Photosphere

Point where a star’s atmosphere appears to become completely opaque. Thus, the photosphere may be thought of as the imaginary surface from which the starlight that we see appears to be emitted. The photosphere is not a thin surface; the Sun’s photosphere has a thickness of ~100 km. (Within that 100 km, the temperature drops from 6000 K at the bottom to 4000 K at the top.) Diameters quoted for stars are the diameter of the photosphere.