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… (More on Photosphere)