Editing Solar granule

{{Short description|Convection cell in the Sun's photosphere}}
{{Refimprove|date=March 2014}}

[[File:Highest_resolution_photo_of_Sun_(NSF)_as_of_January_20,_2020.jpg|thumb|High-resolution image of the Sun's surface taken by the [[Daniel K. Inouye Solar Telescope]] (DKIST).]]

In [[solar physics]] and [[Solar observation|observation]], '''granules''' are [[convection cell]]s in the [[Sun]]'s [[photosphere]]. They are caused by [[convection current|currents]] of [[Plasma (physics)|plasma]] in the Sun's [[convective zone]], directly below the photosphere. The grainy appearance of the photosphere is produced by the tops of these convective cells; this pattern is referred to as '''granulation'''.

The rising part of each granule is located in the center, where the plasma is hotter. The outer edges of the granules are darker due to cooler descending plasma. (The terms ''darker'' and ''cooler'' are strictly by comparison to the brighter, hotter plasma. According to the [[Stefan–Boltzmann law]], [[luminosity]] increases with the fourth power of temperature, causing even a small loss of heat to produce a large luminosity contrast.) In addition to the visible appearance, which can be explained by [[convection|convective motion]], [[Doppler shift]] measurements of the light from individual granules provides evidence for the [[convective]] nature of the granules.

A typical granule has a diameter on the order of {{convert|1500|km}}<ref name="JBZ">{{cite book |title=Sunquakes: Probing the Interior of the Sun |publisher=Johns Hopkins University Press |first=Jack B. |last=Zirker |page=2 |date=2003 |isbn=0-8018-7419-X |oclc=919928822}}</ref> and lasts 8 to 20 minutes before dissipating.<ref>{{cite journal |title=Lifetime of Solar Granules |journal=[[The Astrophysical Journal]] |first1=J. |last1=Bahng |first2=M. |last2=Schwarzschild |author2-link=Martin Schwarzschild |name-list-style=amp |volume=134 |page=312 |date=September 1961 |doi=10.1086/147160 |bibcode=1961ApJ...134..312B|doi-access=free }}</ref> At any one time, the [[Sun]]'s surface is covered by about 4 million granules. Below the photosphere is a layer of "[[Supergranulation|supergranules]]" up to {{convert|30000|km}} in diameter with lifespans of up to 24 hours.

Solar granules resemble [[Rayleigh–Bénard convection|Bénard cells]], but differ in their temporary nature, which is due to disturbances in the surrounding temperature gradient.<ref>{{cite journal |last1=Plaskett |first1=H. H. |author1-link=Harry Hemley Plaskett |title=Solar Granulation |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=1 March 1936 |volume=96 |issue=5 |page=418 |doi=10.1093/mnras/96.5.402 |doi-access=free |bibcode=1936MNRAS..96..402P }}</ref>

== References ==
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==External links==
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{{The Sun}}

[[Category:Sun]]
[[Category:Solar phenomena]]


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