Wikipedia says that solar core has 15 millions of Kelvins ( https://en.wikipedia.org/wiki/Solar_core ), what translates into ~1.4 keV energy per degree of freedom. For nuclear fusion we need to take the two nuclei to distance of range of nuclear forces: ~1fm ( https://en.wikipedia.org/wiki/Nuclear_force ), Coulomb potential for proton-proton (kE*e*e/r) is ~1.4 MeV in 1fm. So thermally these protons would reach 1pm distance, but fusion distance would require 1000x larger energy. Even worse - it is for 1D, in 3D they not only would need 1000x larger energy, but also have velocity precisely pointing the 10^-15m size second nucleus. Otherwise, it would just bounce from the repulsion and fly away. How is it explained that fusion is actually happening in such relatively low temperature? "Because tunneling" might be a good explanation for electrons, but for much heavier protons we should understand trajectory, forces. Maybe electron assistance might be crucial there (?) - e.g. Coulomb says that p - e - p symmetric initial situation should collapse - fuse into deuteron. Electron remaining between the two closing nuclei could screen their repulsion. More realistically, if electron would have low angular momentum trajectory ( https://en.wikipedia.org/wiki/Michał_Gryziński#Free-fall_atomic_model ): ellipse degenerated into a segment in some direction, if another nucleus would approach from this direction, this electron could stay between them by performing successive backscatterings - screening their repulsion.