In order to understand how the Sun produces heat and light, we first had to find out what it was made of. This was only possible following the discovery that different substances absorb and emit specific colours or wavelengths of light. When sunlight passes through a prism, it splits into a range of colours called a spectrum. Patterns of dark lines in the Sun’s spectrum indicate the presence of particular chemical elements. We now know the Sun is largely made up of hydrogen, with some helium and traces of other elements. Nuclear fusion of hydrogen into helium powers the Sun, producing all of its heat and light.

This is the interior of the experimental JET (Joint European Torus) fusion reactor. It is one of many attempts to harness nuclear fusion, the process which powers the Sun, to provide a clean source of energy for the future. Unlike current fission reactors, fusion does not produce significant amounts of radioactive waste.

Image: JET hydrogen fusion reactor (EFDA – JET)

William Thomson (1824–1907), later Lord Kelvin, was a mathematician who tested the validity of a range of contemporary theories about the power of the Sun. He showed that its enormous energy could not simply be the product of combustion, or burning of known chemicals.

Image: William Thomson, 1870 (F6555)

Albert Einstein (1879–1955) published his theory of relativity in 1905 and this, together with the recent discovery of radioactivity, showed that tiny amounts of matter could potentially produce enormous amounts of energy.

Arthur Stanley Eddington (1882–1944), one-time assistant at the Royal Observatory, combined these ideas to show that the Sun’s energy could come from the nuclear fusion
of hydrogen into helium.