MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
The proton-proton chain is an important nuclear reaction which occurs inside the core of main-sequence stars such as the Sun. It involves the conversion of hydrogen nuclei (protons, or 1H) into helium nuclei (4He) via several steps. First, two protons come together and stick. One of them is converted into a neutron, and debris is emitted in the form of a positron and a neutrino. The result is deuterium - a nucleus consisting of one proton and one neutron. With the addition of another proton and the subsequent emission of a gamma-ray, a new product is formed - an isotope of helium called helium-3 or tralphium. Finally, two of these helium-3 nuclei collide, and two protons are ejected, to make helium. On average, a typical proton can wait up to a billion years in the nucleus of a star before it collides with another to initiate this whole process.
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