When the isotopes of hydrogen fuse together, they form helium.
In the sun's core, hydrogen is fusing to form helium.
We now believe that hydrogen fused to form helium when the Universe was between three and four minutes old.
The hydrogen fuses to form helium and free neutrons.
The process of combining protons to form helium is an example of nuclear fusion.
Eventually, the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other, but instead coalesce to form helium.
The fast protons join together to form hydrogen and then helium.
This was caused by neutrons reacting with nickel to form helium.
In principle, using fusion to produce energy is easy: take hydrogen atoms and press them together to form helium.
The main nuclear burning that is happening on the sun is a hydrogen-hydrogen process, forming helium.