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Created page with "thumb|alt=Orion nebula|right|210px|The [[Orion nebula, a diffuse nebula]] A '''diffuse nebula''' is a form of neb..."
[[File:600px-Orion Nebula - Hubble 2006 mosaic 18000.jpg|thumb|alt=Orion nebula|right|210px|The [[Orion nebula]], a diffuse nebula]]
A '''diffuse nebula''' is a form of [[nebula]], composed of large bright clouds of low [[density]] [[gas]] and [[dust]].<ref name=seds>[http://www.messier.seds.org/diffuse.html Diffuse nebulae] from messier.seds.org</ref> There are two types of diffuse nebulae: emission and reflection.<ref name=seds/> They are mainly composed of [[ionization|ionized]] [[Hydrogen]], leading to the sometimes being called H II regions. They often appear red/pink in colour and are associated with large clusters of [[star]]s. Examples of diffuse nebulae include the [[Orion nebula]], the [[Lagoon nebula]] and the [[Omega nebula]].
==History==
In 1610, Nicholas-Claude Fabri de Peiresc discovered the [[Orion nebula]].<ref name=seds/> It was not until around 1731 that Jean-Jacques D'Ortous De Mairan discovered M43. Several more were discovered in the following decades. These nebulae were originally believed to be star clouds or unresolved star clusters. However, the research of William Higgins in the 1860s using spectroscopic methods showed that they were composed of gas.
==Properties==
The two types of diffuse nebula depend on whether the [[light]] emitted from the nebula is produced when excited atoms decay or instead light is reflected. Emission nebulae produce light from the decay of excited atoms. They contain large hot [[star]]s that emit large amounts of high energy radiation. This can excite the surrounding gas, which then re-emits this radiation later when atoms return to a lower energy state.<ref name=seds/> On the other hand, reflection nebulae reflect the light that interacts with [[dust]] particles. This leads reflection nebula to often have a white/blue appearance. Furthermore, diffuse nebulae can be a mixture of both emission and reflection, an example being the [[Triffid nebula]].
===Emission Nebulae===
These clouds of ionized gas typically have masses in the range of 100-10,000 that of the [[sun]].<ref name=swinEmission>[http://astronomy.swin.edu.au/cosmos/E/Emission+Nebula Emission Nebula] from astronomy.swin.edu.au</ref> Their size can vary from being less than a light year across to many hundreds, resulting in a great variation of their [[density|densities]] from 1 to 1,000,000 atoms per cubic centimetre. Unsurprisingly given they contain ionized gas, emission nebulae are very hot. Their average [[temperature]] is of the order of 10,000 [[kelvin]].
Often a cloud of gas is surrounded by surrounded by O and B type stars. These emit large quantities of [[Ultraviolet]] radiation which ionzies [[hydrogen]] present within the nebula forming a cloud of [[proton]]s and [[electron]]s. When these recombine to produce hydrogen, they emit light with a [[wavelength]] of 656.3 nm.<ref name=swinEmission/> This light is red and the reason may emission nebula have a red/pink colour.
A [[planetary nebula]] is an example of an emission nebula. it is comprised of a [[white dwarf]] surrounded by various ionized gas, usually other than hydrogen. This leads to other colours, such as blue caused by singly ionized [[helium]] and green caused by doubly ionized [[oxygen]].<ref>Doubly ionized means that two [[electron]]s have been removed from the atom instead of just one.</ref> A spectacular example of this is the [[Ring nebula]] that contains several colours.
===Reflection Nebulae===
When light is reflected off dust, it is often [[polarized light|polarized]].<ref>[http://astronomy.swin.edu.au/cosmos/R/Reflection+Nebula Reflection nebula] from astrnomy.swin.edu.au</ref> As the size of dust particles tends to be similar to the wavelength of visible light, blue light is scattered more than red. This results in the nebula having a similar spectra to the stars illuminating them but stronger in the blue component. Ultimately, this means they tend to appear blue. It is difficult to estimate the size, as only the portion of the cloud that is reflecting enough light to be detected can be seen.
Reflection nebulae can be determined to be a reflection nebula using spectroscopic methods; if a nebula is a reflection nebula, it should have the roughly same spectra as nearby stars.<ref name=seds/>
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]
A '''diffuse nebula''' is a form of [[nebula]], composed of large bright clouds of low [[density]] [[gas]] and [[dust]].<ref name=seds>[http://www.messier.seds.org/diffuse.html Diffuse nebulae] from messier.seds.org</ref> There are two types of diffuse nebulae: emission and reflection.<ref name=seds/> They are mainly composed of [[ionization|ionized]] [[Hydrogen]], leading to the sometimes being called H II regions. They often appear red/pink in colour and are associated with large clusters of [[star]]s. Examples of diffuse nebulae include the [[Orion nebula]], the [[Lagoon nebula]] and the [[Omega nebula]].
==History==
In 1610, Nicholas-Claude Fabri de Peiresc discovered the [[Orion nebula]].<ref name=seds/> It was not until around 1731 that Jean-Jacques D'Ortous De Mairan discovered M43. Several more were discovered in the following decades. These nebulae were originally believed to be star clouds or unresolved star clusters. However, the research of William Higgins in the 1860s using spectroscopic methods showed that they were composed of gas.
==Properties==
The two types of diffuse nebula depend on whether the [[light]] emitted from the nebula is produced when excited atoms decay or instead light is reflected. Emission nebulae produce light from the decay of excited atoms. They contain large hot [[star]]s that emit large amounts of high energy radiation. This can excite the surrounding gas, which then re-emits this radiation later when atoms return to a lower energy state.<ref name=seds/> On the other hand, reflection nebulae reflect the light that interacts with [[dust]] particles. This leads reflection nebula to often have a white/blue appearance. Furthermore, diffuse nebulae can be a mixture of both emission and reflection, an example being the [[Triffid nebula]].
===Emission Nebulae===
These clouds of ionized gas typically have masses in the range of 100-10,000 that of the [[sun]].<ref name=swinEmission>[http://astronomy.swin.edu.au/cosmos/E/Emission+Nebula Emission Nebula] from astronomy.swin.edu.au</ref> Their size can vary from being less than a light year across to many hundreds, resulting in a great variation of their [[density|densities]] from 1 to 1,000,000 atoms per cubic centimetre. Unsurprisingly given they contain ionized gas, emission nebulae are very hot. Their average [[temperature]] is of the order of 10,000 [[kelvin]].
Often a cloud of gas is surrounded by surrounded by O and B type stars. These emit large quantities of [[Ultraviolet]] radiation which ionzies [[hydrogen]] present within the nebula forming a cloud of [[proton]]s and [[electron]]s. When these recombine to produce hydrogen, they emit light with a [[wavelength]] of 656.3 nm.<ref name=swinEmission/> This light is red and the reason may emission nebula have a red/pink colour.
A [[planetary nebula]] is an example of an emission nebula. it is comprised of a [[white dwarf]] surrounded by various ionized gas, usually other than hydrogen. This leads to other colours, such as blue caused by singly ionized [[helium]] and green caused by doubly ionized [[oxygen]].<ref>Doubly ionized means that two [[electron]]s have been removed from the atom instead of just one.</ref> A spectacular example of this is the [[Ring nebula]] that contains several colours.
===Reflection Nebulae===
When light is reflected off dust, it is often [[polarized light|polarized]].<ref>[http://astronomy.swin.edu.au/cosmos/R/Reflection+Nebula Reflection nebula] from astrnomy.swin.edu.au</ref> As the size of dust particles tends to be similar to the wavelength of visible light, blue light is scattered more than red. This results in the nebula having a similar spectra to the stars illuminating them but stronger in the blue component. Ultimately, this means they tend to appear blue. It is difficult to estimate the size, as only the portion of the cloud that is reflecting enough light to be detected can be seen.
Reflection nebulae can be determined to be a reflection nebula using spectroscopic methods; if a nebula is a reflection nebula, it should have the roughly same spectra as nearby stars.<ref name=seds/>
==References==
{{reflist}}
{{Nebulae}}
[[Category:Astronomy]]
[[Category:Nebulae]]