When hydrogen fuses to form helium, this process releases energy.
Wikipedia explains the reason:
"Comparing the mass of the final helium-4 atom with the masses of the four protons reveals that 0.007 or 0.7% of the mass of the original protons has been lost. This mass has been converted into energy, in the form of gamma rays and neutrinos released during each of the individual reactions. The total energy yield of one whole chain is 26.73 MeV."
That made me think: "Ah, so neutrons are lighter than protons (because in the fusion process two protons become neutrons, losing stuff along the way). How else could it lose mass?"
But then I go and check the mass of a proton, and it's approximately 1.6726×10
−27 kg, while the mass or a neutron is approximately 1.6749×10
−27 kg.
What gives? How can neutrons be more massive than protons, yet Helium-4 be lighter than the four original protons?