View source for Physics:Granule (solar physics)

{{Short description|Convection cell in the Sun's photosphere}}

[[File:Highest_resolution_photo_of_Sun_(NSF)_as_of_January_20,_2020.jpg|thumb|275px|High-resolution image of the Sun's surface taken by the [[Astronomy:Daniel K. Inouye Solar Telescope|Daniel K. Inouye Solar Telescope]] (DKIST).]]
{{Structure of the Sun|right}}

A '''granule''' is a convection cell in the [[Astronomy:Sun|Sun]]'s [[Astronomy:Photosphere|photosphere]]. They are caused by convection currents of [[Physics:Plasma|plasma]] in the Sun's convective zone, directly below the photosphere. The grainy appearance of the solar photosphere is produced by the tops of these convective cells and is called ''granulation''.

The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. (The terms ''darker'' and ''cooler'' are strictly by comparison to the brighter, hotter plasma. According to the [[Physics:Stefan–Boltzmann law|Stefan–Boltzmann law]], [[Astronomy:Luminosity|luminosity]] increases with the fourth power of temperature leading to even a small loss of heat producing a large luminosity contrast.) In addition to the visible appearance, which would be explained by [[Astronomy: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=[[Astronomy:The Astrophysical Journal|The Astrophysical Journal]] |first1=J. |last1=Bahng |first2=M. |last2=Schwarzschild |name-list-style=amp |volume=134 |page=312 |date=September 1961 |doi=10.1086/147160 |bibcode=1961ApJ...134..312B}}</ref> At any one time, the [[Astronomy:Sun|Sun]]'s surface is covered by about 4 million granules. Below the photosphere is a layer of "[[Astronomy:Supergranulation|supergranules]]" up to {{convert|30000|km}} in diameter with lifespans of up to 24 hours.

== Gallery ==
<gallery mode=packed heights=200px style="text-align:left">
Granulation_Quiet_Sun_SST_25May2017.webm|Granulation motions on the Sun's surface observed by the [[Astronomy:Swedish Solar Telescope|Swedish Solar Telescope]].
Sunspot_SST_05Sep2016.png|Detailed view of a [[Astronomy:Sunspot|sunspot]] and surrounding solar surface. The dense pattern of cells (unrelated to the sunspot) is ''granulation''; the individual cells are ''granules''.
</gallery>

== References ==
{{Reflist}}

==External links==

{{The Sun}}

[[Category:Sun]]

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