Most stars use up the hydrogen at their core. When they do, their core becomes smaller and becomes hotter. It becomes so hot it pushes away the outer part of the star. The outer part expands and it makes a red giant star. Astro-physicists think that in about 5 billion years, the Sun will be a red giant. Our Sun will be so large it will eat the Earth. After our Sun stops using hydrogen to make energy, it will use helium in its very hot core. It will be hotter than when it was fusing hydrogen. Heavy stars will also make elements heavier than helium. As a star makes heavier and heavier elements, it makes less and less energy. Iron is a heavy element made in heavy stars. 