This question occurred to me as a result of the discussion in the thread https://www.physicsforums.com/threads/brown-dwarf-minimum-mass.919669.Apparently (1) H+H->D+e+ requires a temperature of >10,000,000 K while (2) H+D->3He requires only about 1,000,000 K. I confess that the references for these temperatures are not high quality, but they are the best i could find. Ref (1) The threshold temperature for hydrogen fusion, sometimes referred to as the proton-proton chain, is 10-14 million K (Kelvin). http://www.answers.com/Q/What_temperature_is_required_for_hydrogen_fusionRef (2) Deuterium is the most easily fused nucleus available to accreting protostars, and such fusion in the center of protostars can proceed when temperatures exceed 10^6 K. https://en.wikipedia.org/wiki/Deuterium_fusion The role of a high temperature is to give the interacting nuclei sufficient energy/momentum that that they can overcome their positive charge repulsion to approach each other close enough for the strong force to have it's effect to complete the process of combining them together. What is puzzling is that the repulsive force is the same for (1) and (2). My question is: Why does the presence of the neutron in D influence the strong force so much that only about 1/10 of the energy/momentum of the pair of interacting nuclei sufficient to allow for the fusion to happen?