When a star gets old enough, or burns through all of its fuel, the elements that it had been fusing together get heavier and heavier (hydrogen to helium to lithium and so on) until they create iron.
However, as a star does not have the energy to fuse iron, the star’s energy gets dumped into an ever-increasing ball of iron and fusion eventually ceases. With fusion gone, gravity takes over and crushes the iron inward with such force that it generates the most powerful explosion in the universe: a supernova.
Once the star begins an attempt to fuse the iron made in the core of a star, it has less than a second to live.
More information on the life cycles of stars here.