Photosphere

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"Solar disk" redirects here. For the disk of the sun in ancient Egyptian mythology, see Aten.

The structure of the Sun

The photosphere of an astronomical object is the region from which externally received light comes. It extends into a star's surface until the gas becomes opaque, equivalent to an optical depth of 2 or 3^{[citation needed]}. In other words, the photosphere is the region where an object stops being transparent to ordinary light.

1 Effective temperature
2 The sun
3 Other layers
4 References

Effective temperature

The effective temperature of the photosphere corresponds to the position where the optical depth becomes 2/3 for a photon of wavelength equal to 500 nanometres, since the total amount of energy emitted by the star is equal to the energy emitted by a gas at that radius^{[citation needed]}. Because stars, excepting neutron stars, have no solid surface,^[1] the photosphere is typically used to describe the Sun or another star's visual surface. The term itself is derived from Ancient Greek roots, φως¨- φωτος/photos meaning "light" and σφαιρος/sphairos meaning "ball," in reference to the fact that it is a ball-shaped surface perceived to emit light

The sun

The Sun's photosphere has a temperature between 4400 and 6400 kelvins ^[2]^[3] and a density of about 2×10^-4 kg m^-3^[4]; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granules—cells of gas each approximately 1000 kilometres in diameter^[5] with hot rising gas in the center and cooler gas falling in the narrow spaces between them. Each granule has a lifespan of only about eight minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules are supergranules up to 30,000 kilometres in diameter with lifespans of up to 24 hours. These details are too fine to see on other stars.

Other layers

The Sun's visible atmosphere has other layers above the photosphere: the 10,000 kilometre-deep chromosphere (typically observed by filtered light, for example H-alpha) lies just between the photosphere and the much hotter but more tenuous corona. Other "surface features" on the photosphere are solar flares and sunspots.

The image of the surface shown in the illustration to the right is actually an ultraviolet image of helium gas at 30.4 nm (from the European Space Agency/NASA SOHO spacecraft), and comes from the chromosphere, which is just above the photosphere, so the "photosphere" label attached to this image is actually incorrect.

References

^ As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet[1]; and only neutron stars are believed to have a solid, albeit unstable[2], crust[3]
^ The Sun - Introduction
^ World Book at NASA - Sun
^ SP-402 A New Sun: The Solar Results From Skylab.
^ NASA/Marshall Solar Physics. NASA.

v • d • e The Sun
Structure	Solar core - Radiation zone - Convection zone
Atmosphere	Photosphere - Chromosphere - Transition region - Corona
Extended structure	Termination Shock - Heliosphere - Heliopause - Heliosheath - Bow Shock
Solar phenomena	Sunspots - Faculae - Granules - Supergranulation - Solar Wind - Spicules - Coronal loops - Solar Flares - Solar Prominences - Coronal Mass Ejections - Moreton Waves - Coronal Holes
Other	Solar System - Solar Variation - Solar Dynamo - Heliospheric Current Sheet - Solar Radiation - Solar Eclipse - Spectral Class: G2