The proton-proton cycle, also known as the proton-proton reaction, can be understood as a chain of reactions following the initial proton collision. Through this chain of thermonuclear reactions, four hydrogen nuclei (or protons) combine to produce one helium nucleus (or alpha particle) and the series of emissions that are radiated by cool stars. Hot stars, in contrast, produce energy through the carbon cycle, in which carbon is changed to nitrogen, oxygen and finally back to carbon.
The collision of two protons begins the proton-proton cycle. The colliding protons create a deuteron, which is a hydrogen nucleus containing one proton and one neutron. To balance the charge of this deuteron, a positron -- an antimatter particle with the same mass but opposite charge of an electron -- is emitted. Also emitted is the almost massless, electrically neutral neutrino. The positron then combines with an electron, leading to the annihilation of both particles. This reaction emits high-frequency electromagnetic radiation in the form of gamma rays.
After emitting the positron and the neutrino, the deuteron collides with a proton. This collision releases another gamma ray and forms a helium nucleus. The helium nucleus then combines with another helium nucleus created from another deuteron-proton collision, emitting two protons. The final result is the formation of an alpha particle, or helium-4, consisting of two protons and two neutrons. Thus, to create the alpha particle, there is a net loss of four protons and two electrons, and the emission of three gamma rays.
While proton-proton cycles occur naturally in cool stars, scientists around the world have also created conditions where proton collisions can be observed. One such space is the Large Hadron Collider (LHC) located in the mountains between France and Switzerland. The LHC creates collisions between protons and allows scientists to observe and learn from the highest-energy collisions created in laboratory conditions.