Do you know the mechanism, what brings about this fusion? Hydrogen atoms, even when pressed tightly together, don't fuse. They repel. As Brian Clegg puts it:
[E]ven in the pressure and temperatures present in the Sun, there isn't enough energy to overcome the repulsive force that keeps protons apart.
Protons make up the nucleus of the hydrogen atom.
It is only because of one of the oddities of quantum physics that stars work at all. Quantum particles like protons don't have an exact location … Each particle is spread out over a range of locations, with a different probability of being in any one of those locations.
While protons repel each other, being positively charged, they have this quantum feature of being in several places at once, even "so close to another proton that they can fuse before they bounce away from each other."
The repulsive force is so powerful that a proton cannot breach it except when it happens to be right on top of another proton, which "has a low probability of occurring," Brian Clegg says, except that "there are so many protons in the Sun that it's happening all the time … When nuclei fuse together the result is a small loss of mass which is converted into energy."
So, if you're following, protons are so powerfully repelled one from the other a proton in one atom cannot approach a proton in another atom. A proton must randomly occur at the particular point in its "range of locations" that happens at that moment to be occupied by another proton. Clegg calls this "tunneling," that is, a proton has appeared on the other side of a formidable barrier "without," he says, "passing through that … barrier."
Such an event is highly unlikely. It happens all the time.
source: Before the Big Bang: the prehistory of our universe by Brian Clegg