A dark nebula is a type of interstellar cloud that is so dense that it obscures the light from the background emission or reflection nebula (e.g., the Horsehead Nebula) or that it blocks out background stars (e.g., the Coalsack Nebula).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.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.
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A region of space in which the density of material is greater than that in the diffuse interstellar medium. Dark nebulae are visible because they block out the light from background stars and 1600 emission nebulae . It is within the dense cores of dark clouds that stars are formed. See also absorption nebula, Bok globule.
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