Hi,
The Sun is so massive that the huge pressures in its center push the material there (mostly the lightest element, hydrogen) together. This process leads to heavier elements and also energy, and it is super efficient! The Sun can keep converting the material in its core to heavier elements and energy for billions of years before it starts running out. Eventually, though, it will run out of hydrogen in its core. At that point it will start a cycle in which heavier and heavier elements are used for fuel, but eventually, after a very, very long time (billions of years) we expect the Sun will run out of fuel and become a very slowly cooling white dwarf star. You can read more about it here:
http://www.windows2univer...ff/tourstars_29.html

best,
Terry