# Sidebar. Energy liberated in proton fusion

Sidebar. Energy liberated in the fusion to two protons and an electron to a deuteron

The energy liberated in each step to proton-proton fusion to create a deuteron and a gamma ray has one problem for direct measurement: How much energy does the neutrino carry away? This seems to vary. Though the total energy release is fixed, the apportioning of part of the total energy between the neutrino and the kinetic of the deuteron can vary. The solution to the question lies in knowing the difference in mass of the particles between the initial state (2 protons and an electron) and the final state (a deuteron). Einstein’s formula, E=mc2, can then be applied.

Mass of two protons and an electron, in three common units

kilograms                     atomic mass units*      MeV

One proton 1.672621924 x 10-27       1.007276466                 938.272088

Two protons 3.345243848 x 10-27    2.014552932              1876.544176

One electron 9.109383701 × 10−31   0.000548580                     0.510999

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2 p + e           3.34615479 x 10-27       2.01510151               1877.05518

Deuteron       3.34358377 x 10-27       2.013553211             1875.612941

Difference     2.57101637 x 10-30        0.001548301                   1.442234