Dark nebula
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A dark nebula is a large cloud which appears as star-poor regions where the dust of interstellar medium seems to be concentrated.
General information
Dark nebulae can be seen if they obscure part of an emission or reflection nebula (eg. the Horsehead Nebula) or if they block out background stars (eg. the Coalsack Nebula).
The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.
The hydrogen of these opaque dark clouds exists in the form of molecular hydrogen. The largest nebulae of this type, the so-called giant molecular clouds (GMC), are more than a million times as massive as the Sun. They contain much of the mass of the interstellar medium, are some 150 light-years across, and have an average density of 100 to 300 molecules per cubic centimetre and an internal temperature of only 7 to 15 K. Molecular clouds consist mainly of gas and dust but contain many stars as well. The cloud cores are completely hidden from view and would be undetectable except for the microwave emissions from their constituent molecules. This radiation is not absorbed by dust and readily escapes the cloud. The material within the clouds is clumped together in all sizes, with some clouds ranging down to the masses of individual stars, small clumps may extend about one light-year across. The clouds have an internal magnetic field that provides support against their own gravity.
GMCs play an important role in the Galaxy dynamics: when a star passes near a GMC, the considerable gravity pull will perturb the star's orbit by a significant amount. After repeated near encounters, a middle-aged star will have significant velocity components in all directions, instead of an almost circular orbit like a newborn star (this is because the newborn star inherits the circular orbit of the GMC where it was born). This gives the astronomer another tool to estimate star ages, and helps to explain the thickness of the galactic disk.
In the inner regions of dark nebulae important events take place: the formation of stars.de:Dunkelwolke fr:Nébuleuse sombre it:Nebulosa oscura fi:Pimeä kaasusumu