What forces the two hydrogen atoms together in nuclear fusion?

Answer 1

High gravitational compression (in stars) or mechanical compression (in weapons) forces two hydrogen atoms together in nuclear fusion. Recall that the only places we see nuclear fusion happeing (so far) are in stars and in fusion weapons. In the star, the thermal energy in which these reactions is happeing is enormous. It's super hot inside the star, and the hydrogen nuclei (two protons) are forced together because of the gravitational compression. Heck, they wouldn't normally want to get anywhere near each other, let alone fuse to become a heavier particle. But the extreme gravity and extreme thermal energy there allow this to happen.

In a nuclear fusion weapon, we use a fission device to create the mechanical compression (in place of the star's gravity) to force hydrogen nuclei together to get them to fuse. The fission device also creates the thermal energy needed. This is the simple explanation of this fundamental nuclear reaction. Links can be found below to learn more.

Answer 2

In nuclear fusion, the strong nuclear force overcomes the electrostatic repulsion between the positively charged protons in the hydrogen atoms, allowing them to come close enough for the force to bind them together into a helium atom. The extreme temperatures and pressures in the core of a star are what provide the necessary conditions for nuclear fusion to occur.