The Sun. Computer graphic of the Sun, showing its turbulent surface. The Sun is a massive nuclear fusion reactor in which hydrogen atoms fuse to helium to release heat and light energy. The core has a temperature of at least 14 million degrees Celsius. This energy radiates out to the surface which is called the photosphere, about 300 kilo- metres thick. The photosphere is affected by magnetic fields, producing phenomena like sunspots (cool areas, black); solar prominences (eruptions of charged particles) and solar wind, not seen here. Its constant output of energy means that the Sun loses about 4 million tons of mass each second.