The primary fusion reaction occurring in the Sun's core is the proton-proton chain reaction. This is a series of nuclear reactions that ultimately convert four protons (hydrogen nuclei) into one alpha particle (helium nucleus), releasing energy in the process.

Here's a simplified breakdown:

1. Two protons collide and fuse to form a deuterium nucleus (one proton and one neutron), releasing a positron (anti-electron) and a neutrino.

2. The deuterium nucleus captures another proton, forming a helium-3 nucleus (two protons and one neutron) and releasing a gamma ray photon.

3. Two helium-3 nuclei fuse to form a helium-4 nucleus (two protons and two neutrons) and releasing two protons.

This cycle can be summarized as:

4 protons → 1 helium-4 nucleus + 2 positrons + 2 neutrinos + 2 gamma ray photons + energy

The energy released in this process is primarily in the form of gamma rays and neutrinos, which contribute to the Sun's immense heat and light.

Important Note: The proton-proton chain reaction is a complex process involving several intermediary steps and reactions. The above description is a simplified version for easy understanding.