Stellar energy

It was soon realised that the energy of the stars was produced by nuclear fusion, the temperatures required in the stars being lower than those needed in the laboratory because of the enormous pressures in the stellar interiors.

One such reaction that occurs in the Sun is the so called PP chain shown below.



Part of the energy in the first reaction is high-speed neutrinos and these can be detected on the Earth.

Note: the apparent imbalance between particles in these equations can be understood if we look at the first of these. On the left we have two protons and no neutrons. On the right we have one proton and one neutron and a positron. One of the protons has been converted into a neutron and a positron. Similar reasoning will explain the other equations.

Other reactions may then occur such as:



Heavier and heavier nuclei can be produced by successive fusion reactions. The types of reaction taking place in the Sun are shown below:


The net result of this series of reactions is:


In other words, 564 million tonnes of hydrogen are being converted into 560 million tonnes of helium every second, producing a staggering output of 3.90x10²⁰ MW - equivalent to some million million million large power stations!

Although the Sun is converting four million tonnes of mass into energy every second it is thought that it will continue to last for a further 10⁹ years.

At higher temperatures (around 10⁸ K) the fusion of helium starts the building of heavier elements.

The reaction is:


This is known as the triple alpha process and is the first stage of 'helium burning'.
Beryllium 8 is very unstable and so can revert to two alpha particles but eventually an equilibrium is set up and maintained.