The Sun. Computer graphic of the Sun, with a swirling abstract heat pattern around it. The Sun is a massive nuclear fusion reactor where hydrogen atoms fuse into helium to release heat and light energy. Its core has a temperature of at least 14 million degrees Celsius. This energy radiates out to form a turbulent surface to the Sun, and a corona of light and heat which extends millions of kilometres into space. Its constant output of energy means that the Sun loses about 4 million tons of mass each second.