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Coma (cometary)
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. This gives a comet a "fuzzy" appearance when viewed in telescopes and distinguishes it from stars. The word coma comes from the Greek "kome", in Greek written as κόμη, and can be translated as "hair".〔( Comet appearance and structure )〕
The coma is generally made of ice and comet dust.〔 Water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun.〔 The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.〔 The solar wind plays a minor role in the destruction of water compared to photochemistry.〔 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.
On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON). On 2 June 2015, NASA reported that the ALICE spectrograph on the ''Rosetta'' space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.
==Size==
Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, even though the density is very low.〔 About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. 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.〔 At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, enlarging the tail.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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