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Photosphere
From Wikipedia, the free encyclopedia
The photosphere of an astronomical object is the region at which the optical depth becomes 2/3 for a photon of wavelength equal to 500 nanometers. In other words, the photosphere is the region where an object stops being transparent to ordinary light. Because stars are believed to have no solid surface, the photosphere is typically used to describe the Sun or another star's visual surface. The term itself is derived from Ancient Greek roots, φοτος/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's photosphere has a temperature of about 5800 kelvins; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed of convection cells called granules—firestorms each approximately 1000 kilometers in diameter with hot rising gas in the center and cooler gases falling in the narrow spaces between them. Each granule has a lifespan of only about eight minutes, resulting in a continually shifting "boiling" pattern. Amid the typical granules are supergranules up to 30,000 kilometers in diameter with lifespans of up to 24 hours. It is unknown whether these features are typical of other stars.
The Sun's visible atmosphere has other layers above the photosphere: the 10,000 kilometer-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.
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| 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 |
Categories: Stars | Sun