Q:  One pc = 3.08 x 10^(16) m.  What is the density of
         stars per cubic meter in the disk?


            density n  =  2.5 x 10^(-51) stars per cubic meter


  Q:  Compute the mean free path of stars in the disk.  Express
         your result in meters.

                                             1
        mean free path L  =  -------------------------------------------------
                            pi * (6.96 x 10^(8) m)^2 * (2.5 x 10^(-51) m^(-3)


                          =   6.4 x 10^(31) m



            That's ... a very large distance.