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Absorption nebula

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Dark nebula
A dark nebula or absorption nebula is a type of interstellar cloud that is so dense it obscures the light from objects behind it, such as background stars and emission or reflection nebulae. The extinction of the light is caused by interstellar dust grains located in the coldest, densest parts of larger molecular clouds. Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules. Like other interstellar dust or material, things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy.

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ASTRONOMY UNBOUND Dictionary

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Absorption nebula
A region of dust and gas which has no stars to illuminate it from within. As such, these objects are completely dark and obscure any light from stars or emission nebulae which may lie behind them. These clouds are usually very cold, typical temperatures lie in the range of 10 to 20° Kelvin. They are composed of approximately cosmic abundance and provide the conditions necessary for some complex chemistry to take place. This chemistry is thought to occur between atoms which adhere to the surfaces of the dust particles and then come into contact with one another. It is in these cold clouds that gravitational instabilities occur which cause regions to collapse and form new stars. See also Barnard objects, Bok globules, emission nebulae, protostars.