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A granule is a convection cell in the
Granulation_Quiet_Sun_SST_25May2017.webm, Granulation motions on the Sun's surface observed by the
A granule is a convection cell in the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
's photosphere. They are caused by convection currents of plasma in the Sun's convective zone
A convection zone, convective zone or convective region of a star is a layer which is unstable due to convection. Energy is primarily or partially transported by convection in such a region. In a radiation zone, energy is transported by radiation ...
, 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 Stefan–Boltzmann law, 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 convective motion, Doppler shift
The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...
measurements of the light from individual granules provides evidence for the convective
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convect ...
nature of the granules.
A typical granule has a diameter on the order of and lasts 8 to 20 minutes before dissipating. At any one time, the Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
's surface is covered by about 4 million granules. Below the photosphere is a layer of " supergranules" up to in diameter with lifespans of up to 24 hours.
Gallery
Swedish Solar Telescope
The Swedish 1-m Solar Telescope (or SST) is a refracting solar telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. It is run by the Institute for Solar Physics of Stockholm University. The primary element is a single ...
Sunspot_SST_05Sep2016.png, Detailed view of a sunspot
Sunspots are solar phenomena, phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibi ...
and surrounding solar surface. The dense pattern of cells (unrelated to the sunspot) is ''granulation''; the individual cells are ''granules''.
References
External links
* {{The Sun Sun Solar phenomena