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


             Since H has an atomic number of Z = 1, 
             and He has an atomic number Z = 2, the threshold
             for He-He fusion will be higher than that of H-H
             by a factor


                    2^2 * 2^2       16
                   -----------  =  -----
                    1^2 * 1^2        1
                    

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


                      16 * 10^(7) K