3 ms·

My understanding was always that fusion _in stars_ ends with iron. Maybe that's why people simplify and assume fusion stops entirely?

• adrian_b 5y ago
  For fusion to be possible the reaction must be exothermic, which means that the average mass per nucleon in the resulting nucleus must be smaller than the average mass per nucleon in the reactants. Until the iron 56 isotope, the average mass per nucleon decreases quickly, then it increases slowly with increasing atomic mass. For reactants heavier than iron 56, fusion is not possible, but for some reactants lighter than iron 56 it is possible to fuse into nuclei a little heavier than iron 56, belonging to the few elements placed immediately after iron, where the average mass per nucleon has not increased much yet, so it is still lower than that of many of the lighter elements.
  • Gauge_Irrahphe 5y ago
    >the average mass per nucleon in the resulting nucleus must be smaller than the average mass per nucleon in the reactants. That doesn't equal to the binding energy per nucleon though, as you add a nucleon.
    • azernik 5y ago
      And then immediately on the addition of a hydrogen, the resulting nucleus is suddenly in a higher-energy (lower binding energy) state than some non-hydrogen splitting of it. And since most of the results of hydrogen hitting something are unstable, this happens almost immediately, leaving the total process looking more like fission (a proton hitting a nucleus, and producing two nuclei of smaller mass than the original). Which is, in fact, how uranium fission works, only with neutrons rather than protons.