Photosphere


Photosphere

The photosphere of an astronomical object is the region from which externally received light comes. 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. It extends into a star's surface until the gas becomes opaque, equivalent to an optical 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 a photon of wavelength equal to 500 nanometers, 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, excepting neutron stars, 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 the Sun or another star's visual surface.

The sun

The Sun's photosphere has a temperature between 4500 and 6000 kelvins [ [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 a density 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 of convection cells called granules—cells of gas each approximately 1000 kilometers 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 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

*Meaning of sentence need to be clarified, please.


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  • Photosphere — Pho to*sphere, n. [Photo + sphere.] A sphere of light; esp., the luminous envelope of the sun. [1913 Webster] …   The Collaborative International Dictionary of English

  • photosphere — 1660s, from PHOTO (Cf. photo ) + SPHERE (Cf. sphere). Astronomical sense is from 1848 …   Etymology dictionary

  • photosphere — [fōt′ō sfir΄] n. [ PHOTO + SPHERE] the visible surface of the sun photospheric [fōt′ōsfer′ik, fōt′əsfer′ik] adj …   English World dictionary

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