"Why does the Sun shine?" is one of those questions asked by curious children to which adults struggle to provide a convincing answer. Hans Bethe received the 1967 Nobel Prize in Physics for revealing how the Sun behaves like a giant nuclear reactor to produce the vast amount of heat and light that supports life on Earth.
Bethe's breakthrough work stems from the British astronomer Sir Arthur Eddington's hypothesis in the 1920s that the intense temperatures and pressures within the Sun can force the nuclei of atoms to fuse and create heavier atoms, releasing a tremendous amount of energy in the process. Drawing upon his series of articles that provided a comprehensive account of the central components of atoms and the manner in which these atomic nuclei interact with each other, Bethe applied this knowledge to understanding the stars.
In two papers in 1938 and 1939 Bethe described the two nuclear reactions that stars use to produce energy, and showed how one predominates over the other depending on the internal conditions. For stars up to and including the size of the Sun, the more dominant energy supply is generated by squeezing four hydrogen nuclei together to form one helium nucleus. In the more extreme temperatures and pressures found in stars that are heavier than our Sun, the dominant energy supply also transforms hydrogen into helium, but this involves a more complex cycle of nuclear reactions in which carbon acts as a catalyst. Albert Einstein's most famous equation, E=mc2, showing that mass and energy are interchangeable, explains why these fusion reactions create heat and light. The mass of helium is less than the sum of the hydrogen nuclei, and the difference in mass is converted into large quantities of energy.
According to Bethe's theories, these reactions ensure there is enough energy to allow the Sun to shine for billions of years, considerably longer than previous estimates had predicted. Bethe's theories have also helped us to imagine stars existing in a form of life cycle. Stars are born, they grow and develop by burning fuel, but at some point this energy source must burn out, and eventually they die.