The core is the innermost layer of the Sun.At the core, gravity pulls all of the mass inward and creates an extremely high pressure. The pressure is high enough to force atoms of hydrogen to come together in nuclear fusion reactions. Nuclear reaction takes place here and the temperature is as high as15×15 •C.During the nuclear fission,four hydrogen atoms combine to form a helium atom and large amount of energy is rreleased in the form of heat and light.