When atoms combine to form new, heavier atoms.
The reason we have to accelerate atoms in particle accelerators is to super high speeds is so that they have enough momentum (I think) so that their nuclei overcome the weak force that normally keeps atom nuclei apart and actually come together, then the strong force takes over and they form a new, heavier atom whose atomic number is the sum of the two atoms that smashed.
In stars it’s typically hydrogen fusing to helium. Gravity is forcing the atoms together, which causes them to heat up, which causes them to move faster and faster until they reach that critical threshold necessary to create fusion.
Source
- Brainiac Kids TV - yes seriously.