proton-proton chain
/ p-p chain /
The proton-proton chain is the main set of nuclear reactions that powers the Sun. Step by step, it takes the lightest and most common nucleus in the universe — a single proton, which is simply the nucleus of a hydrogen atom — and builds it up into helium, releasing energy at each stage.
A proton is just a hydrogen nucleus, so the chain is essentially turning hydrogen into helium. It cannot happen in one leap. First two protons must fuse, which is extremely rare because they repel each other electrically and one of them must change into a neutron at the exact instant of contact; this slow first step is why the Sun burns its fuel so gradually. The product then captures another proton to form a light form of helium (helium-3), and finally two helium-3 nuclei combine to make ordinary helium (helium-4), spitting two protons back out. The net result is that four protons become one helium-4 nucleus, and the helium weighs about 0.7 percent less than the four protons did. That missing mass is released as energy, following Einstein's E = mc^2.
The proton-proton chain matters because it is the dominant energy source not just for the Sun but for all stars up to about the Sun's mass, and it is the universe's main factory for helium. It is also extraordinarily steady: the rarity of that first fusion step acts as a brake that lets a star shine evenly for billions of years rather than blowing itself apart.
The first step — two protons fusing — is so unlikely that an average proton in the Sun's core waits billions of years for its turn, even amid 15-million-degree heat.
This bottleneck is precisely why the Sun lasts billions of years instead of burning out in a flash.
A common confusion: fusion releases energy, while splitting heavy atoms (fission, used in power plants and bombs) is the opposite process. Stars fuse light elements; they do not split them.