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 °F (10,000 °C) at the bottom to 8,000 °F (4,000 °C) at the top; its density is about 1/1,000 that of air at the surface of Earth. Sunspots 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.