Atomic nuclei cannot just combine randomly. Rather, only a small number of specific fusion reactions can occur, and even then only under very extreme circumstances. In the Sun’s core, temperatures exceed 27 million degrees Fahrenheit (15 million degrees Celsius) and pressures exceed one hundred billion Earth atmospheres. In these circumstances, there is a minute chance—less than a billion to one!—that, in any given year, a proton will fuse with another nearby proton to form a deuteron, also known as a deuterium nucleus or “heavy hydrogen” nucleus. The deuteron then fuses quickly with another proton to produce a helium-3 nucleus. Finally, after waiting around (on average) for another million years or so, two nearby helium-3 nuclei can fuse to form a helium-4 nucleus and release two protons.

In this multi-step sequence, called the “proton-proton chain,” hydrogen is transformed into helium-4, and a tiny bit of matter is converted into energy. Even though it is very hard for any given pair of protons to fuse into a deuteron, there are so many protons in the core of the Sun that more than one trillion trillion trillion such fusion reactions occur there each second. The amount of mass converted into energy is thus huge—about 4.5 million tons per second—and provides enough outward push to keep the Sun in a stable size and shape, thus allowing it to shine its stellar glow out into space.