Coma (comet): Difference between revisions
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[[image:ikeya-zhang-comet-by-rhemann.png|frame|right|The [[153P/Ikeya-Zhang|comet Ikeya-Zhang]] exhibiting a bright, condensed coma (March 2002)]] |
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[[File:X-rays from Hyakutake.jpg|thumb|left|[[X-ray]] emission from Hyakutake, as seen by the [[ROSAT]] satellite.]] |
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|last=Combi |first=Michael R. |
|last=Combi |first=Michael R. |
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|coauthors=Harris, W. M.; Smyth, W. H. |
|coauthors=Harris, W. M.; Smyth, W. H. |
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}}</ref> This gives a comet a "fuzzy" appearance when viewed in [[telescope]]s and distinguishes it from [[star]]s. |
}}</ref> This gives a comet a "fuzzy" appearance when viewed in [[telescope]]s and distinguishes it from [[star]]s. The word coma comes from the Greek κόμη, "hair") |
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The coma is generally made of [[Volatiles|ice]] and [[dust]].<ref name=Combi2004/> Water dominates up to 90% of the [[volatiles]] that outflow from the nucleus when the comet is within 3-4 [[Astronomical unit|AU]] of the Sun.<ref name=Combi2004/> The [[H2O|H<sub>2</sub>O]] parent molecule is destroyed primarily through [[photodissociation]] and to a much smaller extent [[photoionization]].<ref name=Combi2004/> The [[solar wind]] plays a minor role in the destruction of water compared to [[photochemistry]].<ref name=Combi2004/> Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's [[Comet tail|tail]] by [[radiation pressure|light pressure]]. |
The coma is generally made of [[Volatiles|ice]] and [[dust]].<ref name=Combi2004/> Water dominates up to 90% of the [[volatiles]] that outflow from the nucleus when the comet is within 3-4 [[Astronomical unit|AU]] of the Sun.<ref name=Combi2004/> The [[H2O|H<sub>2</sub>O]] parent molecule is destroyed primarily through [[photodissociation]] and to a much smaller extent [[photoionization]].<ref name=Combi2004/> The [[solar wind]] plays a minor role in the destruction of water compared to [[photochemistry]].<ref name=Combi2004/> Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's [[Comet tail|tail]] by [[radiation pressure|light pressure]]. |
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About a month after an outburst in October 2007, comet [[17P/Holmes]] briefly had a tenuous dust atmosphere larger than the Sun.<ref name=atmosphere>{{cite web |authorlink=David C. Jewitt |last=Jewitt |first=David |date=2007-11-09 |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=Comet Holmes Bigger Than The Sun |publisher=Institute for Astronomy at the University of Hawaii |accessdate=2007-11-17}}</ref> The [[Great Comet of 1811]] also had a coma roughly the diameter of the Sun.<ref name="primer">{{cite web |
About a month after an outburst in October 2007, comet [[17P/Holmes]] briefly had a tenuous dust atmosphere larger than the Sun.<ref name=atmosphere>{{cite web |authorlink=David C. Jewitt |last=Jewitt |first=David |date=2007-11-09 |url=http://www2.ess.ucla.edu/~jewitt/holmes.html |title=Comet Holmes Bigger Than The Sun |publisher=Institute for Astronomy at the University of Hawaii |accessdate=2007-11-17}}</ref> The [[Great Comet of 1811]] also had a coma roughly the diameter of the Sun.<ref name="primer">{{cite web |
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|title=The Comet Primer |
|title=The Comet Primer |
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In some cases, such as the [[Great Comet of 1882]], a comet develops a visible [[antitail]] or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail. |
In some cases, such as the [[Great Comet of 1882]], a comet develops a visible [[antitail]] or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail. |
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==X-rays== |
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[[File:PIA02118.jpg|thumb|150px|Tempel 1 in X-ray light by [[Chandra X-ray Observatory|Chandra]]]] |
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Comets were found to emit [[X-rays]] in 1996.<ref> |
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{{cite web |
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|author= |
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|title=First X-Rays from a Comet Discovered |
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|url=http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html |
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|publisher=[[Goddard Spaceflight Center]] |
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|date= |
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|accessdate=2006-03-05 |
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}}</ref> This surprised researchers, because X-ray emission is usually associated with very [[black body radiation|high-temperature bodies]]. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged [[ions]] fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping of" one or more electrons from the comet. This ripping off leads to the emission of X-rays and [[far ultraviolet]] [[photon]]s.<ref> |
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{{cite web |
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|title=Interaction model – Probing space weather with comets |
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|url=http://www.kvi.nl/~bodewits |
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|archiveurl=http://web.archive.org/web/20060213232726/http://www.kvi.nl/~bodewits |
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|archivedate=2006-02-13 |
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|publisher=KVI atomics physics |
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|accessdate=2009-04-26 |
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}}</ref> |
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==See also== |
==See also== |
Revision as of 14:56, 12 December 2013
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/96/17P-Holmes_Auvergne_2007_11_02.jpg/200px-17P-Holmes_Auvergne_2007_11_02.jpg)
Coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on its highly elliptical orbit; as the comet warms, parts of it sublimate.[1] This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars. The word coma comes from the Greek κόμη, "hair")
The coma is generally made of ice and dust.[1] Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun.[1] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.[1] The solar wind plays a minor role in the destruction of water compared to photochemistry.[1] Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure.
About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun.[2] The Great Comet of 1811 also had a coma roughly the diameter of the Sun.[3] Even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun.[3] At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.[3]
Stardust was a NASA mission to recover samples of a comet's coma.
In some cases, such as the Great Comet of 1882, a comet develops a visible antitail or dust tail, which points in a different direction and when the viewing angle and parallax are just right may appear to point in the opposite direction from the normal ion tail.
X-rays
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/PIA02118.jpg/150px-PIA02118.jpg)
Comets were found to emit X-rays in 1996.[4] This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping of" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.[5]
See also
References
- ^ a b c d e Combi, Michael R.; Harris; Smyth (2004). "Gas Dynamics and Kinetics in the Cometary Coma: Theory and Observations" (PDF). Lunar and Planetary Institute (Comets II). 745: 523–552. Bibcode:2004come.book..523C.
{{cite journal}}
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suggested) (help) - ^ Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. Retrieved 2007-11-17.
- ^ a b c Gary W. Kronk. "The Comet Primer". Cometography.com. Retrieved 2011-04-05.
- ^ "First X-Rays from a Comet Discovered". Goddard Spaceflight Center. Retrieved 2006-03-05.
- ^ "Interaction model – Probing space weather with comets". KVI atomics physics. Archived from the original on 2006-02-13. Retrieved 2009-04-26.