The photosphere of an astronomical object's the region from which externally received light originates. The term itself's derived from Ancient Greek roots, φως¨- φωτος/photos meaning "light" and σφαιρα/sphaira meaning "ball," in reference to the fact that it's a ball-shaped 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.… (More on Photosphere)