A star's energy comes from the combining of light elements into heavier elements in a process known as fusion. Our Sun is currently burning, or fusing, hydrogen to helium. After the hydrogen in the star's core is exhausted, the star can burn helium to form progressively heavier elements, carbon and oxygen and so on, until iron and nickel are formed. Supernova explosions result when the cores of massive stars have exhausted their fuel supplies and burned everything into iron and nickel. Image Credit: NASA