Q:  The threshold for H-H fusion is roughly 10^7 K.
          What is the threshold for He-He fusion?


             H has an atomic number of Z = 1, 
             He has an atomic number Z = 2, and
             Be has an atomic number of Z = 4.  

             Using the scaling relationship based on quantum
             tunneling, the threshold for for Be-He fusion will be 
             higher than that of H-H by a factor


                    4^2 * 2^2       64
                   -----------  =  -----
                    1^2 * 1^2        1
                    

             So, the threshold for Be-He fusion will be of order


                      64 * 10^(7) K   =  6.4 x 10^(8) K