This reaction is part of the proton-proton chain reaction, one of the fusion reactions by which stars convert Hydrogen to Helium.
The reaction chain consists of three steps and concludes with the creation of a Helium-4 nucleus (the most common Helium isotope), also called an
- Two protons fuse into heavy Hydrogen (deuterium);
- A proton fuses with the deuterium nucleus forming Helium-3 and releasing a Gamma Ray;
- Two Helium-3 nuclei fuse into Helium-4 and 2 protons are released:
(3/2)He + (3/2)He -> 2(1/1)H + (4/2)He
This process represents the most likely pathway to produce Helium-4.
Here's a chart of the proton-proton chain reaction: