The Sun is not a giant ball of fire in the traditional sense. It is a giant ball of plasma, which is a state of matter that is neither a solid, liquid, nor gas. The plasma in the Sun is so hot and dense that it undergoes a process called nuclear fusion. In nuclear fusion, two atoms are fused together to form a new atom, releasing a large amount of energy in the process.

The Sun is made up mostly of hydrogen, and the nuclear fusion reaction in its core fuses hydrogen atoms together to form helium atoms. This process releases a huge amount of energy, which is what powers the Sun.

The Sun is so large that it has enough hydrogen to keep fusing for billions of years. However, eventually the Sun will run out of hydrogen, and it will begin to fuse helium atoms together. This process will release less energy than the fusion of hydrogen atoms, and the Sun will start to get smaller and cooler. Eventually, the Sun will become a white dwarf star.

So, the Sun does not burn up all its fuel because it is not burning in the traditional sense. It is undergoing a process called nuclear fusion, which is a much more efficient way to produce energy.