As a star expends its supply of hydrogen fuel, the helium produced in hydrogen fusion builds up steadily in the star's core. This type of fusion is much more energetic than hydrogen burning and consequently lasts for a much shorter time. For low-mass stars like the Sun, this is the final stage in its energy production. When the helium fuel is expended, the star puffs away its outer atmosphere in a few, final bursts of activity before quietly cooling off as a white dwarf.
More massive stars share a more dramatic fate.
Each of these successive stages of nuclear fusion lasts an even shorter period of time than the previous one, and the race to the finish is on. With the burning of silicon, iron collects at the star's core, and this stage cues the grand finale. As the central source of energy dies out, the outer layers of the star come crashing onto the iron core, crushing it into an exotic object called a neutron star, so dense that a spoonful weighs more than 100 million tons. For stars more than 10 times the mass of the Sun, a slightly different fate awaits. The neutron star that forms at the star's core is incapable of supporting the weight of the collapsing material, and it compresses further still.