Twinkle, twinkle, little star,
We know exactly what you are:
Nuclear furnace in the sky,
You'll burn to ashes by and by.

The source of all energy radiated by the Sun lies in its core, a central region comprising only 1.5% of the total solar volume. This is essentially a large thermonuclear reactor where hydrogen atoms are fused together forming hydrogen, releasing energy at the prodigious rate of 3.86 x 1026 Joules per second. The fusion reactions in the solar core take place because of the very high temperatures and very large pressures present in this region of the Sun. Although we currently have no way of measuring these quantities, physical models of the Sun lead us to believe that the core temperature is around 15 million K and that the central pressure is about 250 billion times the pressure exerted by the Earth's atmosphere at sea level.

It is the massive gravity of the Sun that compresses the core to such a high pressure and resultant high temperature, which then is sufficient to ignite the thermonuclear or fusion reactions which take place. The overall result of these reactions is to convert 4 hydrogen atoms into one helium atom. For every 1 kilogram of hydrogen that is consumed, 0.007 kg is turned into energy. Using the famous energy-mass equivalence formula (E = m c2) developed by Einstein, we can calculate that this releases a little over 600 trillion Joules (6 x 1014 J). When related to the total energy output of the Sun, this means that the solar reactor is consuming mass at almost 5 million metric tons per second! The "ashes" of these reactions also mean that the core is deficient in hydrogen and more abundant in helium than are other regions of the Sun.

There are two quite separate types of detailed reactions in which 4 H atoms may eventually result in one He atom. The first of these are:

This reaction sequence is believed to be the most important one in the solar core. The total energy released by these reactions in turning 4 hydrogen atoms to 1 helium atom is 26.7 MeV. However, there is a second series of reactions which may generate less than 10% of the total solar energy. These involve carbon atoms, which however, are not consumed in the overall process. The formulae for this "carbon cycle" are:

All the energy that is generated in the solar core escapes mostly in the form of very high energy gamma rays. This energy is absorbed and re-emitted many many times by the layers overlying the core, as the photons diffuse out toward the surface. In doing so, the energy is degraded: gamma photons are turned into X-ray photons, and then into UV photons, and finally into visible light and infrared photons. And so it is light and heat that is finally radiated from the surface of the Sun into interplanetary space. This same heat and light has a flux density of 1370 watts per square metre by the time it arrives at the top of the Earth's atmosphere. It is this energy that makes life possible on the surface of the Earth, that produces our terrestrial weather, and that photovoltaic cells can convert into electrical energy. Solar energy is, in actuality, nuclear energy!

ASAAustralian Space Academy