What fuels the Sun?

This was a question that had puzzled scientists for thousands of years. In the 1800s, astronomers thought it was gravity: the slow contraction of the Sun's mass to a smaller volume. This is indeed a source of energy in Jupiter, but such a source in the Sun would only last a few million years. Geologists in the early 1900s began to claim the Earth was billions of years old, something that posed a difficult situation for astronomers to explain!

Luckily, physicists were were beginning to understand the Four Forces of Nature:

1. Gravity - This force acts between all mass in the universe and it has infinite range.

2. Electromagnetic - This acts between electrically charged particles. Electricity, magnetism, and light are all produced by this force and it also has infinite range.

3. The Strong Force - This force binds neutrons and protons together in the cores of atoms and is a short range force.

4. Weak Force - This causes Beta decay (the conversion of a neutron to a proton, an electron and an anti-neutrino) and various particles are formed by strong interactions but decay via weak interactions. Like the strong force, the weak force is also short range.

Nuclear Reactions
The sun generates its energy by breaking and reconnecting the bonds between particles inside the atomic nuclei. Particles, being: protons and neutrons, located in the nuclei of atoms. Thus, the Sun uses the Strong Force to derive its energy.

Nuclear Fission reactions split the large nuclei to smaller nuclei, releasing energy.

Nuclear Fusion reactions combine light nuclei into heavier nuclei to release energy. Since protons have like charges, they don't want to combine! However, in the center of the Sun, densities are extremely high and the particles hit each other with such speed, as to overcome this Coulomb Barrier.

Iron has the most tightly bound nucleus. You can not generate ANY energy once you arrive at Iron, via fusion or fission.