This question is probably very basic for those who understand nuclear fusion but it has been annoying me for a while now and I would like an answer. I have tried searching these forums but I can't find what I'm looking for. The sun is constantly fusing hydrogen atoms to form helium atoms, correct? Well, if hydrogen nuclei only contain a single proton, where do the neutrons for the helium atom come from? An answer would be much appreciated, try not to make it too complicated, thanks.