Q:  What is the velocity of the atom just as it reaches
          the surface of the star?  What is its kinetic energy?


          The kinetic energy is equal to the change in gravitational
          potential energy:


             KE   =   GPE(final)  -   GPE(initial)

                         G M m               G M m
                  =  - ----------    -   - ------------
                        infinity               R


                       G M m 
                  =   --------
                         R
                     
  Q:  Assume that this energy is completely thermalized
          in a shock near the surface of the star.
          Express this energy in keV for 

            a) a star like the Sun      

                  M = 1.99 x 10^(30) kg     R = 6.96 x 10^(8) m

                    --> KE  = 1.99 keV


            b) a white dwarf star (assumed to have the same mass
                  as the Sun, but only 1/100 the radius)

                  M = 1.99 x 10^(30) kg     R = 6.96 x 10^(6) m

                    --> KE  = 199  keV