Thephotosphere is a star's outer shell from whichlight is radiated. It extends into a star's surface until theplasma becomes opaque, equivalent to anoptical depth of approximately2⁄3,[1] or equivalently, a depth from which 50% of light will escape without being scattered.
Stars, except perhapsneutron stars, have no solid or liquid surface.[note 1] Therefore, the photosphere is typically used to describe theSun's or anotherstar's visual surface.
The termphotosphere is derived fromAncient Greek roots, φῶς, φωτός/phos,photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light.[citation needed]
Solar atmosphere: temperature and density.[5] Seehere for meanings of extra lines in the graph.
TheSun's photosphere has a temperature between 4,400 and 6,600 K (4,130 and 6,330 °C) (with an effective temperature of 5,772 K (5,499 °C))[6][7] meaning human eyes perceive it as an overwhelmingly bright surface, and with a sufficiently strong neutral density filter, as a hueless, gray surface. It has adensity of about 3×10−4kg/m3;[8] increasing with increasing depth.[5] The Sun's photosphere is 100–400 kilometers thick.[9][10]
In the Sun's photosphere, the most ubiquitous phenomenon aregranules—convection cells ofplasma each approximately 1,000 km (620 mi) in diameter with hot rising plasma in the center and cooler plasma falling in the spaces between them, flowing at velocities of 7 km/s (4.3 mi/s). Each granule has a lifespan of only about twenty minutes, resulting in a continually shifting "boiling" pattern. Grouping the typical granules aresupergranules up to 30,000 km (19,000 mi) in diameter with lifespans of up to 24 hours and flow speeds of about 500 m/s (1,600 ft/s), carryingmagnetic field bundles to the edges of the cells. Other magnetically related phenomena in the Sun's photosphere includesunspots and solarfaculae dispersed between granules.[11] These features are too fine to be directly observed on other stars; however, stellar spatial structures have been indirectly observed, and can behave like sunspots.We can refer to these asstarspots, although clearly non-analogous features also may be present as well.
^As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet;[2] and only neutron stars are believed to have a solid, albeit unstable,[3] crust[4]
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