Type of interstellar cloud that obscures visible light
For other uses, see Dark Nebula.
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The dark nebula LDN 1768 in front of background stars
Star formation
Object classes
Interstellar medium
Molecular cloud
Bok globule
Dark nebula
Young stellar object
Protostar
Pre-main-sequence star
T Tauri star
Herbig Ae/Be star
Herbig–Haro object
Theoretical concepts
Accretion
Initial mass function
Jeans instability
Kelvin–Helmholtz mechanism
Nebular hypothesis
Planetary migration
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A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of 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 in the coldest, densest parts of 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, the things it obscures are visible only using radio waves in radio astronomy or infrared in infrared astronomy.
Dark clouds appear so because of sub-micrometre-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 (CO with oxygen as the 18O isotope), 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 closest and largest dark nebulae are visible to the naked eye, since they are the least obscured by stars in between Earth and the nebula, and because they have the largest angular size, appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great Rift. These naked-eye objects are sometimes known as dark cloud constellations and take on a variety of names.
In the inner molecular regions of dark nebulae, important events take place, such as the formation of stars and masers.
^Di Francesco, James; Hogerheijde, Michiel R.; Welch, William J.; Bergin, Edwin A. (November 2002). "Abundances of Molecular Species in Bernard 68". The Astrophysical Journal. 124 (5): 2749–2755. arXiv:astro-ph/0208298. Bibcode:2002AJ....124.2749D. doi:10.1086/344078. S2CID 119078546.
^ESO - eso9934 - Secrets of a Dark Cloud Archived 2009-02-04 at the Wayback Machine
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