Q:  What is the mean molecular weight of a pure helium core,
          in which all atoms are fully ionized?


          Each helium atom produces 3 particles when fully ionized:
          the nucleus, plus 2 electrons.  The mean molecular weight
          is thus

                  mu   =   average mass per particle, in unit of H atoms



                            mass of 1 He atom       4 * mass of H atom
                  mu   =   ------------------   =  ------------------
                              3 particles             3 particles


                             4
                       =   -----   =  1.33
                             3