The nucleus of the sun is under immense pressure and extreme temperatures, reaching up to 10 million Kelvin, enabling nuclear fusion, the process that powers the sun. 

Nuclear fusion is also the reason why stars do not collapse under their own gravity. The energy produces by fusion creates an outward pressure that balances the inward gravitational, giving the sun its stability.

Reaction

Hydrogen (¹₁H) is the simplest atom, consisting of one proton and one electron. In the sun, hydrogen consists in a state of plasma, where electrons and protons swarm independently, creating an ionized gas of positive and negative particles. 

Under the immense pressure of the sun, protons overcome their repulsion and fuse together:

1)        ¹H +¹H -> ²H + e* + n

2)        ²H+¹H -> ³He + γ

3)        ³He +³He -> ⁴He +¹H + ¹H

When two protons collide, one of them undergoes the β-decay, transforming into a neutron while emitting a positron and a neutrino (1). 

The positron is the antiparticle of the electron and immediately collides with an electron, producing two photons (γ) which eventually provide you right now with energy (Click here to leran more about elementary particles). This forms deuterium, a proton and neutron bounded together:  ²₁D. It then fuses with another proton, forming an isotope of helium (³He) and releasing more photons. (2)

Two of ³He collide, forming a stable helium-4 nucleus and releasing two protons. (3)

All in all, six individual protons are involved in the process, but two are released, resulting in the production of one helium nucleus, two positrons, two neutrinos, and a significant amount of energy.

The role of nuclear fusion

Step 1) of this process occurs relatively slowly. This slow rate of fusion controls the suns energy output and ensures it burns steadily over billions of years which gave humanity enough time to evolve and now still be fueled by the sun.

In larger Stars the process of forming heavier elements continues up to iron.