Everything about The Photosphere totally explained
The
photosphere of an
astronomical object is the region from which externally received light comes. It extends into a star's surface until the gas becomes opaque, equivalent to an
optical depth of 2 or 3. 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 radius. Because stars, excepting
neutron stars, 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,
φως¨- φωτος/
photos meaning "light" and
σφαιρος/
sphairos meaning "ball," in reference to the fact that it's a ball-shaped surface perceived to emit light
The sun
The Sun's photosphere has a temperature between 4500 and 6000
kelvins (5800 kelvin average) and a
density of about
2 kg m-3 ; 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 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.
Further Information
Get more info on 'Photosphere'.
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