The Horsehead Nebula is a famous diffuse dark nebula found in the Orion Molecular Cloud Complex in the constellation Orion.
It is a dark cloud composed of dust and gas where star formation is taking place. The nebula is also known as Barnard 33, and is located in the emission nebula IC 434. The Horsehead Nebula is approximately 1,500 light years distant from Earth.
The Horsehead Nebula is one of the most easily identifiable nebulae in the sky as its shape resembles that of a horse’s head and neck when observed from Earth. The swirling clouds of gas and dark dust are lit by a pinkish glow of hydrogen gas located behind the nebula and ionized by the nearby bright star Sigma Orionis. Sigma Orionis, which is in fact a five-star system, illuminates the entire region.
The brighter star visible in this area of the sky in images is Zeta Orionis, but it is located in the foreground and not related to the Horsehead Nebula.
The nebula formed from a collapse of an interstellar cloud of material and appears dark mainly because of the thick dust in the neighbouring area, with the bright spots at the base marking hidden protostars, newly formed or forming young stars.
The Horsehead can only be seen because its obscuring dust is silhouetted against the brighter nebula IC 434. The nebulous region that forms the horse’s head is just part of a larger dust cloud.
The nebula is really an exceptionally dense cloud projecting in front of the ionized hydrogen gas. This is revealed by the fact that lower side of the horse’s neck is particularly dark and casts a shadow to the east.
The glowing hydrogen gas marks the edge of the dark cloud that forms the nebula, which is evident in the notable change in the density of visible stars on either side of the cloud. The brighter region at the top left edge of the Horsehead is a young star still hidden in the gas and dust in which it was formed.
The jutting pillar that forms the Horsehead has a high density of helium and hydrogen, which makes it hard to erode. While the gas clouds that surrounded the nebula have already dissipated, it will be another five million years before the pillar dissipates too.
FACTS, LOCATION AND SIZE
The Horsehead Nebula is one of the best known and most photographed stellar nurseries in the night sky. It can contain more than a hundred known gases, both organic and inorganic, and also complex, large organic molecules of dust.
The nebula was first recorded on a photographic plate in 1888 by the Scottish astronomer Williamina Fleming at the Harvard College Observatory.
In April 2013, the Hubble Space Telescope photographed the nebula in astounding detail, revealing two new stars located in the top ridge. NASA explained in a release that one of these stars is emitting “harsh ultraviolet glare” which is slowly stripping away the cloud. “Gas clouds surrounding the Horsehead already have dissipated, but the tip of the jutting pillar contains a slightly higher density of hydrogen and helium, laced with dust,” said NASA. “This casts a shadow that protects material behind it from being stripped away by intense stellar radiation evaporating the hydrogen cloud, and a pillar structure forms.”
As one of the nearest regions where massive stars are formed, the cloud complex is also most easily photographed, making these some of the most recognizable objects in the sky.
The Horsehead Nebula can be found just to the south of Alnitak, Zeta Orionis, the easternmost star of Orion’s Belt. It is 2-3 light years across and a challenging object to find with a small telescope.
The nebula appears shadowy in optical light, but transparent at infrared wavelengths, revealing the Milky Way and galaxies in the background.
Horsehead Nebula (Barnard 33)
Location: 05h40m59.0s (right ascension), -02°27’30.0” (declination)
Constellation: Orion, the Hunter
Distance: 1,500 light years
Apparent size: 8 x 6 arcminutes
Designations: Barnard 33, IC 434, LDN 1630, M3T 31, [OS98] 52