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Photosphere
From Wikipedia, the free encyclopedia
The photosphere of an astronomical object is the region from which externally received light originates. 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 2/3[1]. In other words, the photosphere is the deepest region of an object, usually a star, which is transparent to photons of a given wavelength.
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[edit] The Sun
The Sun's photosphere has a temperature between 4500 and 6000 kelvin [2] (with an effective temperature of 5800 kelvin) [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 kilometers 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 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.
[edit] Other layers
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.
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.
[edit] References
- ^ Carroll and Ostlie (1996). Modern Astrophysics, Addison-Wesley.
- ^ The Sun - Introduction
- ^ World Book at NASA - Sun
- ^ "SP-402 A New Sun: The Solar Results From Skylab".
- ^ "NASA/Marshall Solar Physics". NASA.
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