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IC 434, NGC 2024 and Barnard 33 - the Horsehead Nebula

Description of object:

South of the bright star Alnitak (Zeta Orione), the eastern of Orion's 3 belt stars, lies one of the most colorful and beautiful nebulous regions in the sky. In front of the reddish H-II region IC 434 a silhouette of a dark nebula consisting of cold dust and gas, the well-known Horsehead Nebula Barnard 33 (LDN 1630), stands out. Alnitak is a blue supergiant of spectral class O9 and is located at a distance of approximately 1100 light years from the solar system. Alnitak is not associated with the nebular region, it is much further away from the solar system with a distance of 1500 to 1600 light years. The Horsehead Nebula has a real diameter of about 3 light years and contains about 27 solar masses.

At the upper edge of our image, a bit right of the center, there is another blue O9 supergiant, the star Sigma Orionis which is a multiple system. It belongs to a small loose group of blue stars, which is classified as an open cluster. Remarkable is the long, straight ionization edge of IC 434, where the high-energy UV radiation of the Sigma Orionis cluster meets a large molecular cloud complex. East of IC 434 (below) it is Orion B, south (right) it is Orion A.

Inside Orion B are two prominent blue reflection nebulae, the larger one is NGC 2023 and the smaller one, the one further away from the Horsehead Nebula, is IC 435. In the center of NGC 2023 is the star HD 37903, the nebular region is classified as a star forming region of low mass stars, which is manifested by the absence of red H-alpha emission.

East of the belt star Alnitak is NGC 2024, sometimes also called the Flame Nebula. NGC 2024 is a mixture of emission and reflection nebulas.
Chaotic, thin dust regions extend through nebulous regions and "fissure" the nebula. With a size of 30 arcminutes, its size corresponds approximately to the full moon when viewed with the naked eye. The Flame Nebula was discovered by Friedrich Wilhelm Herschel on 1 January 1786.

Northwest of NGC 2024 are the two reflection nebulae IC 432 (LBN 946) and IC 431 (LBN 944). At the center of IC 432 is a star of spectral class B5, the variable V 901 Orionis. The region is interesting because the nebula also shows clear red H-alpha emission, but an ionizing hot star is missing. The central star in IC 431 is also a B5 star, HD 37674.

« The annotated image shows also a comparison to the size of the full moon.
Interesting is the story about the discovery of IC 434 and the Horsehead Nebula. The extensive nebula region was not visually observed for the first time, it was one of the first discoveries of the then new photography. In 1890 Edward Charles Pickering - about one year after the photograph was taken by his brother - reported about the nebular region. The discoverer is considered to be a colleague of Pickering, Williamina Fleming, in 1889.

In 2001, NASA conducted an international contest to find out which object the Hubble Space Telescope should photograph for its 11th birthday and the result was clear: the Horsehead Nebula. For its 23rd birthday (2013), the object was again recorded with the new WFC3 wide-angle camera in the near infrared spectral range. In this spectral range the emission nebula IC 434 is filtered out, so that the true structure of the dark cloud is more clearly visible.

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Source/Credit: ESA/Hubble »

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