What is fusion? What elements are involved in this reaction in the sun?
(Another YahooAnswer of mine to a good question)

Fusion is the name of the nuclear reaction where one or more nucleons (a proton or a neutron) combine to form a larger atomic nuclei.

The smallest and simplest atom is Hydrogen, with a single proton making up it’s nucleus. When two protons are brought close enough together so that they almost touch, a force called the ’strong nuclear force’ pulls them together tightly, and in the process releases energy in the form of light, kinetic energy, and a positron (the anti-matter equivalent of an electron).

The new atom however is not made up of two protons, one of the protons decays into a neutron by releasing a positron (that is how it loses it’s electrical charge) This new atom, made up of a proton and a neutron, is still a form of Hydrogen, but it is an isotope called deuterium. So when two protons fuse, they create deuterium, this is the start of the major reactions in stars the mass of our sun or less, called the Proton-Proton Chain.

The protons have an electrostatic positive charge so they repel each other. It is the heat and gravitational pressure of the sun that get individual protons close enough together so that the strong nuclear force exceeds the electrostatic repulsive force. It turns out that the equivalent of over 100 pounds of force need to be applied to each individual proton to overcome the electrostatic repulsion. And even in a large star like our sun, the mass and heat are actually still not enough to make two protons fuse together.

Before the era of quantum mechanics this was a big mystery. But it was soon realized that sub atomic particles like protons can undergo an effect called ‘quantum tunneling’ which enables it to pass through barriers that classical physics would not allow. The smaller the energy barrier the more likely the particle will tunnel through it. The large energy barrier still preventing proton-proton fusion in the sun makes the tunneling a rare event, but there are so many protons that enough are tunneling at any moment to fuse and start the fusion chain in the sun.

This is a very important reaction in the sun and all stars with long lives because it essentially limits the rate at which the star can consume it’s fuel. It takes about 10 billion years for two proton to get close enough to each from quantum tunneling to fuse in a star like our sun.

Once two protons have been brought close enough together through random quantum mechanical fluctuations enabling a fusion reaction, a steady supply of deuterium is created. As soon as there is any deuterium in the sun the heat and gravitational pressure are enough to cause fusion reactions. At this point, there are many different reactions which take place, in some a proton fuses with a deuterium atom, in other cases two deuterium will fuse to create helium. Helium can then fuse with either individual protons, deuterium, or another helium to make larger elements. The types of available reactions increase rapidly and other elements like Boron, Beryllium, and Lithium are produced and also take place in still more reactions. Three helium nuclei can fuse to create a carbon atom.

In this way larger and larger atoms are created, but the dominant energy in stars the size of our sun come from the proton-proton chain reaction. In large stars, the Carbon-Nitrogen-Oxygen cycle dominates.

Very large stars can burn through there fuel very quickly, and some stars after they consume their fuel and lose the outward pressure from the fusion reaction that balances their inward gravitational pressure, suddenly collapse initiating a runaway series of nuclear reactions, like a nuclear bomb the size of a star. These are supernovae explosions and for a few minutes can create more light and energy than entire galaxies. It is these explosions that fuse smaller elements like carbon into larger ones like iron, aluminum, and uranium. These explosions throw these heavy elements out into space, and those elements later collect and form rocks, then asteroids, then planets, and all the things that live on the planets. That is why, in the immortal words of the famous scientist Carl Sagan, we are all made of ‘Star Stuff’

Further Reading:

The Proton - Proton chain
http://hyperphysics.phy-astr.gsu.edu/hba…

Quantum Tunneling
http://en.wikipedia.org/wiki/Quantum_tun…

Proton Proton fusion in the sun
http://hyperphysics.phy-astr.gsu.edu/hba…

The CNO Cycle
http://en.wikipedia.org/wiki/CNO_cycle

Supernova
http://en.wikipedia.org/wiki/Supernova