If the half-width of the lines is 100 Angstroms,
     and the central wavelength is 6000 Angstroms,
     then the fractional width is

            (half-width)           100       1
         ------------------    ~   ----  ~  ---
          central wavelength       6000      60


   This fraction is roughly equal to the ratio of the 
     gas speed v to the speed of light c

                 v  ~  c / 60  ~  5000 km/s