Q:  Using a value of mu_e = 0.61 (appropriate for
          a hydrogen-rich envelope) and the value of mu_c = 1.33
          you calculated earlier, what is the critical mass for a helium
          core?
                                               2
                                     (  0.61  )
              M_core  =  M  * 0.37 * ( ------ )
                                     (  1.33  )


                      =  0.08 * M


          So, if the helium core grows to about 8 percent of the
          total mass of the star, thermal pressure will not be able
          to support it.