The Sun. Illustration of the surface of the Sun showing sunspots (dark) and solar prominences (loops). Solar prominences occur where vast eruptions of charged particles (plasma) shoot out at high speed. The eruptions are confined within tube-shaped magnetic fields, producing the loops seen here. Sunspots are transient patches of relative coolness on the Sun's surface. The red layer above the surface, called the chromosphere, consists of hydrogen. The Sun is powered by a vast nuclear fusion reactor in its core, where the tem- perature is at least 14 million C. Constant output of energy means that the Sun loses about 4 million tons of mass each second.