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A dark nebula or absorption nebula is a type of interstellar cloud that is so dense it obscures the 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, 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.〔(James Di Francesco ''et al.'', 2002, "Abundances of Molecular Species in Barnard 68", ''The Astronomical Journal'', 124, 2749 )〕〔(ESO - eso9934 - Secrets of a Dark Cloud )〕 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. == Gallery == File:A Hole in the Sky.jpg|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.〔(【引用サイトリンク】url=http://www.eso.org/public/images/potw1518a/ )〕 File:The dark nebula LDN 483.jpg|Dark nebula LDN 483 is located about 700 light-years away in the constellation of Serpens (The Serpent). File:The Dark Cloud Lupus 4.jpg|Lupus 4 is a dense pocket of gas and dust where new stars are expected to form. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dark nebula」の詳細全文を読む スポンサード リンク
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