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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" (the distance at which the light is diminished by a power of e) of 2 or 3. In other words, the photosphere is the region where an object stops being transparent to ordinary light. 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 nanometres, since the total amount of energy emitted by the star is equal to the energy emitted by a gas at that radius. Because 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 is a ball-shaped surface perceived to emit light.
The Sun's photosphere has an effective temperature of about 5778 Kelvins 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 kilometres in diameter with hot rising gas in the center and cooler gases 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 supergranules up to 30,000 kilometres in diameter with lifespans of up to 24 hours. These details are too fine to see on other stars.
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.