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Created page with "A '''dark nebula''', also known as an absorption nebula, is a type of nebula formed from gas and dust and visible because it blocks out and absorbs light from st..."
A '''dark nebula''', also known as an absorption nebula, is a type of [[nebula]] formed from [[gas]] and [[dust]] and visible because it blocks out and absorbs light from [[star]]s or bright nebulae behind.<ref>[http://www.messier.seds.org/dark.html Dark nebulae] from messier.seds.org</ref> Dark nebulae may also contain stars, but these are obscured by the nebula. Well known examples of dark nebulae include the [[Horsehead nebula]] and the [[Coalsack nebula]]
==Properties==
Dark nebulae come in a range of sizes; the smallest are only a couple of [[light year]]s across, containing only a couple of thousand solar masses of material whereas the largest can be over 600 light years across, some containing more than a million solar masses of interstellar material.<ref name=swin>[http://astronomy.swin.edu.au/cosmos/D/Dark+Nebula Dark nebula] from astronomy.swin.edu.au</ref> The largest dark nebulae are called giant molecular clouds. Typical [[density|densities]] within clouds are 100-300 molecules per cubic centimetre.<ref name=uoregon>[http://abyss.uoregon.edu/~js/glossary/dark_nebula.html Dark nebula] from abyss.uoregon.edu</ref>
Although they are called dark nebulae, they are only dark at [[visible light|visible wavelengths]], in the region of 400-700 nm. The [[temperature]] inside dark nebula is often in the range 10-100 [[kelvin]], meaning [[hydrogen]] within the nebula exists as H<sub>2</sub> [[molecules]].<ref name=swin/> At this lower temperature, dust within the nebula emits strongly at longer [[infrared]] wavelengths.<ref name=uoregon/>
===Structure===
In the outer layers of the nebula, interstellar [[ultraviolet]] radiation causes hydrogen in the cloud to exist as neutral [[atom]]s.<ref name=uoregon/> These layers absorb this radiation, preventing it from reaching further into the cloud. This allows [[molecule]]s to form deeper within the nebula, and over 70 different chemical species have been found within such nebulae.<ref name=uoregon/> The extreme conditions of low temperatures and pressures produce unusual [[chemistry]]. As an example, both hydroisocyanic acid, HNC, and hydrocyanic acid, HCN, have been found in the same cloud in equal quantities, whereas on [[Earth]] hydroisocyanic acid would break down an reform into hydrocyanic acid.<ref name=uoregon/>
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]
==Properties==
Dark nebulae come in a range of sizes; the smallest are only a couple of [[light year]]s across, containing only a couple of thousand solar masses of material whereas the largest can be over 600 light years across, some containing more than a million solar masses of interstellar material.<ref name=swin>[http://astronomy.swin.edu.au/cosmos/D/Dark+Nebula Dark nebula] from astronomy.swin.edu.au</ref> The largest dark nebulae are called giant molecular clouds. Typical [[density|densities]] within clouds are 100-300 molecules per cubic centimetre.<ref name=uoregon>[http://abyss.uoregon.edu/~js/glossary/dark_nebula.html Dark nebula] from abyss.uoregon.edu</ref>
Although they are called dark nebulae, they are only dark at [[visible light|visible wavelengths]], in the region of 400-700 nm. The [[temperature]] inside dark nebula is often in the range 10-100 [[kelvin]], meaning [[hydrogen]] within the nebula exists as H<sub>2</sub> [[molecules]].<ref name=swin/> At this lower temperature, dust within the nebula emits strongly at longer [[infrared]] wavelengths.<ref name=uoregon/>
===Structure===
In the outer layers of the nebula, interstellar [[ultraviolet]] radiation causes hydrogen in the cloud to exist as neutral [[atom]]s.<ref name=uoregon/> These layers absorb this radiation, preventing it from reaching further into the cloud. This allows [[molecule]]s to form deeper within the nebula, and over 70 different chemical species have been found within such nebulae.<ref name=uoregon/> The extreme conditions of low temperatures and pressures produce unusual [[chemistry]]. As an example, both hydroisocyanic acid, HNC, and hydrocyanic acid, HCN, have been found in the same cloud in equal quantities, whereas on [[Earth]] hydroisocyanic acid would break down an reform into hydrocyanic acid.<ref name=uoregon/>
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]