When four protons collide to form helium, what fraction of the original mass of the protons is converted to energy?

0.7 %

Explanation:

In a star like the Sun, or of smaller mass, the most frequent chain of nuclear reactions constitutes the cycle called p-p (proton-proton). This cycle begins with the fusion of two hydrogen nuclei (two protons) into one of deuterium, with the emission of a positron (the positron is the antiparticle of the electron, it is a positive electron) and a neutrino.

In this reaction 0.7% of the mass of the protons of the initial state is converted into energy. This number, 0.7%, is huge compared to that of a chemical reaction (a combustion, for example), which is typically of the order of 0.000000001%. Deuterium can in turn be fused with another hydrogen, this with helium.

From an energy point of view, what happens as a result of all these reactions is that four hydrogen nuclei fuse together, producing one of helium, two positrons, 2 neutrinos and 24.68 MeV of kinetic energy.