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[[File:17P-Holmes Auvergne 2007 11 02.jpg|Comet [[17P/Holmes]], 2007/11/02|thumb|200px]]
[[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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'''Coma''' is the nebulous envelope around the [[Comet nucleus|nucleus]] of a [[comet]]. It is formed when the comet passes close to the [[Sun]] on its highly [[ellipse|elliptical]] [[orbit]]; as the comet warms, parts of it [[Sublimation (chemistry)|sublimate]].<ref name=Combi2004>{{cite journal
[[File:X-rays from Hyakutake.jpg|thumb|left|[[X-ray]] emission from Hyakutake, as seen by the [[ROSAT]] satellite.]]
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In [[astronomy]], a '''coma''' (from the Greek κόμη, "hair") is the nebulous envelope around the [[Comet nucleus|nucleus]] of a [[comet]]. It is formed when the comet passes close to the [[Sun]] on its highly [[ellipse|elliptical]] [[orbit]]; as the comet warms, parts of it [[Sublimation (chemistry)|sublimate]].<ref name=Combi2004>{{cite journal
|last=Combi |first=Michael R.
|last=Combi |first=Michael R.
|coauthors=Harris, W. M.; Smyth, W. H.
|coauthors=Harris, W. M.; Smyth, W. H.
Line 11: Line 10:
|last2=Harris
|last2=Harris
|last3=Smyth
|last3=Smyth
}}</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")


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&nbsp;[[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&nbsp;[[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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[[File:17P-Holmes Auvergne 2007 11 02.jpg|Comet [[17P/Holmes]], 2007/11/02|thumb|200px]]
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
|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.

==X-rays==
[[File:PIA02118.jpg|thumb|150px|Tempel 1 in X-ray light by [[Chandra X-ray Observatory|Chandra]]]]
Comets were found to emit [[X-rays]] in 1996.<ref>
{{cite web
|author=
|title=First X-Rays from a Comet Discovered
|url=http://heasarc.gsfc.nasa.gov/docs/rosat/hyakutake.html
|publisher=[[Goddard Spaceflight Center]]
|date=
|accessdate=2006-03-05
}}</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>
{{cite web
|title=Interaction model – Probing space weather with comets
|url=http://www.kvi.nl/~bodewits
|archiveurl=http://web.archive.org/web/20060213232726/http://www.kvi.nl/~bodewits
|archivedate=2006-02-13
|publisher=KVI atomics physics
|accessdate=2009-04-26
}}</ref>


==See also==
==See also==

Revision as of 14:56, 12 December 2013

Comet 17P/Holmes, 2007/11/02

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

Tempel 1 in X-ray light by Chandra

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

  1. ^ 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}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. Retrieved 2007-11-17.
  3. ^ a b c Gary W. Kronk. "The Comet Primer". Cometography.com. Retrieved 2011-04-05.
  4. ^ "First X-Rays from a Comet Discovered". Goddard Spaceflight Center. Retrieved 2006-03-05.
  5. ^ "Interaction model – Probing space weather with comets". KVI atomics physics. Archived from the original on 2006-02-13. Retrieved 2009-04-26.
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