Dark nebula
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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.[clarification needed] Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds. Isolated small dark nebulae are called Bok globules.
Dark clouds appear so because of submicrometre-sized dust particles, coated with frozen carbon monoxide and nitrogen, which effectively block the passage of light at visible wavelengths. Also present are molecular hydrogen, atomic helium, C18O, CS, NH3 (ammonia), H2CO (formaldehyde), c-C3H2 (cyclopropenylidene) and a molecular ion N2H+ (diazenylium), all of which are relatively transparent. These clouds are the spawning grounds of stars and planets, and understanding their development is essential to understanding star formation. [1][2]
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.
In the inner regions of dark nebulae important events take place, such as the formation of stars and masers.
