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming.
Close up view of the Horsehead Nebula, taken by theHubble Space Telescope in 2001. The image was captured upon popular demand to celebrate the eleventh anniversary of the telescope.[12]The three bright stars ofOrion's Belt with the Horsehead Nebula to the lower left of the belt starAlnitak
^Sharp, Nigel (2014)."The Horsehead Nebula".National Optical Astronomy Observatory. Association of Universities for Research in Astronomy. Archived fromthe original on March 13, 2020. RetrievedJuly 21, 2014.
