Protonprotonketjut

Protonprotonketjut, often abbreviated as the pp-chain, is a set of nuclear fusion reactions by which stars, including our Sun, convert hydrogen into helium. This process is the primary energy source for stars within a certain mass range. The overall reaction involves four protons (hydrogen nuclei) combining to form one helium nucleus, releasing energy in the form of gamma rays and neutrinos.

The pp-chain occurs in several steps. The most common pathway begins with two protons fusing to form deuterium, a heavier isotope of hydrogen, along with a positron and an electron neutrino. The positron quickly annihilates with an electron, releasing energy. In the next step, deuterium fuses with another proton to form helium-3, a lighter isotope of helium, releasing a gamma ray. Finally, two helium-3 nuclei fuse to produce helium-4, the most common isotope of helium, expelling two protons in the process, which can then participate in further pp-chain reactions.

There are also less common branches of the pp-chain, such as the ppI, ppII, and ppIII chains, which involve intermediate steps with isotopes like lithium, beryllium, and boron. These branches become more significant at different stellar core temperatures and contribute to the production of certain elements. The energy released by the pp-chain is what powers stars, providing the light and heat that sustain planetary systems. The neutrinos produced escape the star almost unimpeded, carrying away a significant portion of the reaction's energy.