What is a dark nebula?
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Updated June 01, 2014
A dark nebula is a type of interstellar cloud so dense that it obscures the light from background stars. Fortunately a few stars of the Milky Way come to pierce the vast dark clouds that separates the plane of our galaxy. In all nebulae there are dark clouds, it is also they who embellish the nebulae, the best example is the HorseHead Nebula or Serpent Nebula.
The extinction of light is caused by interstellar dust solid grains constituting the cloud.
Large complex of dark nebulae are associated with giant molecular clouds. The isolated small dark nebulae are called "Bok globules".
So the dark clouds appear due to the micron size particles of dust, covered with carbon oxide and frozen nitrogen, which effectively blocks the passage of light in the visible wavelengths. These clouds are rich in elements, there are also molecular hydrogen (H2), atomic helium, ammonia.
These clouds are nurseries of stars and planets. The largest dark nebulae are visible with the naked eye, they appear as black spots more or less luminous on the bottom of the Milky Way. This is fantastic pillars of absorbing dust and transparent gas sculpted by winds radiating nearby stars.
Depending on the temperature and density of the cloud, the gas, mainly hydrogen, can be found in the form of atom, ion or molecule. Thus ordinary matter is mainly composed of ionized hydrogen (H +), atomic (H1) and molecular (H2) and solid dust grains. Interstellar dust grains are actually simple assemblies of molecules which become more and more complex to achieve dimensions of 0.1 μ (approximately 10 000 molecules).
nota: Molecular hydrogen (H2) is formed of two hydrogen atoms chemically associated. Molecular clouds of interstellar nebulae whose density allows the formation of H2. The H2 molecule is not easily detectable but scientists have a tracer that allows us to say that there is molecular hydrogen in a cloud, this tracer is carbon monoxide (CO). Indeed, the relationship between the brightness of CO and H2 mass is almost constant. It is in molecular clouds that form stars, however they also form themselves in group from the gravitational collapse of several parts of the Bok globule (dark mass of dust).
image: Its dusty molecular clouds probably contain enough raw materials to form hundreds of thousands of stars. This region of the dark part of the constellation Aquila covers a field a little wider than the full moon vision. Despite these dark nebulae, a small nebula (RNO 109) tinted red is visible in the upper left and Herbig Haro (HH32) right above the center. The dark clouds of the Eagle are estimated at about 600 light years. At this distance, the field of view spans about 7 light-years. Credit & Copyright: Adam Block, Mt. Lemmon SkyCenter, University of Arizona.
Dark Nebula Horsehead
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Nebula Horsehead is a dark nebula in the constellation Orion. The nebula is located just below Alnitak (ζ Ori), the star to the east of Orion's belt. The nebula, located 1,350 light-years was discovered in 1888 on a photographic plate taken at the Harvard College Observatory.
It is easily recognizable by the shape horse head which gave it its name. Nebula Horse Head is part of a large molecular cloud black. Behind the nebula is hydrogen ionized by the nearby bright star Sigma Orionis which gives this beautiful red color.
The darkness of the horse's head is caused by the presence of a dense cloud of gas and dust. It strongly absorbs visible radiation emitted by the ionized gas background (red in the photo). At the base of the head, there are young stars in training.
Image: Horsehead Nebula in the constellation of Orion.
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The light and the dark of the Black river
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The central band of the Milky Way is related to Antares by a dust lane named "Black River". Black River is a cosmic cloud that connects the Pipe Nebula, the colored region located near the bright star Antares in the constellation Scorpius.
The opacity of the Black River is caused by the absorption of the starlight background by cosmic dust. This nebula contains mostly hydrogen and molecular gas. The red supergiant Antares, is surrounded by dust which forms a reflection nebula, whose color is yellow, bottom right image. Just above the bright blue double star Rho Ophiuchi is wrapped in a molecular cloud, a blue reflection nebula, while the emission nebulae, red, dot the area.
The globular cluster M4 red is visible just above and right of Antares, with a distance estimated at about 7,000 light-years more distant than the other colored clouds. As for the Black River, it lies about 500 light years. This colorful celestial landscape is in the constellation Scorpius, this mosaic of images taken with the telescope covers a field of almost 10 degrees, the equivalent of 20 full moons.
Image: Credit: ESA, Hubble, NASA
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The Pipe Nebula is a dark nebula in the constellation Ophiuchus. This cloud of gas and dust spread over a vast territory in the sky, and it belongs to an even larger complex known as Dark Horse Nebula. Despite its darkness, observers can easily locate the Pipe Nebula to the naked eye, away from city lights. It is located about one third of the way between the Lagoon Nebula and the star Antares. When we look at the Pipe Nebula, two elements stand out distinctively. One is the pipe and the other bowl of the pipe.
The nebula opaque drawn by the smoke of the pipe is a cloud that absorbs the light of background stars in the Milky Way. Edward Emerson Barnard, the pioneer of astrophotography, cataloged a series of dark nebula, Barnard 59, 65, 66, 67 (hose pipe), also known as LDN 1773 and Barnard 78 (home to the pipe) , also known as LDN 42.
Image: The dark cloud of the Pipe Nebula. Credit ESO GigaGalaxy Zoom