Any star, including our Sun, begins its life with hydrogen as a fuel. The mass of all that gas pushing in on the center (core) of the star causes the temperature to rise to temperatures high enough that hydrogen fuses (fusion reaction) with itself to make helium. When all the hydrogen is used up in the core, the star fuses helium to make carbon. After that, if the star is about the mass of the Sun, its outer atmosphere puffs out (almost to or to Earth), it reddens, and the star simply cools off over millions of years. For stars that are around eight or more times as massive as the Sun, there is enough mass and pressure to fuse carbon, eventually these stars implode, layers hit the core and bounce back out, forming a supernova.