Dark nebula

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The dark nebula LDN 1768 in front of background stars

Template:Star formation 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.

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, events such as the formation of stars and masers take place.

Complexes and constellations

Along with molecular clouds, dark nebula make up molecular cloud complexes.

Dark nebula form in the night sky apparent dark cloud constellations. Template:Excerpt

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The Milky Way as seen by Gaia, with prominent dark features labeled in white, as well as prominent star clouds labeled in black

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Main dark nebulae of the solar apex half of the galactic plane

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Main dark nebulae of the solar antapex half of the galactic plane

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Dust extinction within 1250 pc

See also

• List of dark nebulae
• Bok globule
• Dark Cloud (disambiguation)