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. 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.
