The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived to emit light. It extends into a star's surface until the gas becomes opaque, equivalent to an optical depth of approximately 2/3. In other words, a ph...
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The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived to emit light. It extends into a star's surface until the gas becomes opaque, equivalent to an optical depth of approximately 2/3. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.
The surface of a star is defined to have a temperature given by the effective temperature in the Stefan–Boltzmann law. By using a simple model for stellar atmospheres, assuming local thermal equilibrium in a plane parallel geometry and the Eddington approximation, the effective temperature of the sun can be shown to occur at an optical depth of 2/3. This indicates the surface of a star is not at the top of the atmosphere...
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