Q:  Suppose the wind of zeta Oph, travelling at speed v,
         smashes into the ISM, and all of the kinetic energy
         of its atoms are converted into single X-ray photons.

      What velocity would the wind have?


    
        The spectrum has a peak at energy E = 0.9 keV = 1.4 x 10^(-16) J.
        So if the original kinetic energy of a proton is equal to this
        photon's energy, 

 
                 0.5 * mH * v^2   =   1.4 x 10^(-16) J


                             (   2 * 1.4 x 10^(-16) J    )
                  v  =   sqrt( ------------------------- )
                             (      1.67 x 10^(-27) kg   )


                     =   420 km/s