Q:  Estimate the minimum separation b_min which is
          consistent with our assumptions.  Assume that all stars have
          the mass of the Sun, and move with the local velocity
          dispersion.

             m  =  1.99 x 10^(30) kg
  
             v  =  25,000 m/s



                      2 * G * (1.99 x 10^(30) kg)
           b_min  =  ------------------------------  *   (some factor)
                            (25,000 m/s)^2



                  =    4.2 x 10^(11) m   *   (some factor)  

 
                  =   about 3 AU  *  (some factor)


        If we declare that b_min must be at least 10 times larger
        than the combination of displacements of the two stars
        toward each other, we find that weak encounters require
        an impact parameter of at least 30 AU or so.