- Photosphere
The photosphere of an
astronomical object is the region from which externally received light comes. The term itself is derived fromAncient 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. It extends into a star's surface until the gas becomes opaque, equivalent to anoptical depth of 2/3Fact|date=January 2008. In other words, the photosphere is the region where an object stops being transparent to ordinary light.Effective temperature
The effective temperature of the photosphere corresponds to the position where the
optical depth becomes 2/3 for aphoton ofwavelength equal to 500nanometer s, since the total amount of energy emitted by the star is equal to the energy emitted by a gas at that radiusFact|date=January 2008. Because stars, exceptingneutron star s, have no solid surface, [As of 2004, although white dwarfs are believed to crystallize from the middle out, none have fully solidified yet [http://arxiv.org/abs/astro-ph/0411199v1] ; and only neutron stars are believed to have a solid, albeit unstable [http://www.journals.uchicago.edu/doi/abs/10.1086/377351] , crust [http://www.ingentaconnect.com/content/bsc/mnr/2004/00000351/00000003/art00021] ] the photosphere is typically used to describe theSun or anotherstar 's visual surface.The sun
The Sun's photosphere has a temperature between 4500 and 6000
kelvin s [ [http://imagine.gsfc.nasa.gov/docs/science/know_l1/sun.html The Sun - Introduction] ] (5800 kelvin average) [ [http://www.nasa.gov/worldbook/sun_worldbook.html World Book at NASA - Sun] ] and adensity of about 2e|-4 kg m-3 cite web| url=http://history.nasa.gov/SP-402/p2.htm| title=SP-402 A New Sun: The Solar Results From Skylab] ; other stars may have hotter or cooler photospheres. The Sun's photosphere is composed ofconvection cell s called granules—cells of gas each approximately 1000kilometer s in diameter [cite web| url=http://solarscience.msfc.nasa.gov/feature1.shtml| title=NASA/Marshall Solar Physics| publisher=NASA ] 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 super granules up to 30,000 kilometers 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 exampleH-alpha ) lies just between the photosphere and the much hotter but more tenuouscorona . Other "surface features" on the photosphere aresolar flare s andsunspot s.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
*Meaning of sentence need to be clarified, please.
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