Q:  What is the typical radius of an HII region?
        Express your answer in both meters and parsecs.
        Recall 1 pc = 3.08 x 10^(16) meters.

                                               1/3                         1/3
                [   3 * 10^(49) photon/s     ]       (         1         ) 
          R  =  [ -------------------------- ]       ( ----------------- ) 
                [   4 * pi * 10^(7) per m^3  ]       (  2.5 x 10^(11) s  )



             =   3.9 x 10^(17) m

             =   13 pc