Thermonuclear fusion: deep inside the heart of stars [2]

The process of Thermonuclear fusion requires high temperatures and high pressure. Those conditions until now  have been found only inside the hearth of stars. Here the elements are under the physical form of a ionized gas. It means that the  different particle have a certain electric charge, and so before nuclear interaction could begin to work, coulombian repulsion has to be defeated.

Considering the simplest situation, with two hydrogen atoms, the energy required to exceed the coulombian barrier is 1000 KeV. Inside a star the temperature is usually around 10^7 K. Heat energy of mono-atomic gas can be calculated as follow:

According to this data, nuclear reaction should be impossible at those condition. However three other factors can combine to bring a certain probability of success for those reactions:

• Particles are characterized by a Maxwell speed distribution. It means that a certain amount of particles have a energy greater than the medium one, and a certain amount can reach the level required;
• According to quantum mechanics there's a little probability that a particle with low energy could  exceed the coulombian barrier, by Quantum Tunnelling;
• Stars are made by a large amount of particles. Even though the medium level isn't enough to exceed the barrier, a great amount of particles could have enough energy.

Nevertheless most of the energy produced by a star is related to the Proton-Proton chain, that occurs at 3x10^7 K. The reaction is basically the transformation of 4 proton in a nucleus of Helium, according to the following process:

The total energy produced by this process is 26 MeV per cicle, even though 0,26 MeV are emitted under  the form of particles known as Neutrino.

This cycle is prevalent in star only through the first period of their life. When temperature gets closer to 10 million degrees another chain takes place, and it is known as CNO chain (Carbon Nitrogen Oxigen).

This chain consists, like the previous one, in the transformation of 4 protons in an atom of Helium. In total the energy production is similar to the previous one, 25 MeV. So what are the factors that mark the difference between one chain and the other? In the CNO chain heavier atoms are used as catalyst. Because of that higher energies are require to exceed the coulombian barrier, and so also higher temperatures.

Anyhow the process that is fundamental for  the production of energy by thermonuclear fusion on earth is the p-p chain. The CNO chain in fact requires higher temperatures, too high for our actual technologies. Consequently the following article will always refer to this cycle.