Here's how it works:

1. Hydrogen atoms collide: In the sun's core, immense pressure and heat cause hydrogen atoms to collide at incredibly high speeds.

2. Fusion: The collisions are so powerful that they overcome the natural repulsion between the positively charged nuclei of the hydrogen atoms. This allows the nuclei to fuse together, forming helium.

3. Energy release: This fusion process releases a tremendous amount of energy in the form of light and heat.

The specific reaction involved is called the proton-proton chain reaction:

* Two protons (hydrogen nuclei) fuse to form deuterium (one proton and one neutron) and a positron (a positively charged electron) and a neutrino.

* Deuterium fuses with another proton to form helium-3 (two protons and one neutron).

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

This chain reaction, along with other nuclear fusion processes, provides the sun with its immense energy output.