After the supply of Hydrogen in the core is depleted the corresponding nuclear reactions stop (there are other fusion reactions, but they can occur only at higher temperatures than the ones present at the center of the star at this stage). Then the pressure drops and the gravitational collapse proceeds. During this process the center of the star is compressed more and more, increasing the central temperature until, finally, it becomes so hot that nuclear reactions involving Helium start up: Helium atoms slam together, and, after a complicated reaction produce Carbon. When these reactions turn on the energy output is enormous, the core becomes extremely hot and radiates a very large amount of energy. This radiation pushes out the outer layers of the star, and as they are pushed out they become a bit cooler and thus look redder. The star then becomes a red giant a bloated result of the burning of Helium Our sun will eventually go through this process and will grow to the point that it will engulf the orbits of Mercury, Venus and, possibly, the Earth.