We are, as I like to say, starstuff."
Stars convert hydrogen to helium to produce light (and other radiation). As time progresses, the heavier helium sinks to the center of the star, with a shell of hydrogen around this helium center core. The hydrogen is depleted so it no longer generates enough energy and pressure to support the outer layers of the star. As the star collapses, the pressure and temperature rise until it is high enough for helium to fuse into carbon, i.e. helium burning begins. To radiate the energy produced by the helium burning, the star expands into a Red Giant.
The Sun will run out of fuel and become a red giant in about 4 billion years. What do you think will happen to the Earth then? Click here for the answer.