The Horsehead Nebula (B33) is a famous dark nebula located in the constellation Orion. It is part of the Orion Molecular Cloud Complex, one of the nearest active regions of star formation to the Sun. The nebula lies at an approximate distance of 1,375 light-years. It appears in the southern portion of a dense dark dust cloud catalogued as Lynds 1630 (LND 1630).
The Horsehead Nebula is one of the most identifiable nebulae in the sky. The swirling clouds of gas and dark dust that form the nebula have the shape of a horse’s head, giving the nebula its name. The dark dust clouds are made visible by the pinkish glow of hydrogen gas located behind the nebula.
In images, the darkness of the Horsehead Nebula stands out against the emission nebulosity in the background. The red glow of the ionized hydrogen gas behind the nebula is caused by the nearby bright star Sigma Orionis, which partly illuminates the Horsehead.Sigma Orionis is a multiple star system composed of exceptionally hot stars of the spectral types O, B, and A. The brightest components are over 10 times more massive than the Sun and destined to go out as supernovae when they reach the end of their short life cycles. The stars are less than 2 million years old.
The brighter star visible in this area of the sky in images is Alnitak (Zeta Orionis), the easternmost star of the Belt of Orion. The hot blue supergiant is located in the foreground and is not related to the Horsehead.
The Horsehead Nebula is one of the most popular astrophotography targets in the sky. Its unusual shape and the colourful surroundings of the Orion’s Belt region make it a favourite for amateur astronomers.
The dark nebula appears along the edge of the large emission nebula IC 434. IC 434 is an active star-forming region located approximately 1,500 light-years away. It has an apparent magnitude of 7.3. The Horsehead appears silhouetted against it. The nebula IC 434 was discovered by the German-born British astronomer William Herschel in 1786 and the Horsehead was first recorded a full century later, in the late 1800s.
The Horsehead Nebula formed from a collapse of an interstellar cloud of material and appears dark mainly because it consists of thick dust. It can only be seen because its obscuring dust is backlit by the brighter nebula IC 434. The nebulous region that forms the horse’s head is part of a larger dust cloud that blocks the light from the star-forming region behind it.
The Horsehead Nebula is mostly composed of cold molecular hydrogen, which does not emit any light and radiates very little heat. The nebula’s clouds are very thick and block the light of any stars within and behind the nebula. A near-infrared image taken by the Hubble Space Telescope (HST) provided astronomers with a better look at the nebula’s structure and the stars embedded within it as well as those located in the background.
The nebula appears shadowy in optical light, but transparent at infrared wavelengths, revealing the Milky Way and galaxies in the background. The bright spots at the Horsehead Nebula’s base mark very young stars that are still in the process of forming.
The Horsehead contains protostars, but its clouds are so dense that they block the visible light of newly formed stars. If there are young stars in the nebula, they will consume a lot of the nebula’s material in the process of forming.
The nebula will dissipate in the next 5 billion years. Its dust clouds will be gradually eroded away by the ultraviolet light of the luminous young stars in the vicinity. The strong radiation of the young stars will tear apart the nebula’s gas molecules and blow away the dust through a process known as photodissociation. Eventually, the view of the nebula will be replaced by one of massive young blue stars.
A young stellar object, catalogued as IRAS 05383−0228, was identified in the nebula in a study published in 1984. The peak temperatures of the Horsehead’s neck, mane and western ridge occur along the western rim, near the young stellar object.
A study published in The Astronomical Journal in 2009 identified two bona fide young stellar objects (YSOs) – designated B33-1 and B33-28 – and five candidate protostars in the Horsehead Nebula. The protostars lie at the nebula’s western limb. The data indicated that they are in the same phase of their life cycles and formed at about the same time. B33-1 appears to be coming out of a cavity that is visible in optical wavelengths, while B33-28 is optically invisible. The candidate YSOs range from foreground to partly embedded to fully embedded objects. The ages of the two protostars are similar to the formation timescale of the nebula. The study found no evidence of sequential star formation in the Horsehead.
The streaks of nebulosity in the foreground of the Horsehead are likely caused by strong magnetic fields, which funnel the streams of gas from the nebula.
The Horsehead Nebula is one of the best-known dark nebulae in the sky, along with the Coalsack Nebula in the constellation Crux, the Great Dark Horse in Ophiuchus, and the Dark Doodad Nebula in Musca.
The nebula was discovered by the Scottish astronomer Williamina Fleming on a photographic plate at the Harvard College Observatory in 1888. Fleming worked as a human “computer,” helping with the processing of astronomical data and the photographic classification of stellar spectra. She was one of the founding members of the Harvard Computers. She was hired by Edward Charles Pickering, the director of the observatory, in whose home she had previously worked as a maid.
Fleming described the bright nebula around the Horsehead (IC 434) as having “a semicircular indentation 5 minutes in diameter 30 minutes south of Zeta Orionis.” She was not credited for the discovery of the Horsehead until 1908, when the second Dreyer Index Catalogue was published. Her entire work had until that point been attributed to Pickering.
The Horsehead Nebula is catalogued as Barnard 33 (B33) in the Barnard Catalogue of Dark Markings in the Sky (1919), compiled by the American astronomer Edward Emerson Barnard. E. E. Barnard was one of the first astronomers to describe the nebula. He wrote, “Dark mass, diam. 4′, on nebulous strip extending south from ζ Orionis.” He photographed the nebula from the Lick Observatory in California in 1894. The first person to use the name Horsehead is unknown.
The Horsehead Nebula is part of the Orion Molecular Cloud Complex, a vast star-forming region that contains a multitude of stars less than 12 million years old. The Orion complex contains two giant molecular clouds, Orion A and Orion B. The Horsehead is part of the Orion B molecular cloud, which also contains the emission nebula IC 434, the Flame Nebula (NGC 2024), the reflection nebula Messier 78, and the nearby McNeil’s Nebula.
The Orion B cloud hosts several star-forming regions, the largest of which is the open cluster within the Flame Nebula. The dark lanes intersecting the Flame Nebula may be part of the same dark cloud that forms the Horsehead.
The Orion B molecular cloud was imaged by ESA’s Herschel Space Observatory in 2010 and 2011. The image provided astronomers with a far-infrared view of the star-forming regions within Orion B and allowed them to map the cloud. The Horsehead Nebula appears at the right edge of the cloud. It was sculpted by the strong stellar winds from the young clusters of massive stars that were born in this region.
Hubble Space Telescope image
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.”
At near-infrared wavelengths, the nebula’s material absorbs much less light, revealing the distribution of gas that will be accreted by protostars in the process of forming. The young star at the top left edge of the Horsehead Nebula has gathered material from the surrounding clouds of gas and dust. Its powerful radiation is eroding the remaining material in the vicinity.
The jutting pillar that forms the Horsehead has a higher 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.
The Horsehead Nebula is easy to find because it lies in the region of Orion’s Belt, one of the most recognizable asterisms in the sky. The nebula appears just south of Alnitak, the left (or easternmost) star of the Belt. It forms a triangle with Alnitak and the fainter Sigma Orionis.
The Horsehead Nebula appears in the same area as the emission and reflection nebula NGC 2023, the reflection nebula IC 435, and the emission nebula NGC 2024, better known as the Flame Nebula, which is ionized by Alnitak.
Located just east of Alnitak, the Flame Nebula is a good starting point for finding the Horsehead. If the Flame Nebula is visible in a telescope, then the Horsehead may be as well. If it is not, then either the sky is not dark enough or a larger telescope is required.
The Horsehead Nebula is a challenging target for amateur telescopes. It appears as a small, faint greyish patch surrounded by brighter regions of nebulosity, and it is somewhat washed out by the light of the bright Alnitak. Visually, it is best observed in 10-inch or larger telescopes with H-beta filters. Because it appears silhouetted against the faint IC 434, it requires exceptionally clear, dark skies. The horse’s snout can be spotted in 16-inch telescopes.
The nebula is best seen in long-exposure photographs, which capture details that the human eye cannot.
The nebula NGC 2023 appears about 15 arcminutes northeast of the Horsehead. It is one of the largest reflection nebulae in the sky, about 10 arcminutes across. It lies at a similar distance as the Horsehead, about 1,300 light-years away. The nebula reflects the light of the young hot blue Herbig Ae/Be star HD 37903. The region around the reflection nebula is formed of ionized gases and shines as an emission nebula.
The best time of the year to observe the Horsehead Nebula and other deep sky objects in Orion is during the month of January, when the constellation is prominent in the evening sky. Orion is one of the 15 equatorial constellations, visible from most locations on Earth for at least part of the year. It is particularly conspicuous in the evening during the northern hemisphere winter and southern hemisphere summer but can be seen for most of the year at some point in the night.
Horsehead Nebula – Barnard 33
|Right ascension||05h 40m 59.0s|
|Declination||−02° 27′ 30.0″|
|Apparent size||8 x 6 arcminutes|
|Distance||1,375 ± 54 light-years (422 ± 17 parsecs)|
|Names and designations||Horsehead Nebula, B33, Barnard 33, LDN 1630, IC 434|