Visible surface of the Sun , about 250 mi (400 km) thick.
It emits most of the Sun's light that reaches Earth directly. Temperatures range from about 18,000 kF (10,000 kC) at the bottom to 8,000 kF (4,000 kC) at the top; its density is about 1/1,000 that of air at the surface of Earth. Sunspot s are photospheric phenomena. The photosphere has a granular structure. Each grain (cell), a mass of hot gas several hundred miles in diameter, rises from inside the Sun, radiates energy, and sinks back within minutes to be replaced by others in a constantly changing pattern.