In nuclear fusion, two hydrogen nuclei (protons) combine to form helium in a multi-step process that occurs at extremely high temperatures and pressures. Hydrogen nuclei, being positively charged protons, repel each other due to their electromagnetic forces. However, under the extreme conditions found in the core of stars and other high-energy environments, nuclear fusion can occur through a process known as the proton-proton chain.

The proton-proton chain is the dominant process for hydrogen fusion in stars like our Sun. It involves a series of nuclear reactions that convert four hydrogen nuclei (protons) into one helium nucleus (alpha particle) along with some subatomic particles and energy release. The proton-proton chain proceeds through the following steps:

1. Step 1: Proton-Proton Fusion: Two protons fuse to form a deuterium nucleus (one proton and one neutron), releasing a positron (a positively charged electron) and a neutrino. The positron quickly annihilates with an electron, resulting in the release of gamma rays.

   p + p → d + e+ + νe

2. Step 2: Deuterium-Proton Fusion: The deuterium nucleus (a proton and a neutron) fuses with another proton to form helium-3 (two protons and one neutron) and releasing a gamma ray.

   d + p → He-3 + γ

3. Step 3: Helium-3 Fusion: Two helium-3 nuclei fuse to form helium-4 (two protons and two neutrons) and two protons. This reaction releases two protons and energy in the form of gamma rays.

   He-3 + He-3 → He-4 + 2p