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Dark nebula
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{{Short description|Type of interstellar cloud that obscures visible light}}{{Other uses|Dark Nebula (disambiguation){{!}}Dark Nebula}}{{More citations needed|date=June 2022}} [[File:All Quiet in the Nursery?.jpg|thumb|upright=1.5|The dark nebula [[LDN 1768]] in front of background stars]] {{Star formation}} A '''dark nebula''' or '''absorption nebula''' is a type of [[interstellar cloud]], particularly [[molecular cloud]]s, that is so dense that it obscures the [[Visible spectrum|visible wavelengths]] of [[light]] from objects behind it, such as background stars and [[emission nebula|emission]] or [[reflection nebula|reflection]] nebulae. The [[extinction (astronomy)|extinction]] of the light is caused by [[cosmic dust|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 globule]]s. 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, C<sup>18</sup>O (CO with [[oxygen]] as the [[Oxygen-18|<sup>18</sup>O isotope]]), CS, NH<sub>3</sub> ([[ammonia]]), H<sub>2</sub>CO ([[formaldehyde]]), c-C<sub>3</sub>H<sub>2</sub> ([[cyclopropenylidene]]) and a molecular ion N<sub>2</sub>H<sup>+</sup> ([[diazenylium]]), all of which are relatively transparent. <ref>{{Cite journal|last1=Di Francesco|first1=James|last2=Hogerheijde|first2=Michiel R.|last3=Welch|first3=William J.|last4=Bergin|first4=Edwin A.|date=November 2002|title=Abundances of Molecular Species in Bernard 68|journal=[[The Astrophysical Journal]]|volume=124|issue=5|pages=2749β2755|arxiv=astro-ph/0208298|bibcode=2002AJ....124.2749D|doi=10.1086/344078|s2cid=119078546}}</ref><ref>[http://www.eso.org/public/outreach/press-rel/pr-1999/phot-29-99.html ESO - eso9934 - Secrets of a Dark Cloud] {{webarchive|url=https://web.archive.org/web/20090204200348/http://eso.org/public/outreach/press-rel/pr-1999/phot-29-99.html |date=2009-02-04 }}</ref> 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 (astronomy)|Great Rift]]. These naked-eye objects are sometimes known as [[Constellation#Dark cloud constellations|dark cloud constellations]] and take on a variety of names. In the inner molecular regions of dark nebulae, events such as the [[star formation|formation of stars]] and [[astrophysical maser|masers]] take place. ==Complexes and constellations== Along with molecular clouds, dark nebula make up [[molecular cloud complex]]es. Dark nebula form in the night sky apparent [[dark cloud constellation]]s. {{excerpt|Constellation#List of dark cloud constellations}} [[File:Milkyway360-hemispheres-32k m14-g1 Clouds.jpg|thumb|650x650px|The Milky Way as seen by Gaia, with prominent dark features labeled in white, as well as prominent [[star cloud]]s labeled in black|center]] [[File:Regioni_celesti_scelte_-_Osc_A.png|thumb|650x650px|Main dark nebulae of the [[solar apex]] half of the [[galactic plane]]|center]] [[File:Regioni_celesti_scelte_-_Osc_B.png|thumb|650x650px|Main dark nebulae of the [[solar antapex]] half of the galactic plane|center]] == See also == * [[List of dark nebulae]] * [[Bok globule]] * [[Dark Cloud (disambiguation)]] == References == {{reflist}} {{nebula}} {{Star formation navbox}} {{Portal bar|Astronomy|Outer space}} {{Authority control}} [[Category:Dark nebulae| ]] [[Category:Nebulae]] [[Category:Cosmic dust]]
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