PHOTOSPHERE

visible surface of the Sun, about 400 kilometres (250 miles) thick, from which is emitted most of the Sun's light that reaches the Earth directly. Light generated deeper in the Sun cannot get out without absorption and re-emission. Temperatures in the Sun's photosphere range from about 10,000 K (18,000 F) at the bottom to 4,400 K (8,000 F) at the top; density is about 1/1,000 that of air at the surface of the Earth. Sunspots are photospheric phenomena. Large-scale photographic images of the photosphere show it to have a granular structure. Each grain or cell is a mass of hot gas several hundred kilometres in diameter; each rises from inside the Sun, radiates energy, and sinks back within a few minutes to be replaced by others in a constantly changing pattern, shown particularly well in time-lapse motion pictures of the Sun.