In 1934, Enrico Fermi realized that there must be some process in the Sun that was producing energy at a rate faster than that of nuclear fusion. He proposed what is now known as the “proton-proton chain” reaction, in which four hydrogen nuclei are fused into one helium nucleus, releasing vast amounts of energy. This process is responsible for the Sun’s luminosity. In 1938, Hans Bethe calculated that this same process could account for all the elements heavier than hydrogen in the Universe.
The proton-proton chain reaction has two main steps: first, two protons fuse to form a deuterium nucleus; then, a deuterium and a proton fuse to form a helium-3 nucleus. Each step releases energy, which helps to offset the energy lost to photons escaping from the Sun’s surface. The total energy output of the Sun is about 386 billion megawatts (3.86×10^26 W).
The proton-proton chain reaction occurs in several different places within the Sun. The first step takes place in the core, where temperatures are high enough (about 15 million Kelvin) for protons to overcome their mutual electrostatic repulsion and fuse together. The second step occurs in a shell around the core, where temperatures are slightly lower (about 10 million Kelvin). Helium-3 is much less massive than deuterium, so it fuses more easily with protons to form helium-4—the final product of the proton-proton chain reaction. This last step releases even more energy—enough to power all of civilization on Earth!
The products of nuclear fusion reactions—helium and other heavier elements—are continuously being ejected from the Sun’s surface in what is known as the solar wind. These particles eventually disperse throughout our galaxy and beyond, providing material for new stars and planets to form from. In this way, Enrico Fermi’s discovery not only illuminates our own planet but also plays a role in shaping our entire Universe!