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A dark nebula or absorption nebula is a type of interstellar cloud 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 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.

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 largest dark nebulae are visible to the naked eye, 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 outer molecular regions of dark nebulae, important events take place, such as the formation of stars and masers.
Image gallery

Dark nebula LDN 1768 contains protostars.[3]

Dark nebula called LDN1774, taken by the Wide Field Imager, an instrument mounted on ESO’s 2.2-meter MPG/ESO telescope at La Silla.[4]

Dark nebula LDN 483 is located about 700 light-years away in the constellation of Serpens (The Serpent).[5]

Lupus 4 is a dense pocket of gas and dust where new stars are expected to form.[6]

See also

List of dark nebulae
Bok globule

References

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
"All Quiet in the Nursery?". Retrieved 23 March 2016.
"A Hole in the Sky". Retrieved 14 June 2015.
"The dark nebula LDN 483". www.eso.org. European Southern Observatory. Retrieved 14 January 2015.

"Cosmic Forecast: Dark Clouds Will Give Way to Sunshine". www.eso.org. European Southern Observatory. Retrieved 8 September 2014.

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Nebula
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Dark nebula Diffuse nebula
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Planetary nebula Supernova remnant Nova remnant H II region Reflection nebula
Variable nebula Preplanetary nebula

Pre-stellar nebula

Giant molecular cloud Bok globule Evaporating gaseous globule Solar nebula

Stellar nebula

Nova remnant Protoplanetary nebula Wolf–Rayet nebula

Post-stellar nebula

Planetary nebula Supernova remnant Pulsar wind nebula Supershell

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Star formation
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Interstellar medium Molecular cloud Bok globule Dark nebula Young stellar object Protostar T Tauri star Pre-main-sequence star Herbig Ae/Be star Herbig–Haro object


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Initial mass function Jeans instability Kelvin–Helmholtz mechanism Nebular hypothesis Planetary migration

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