Q:  What is the speed of hydrogen atoms in a cloud of HI?

                           3 x 1.38 x 10^(-23) * 50
             v  =  sqrt [  -------------------------  ]
                               1.67 x 10^(-27)


                =  1100 m/s


  Q:  What is the typical time between collisions?
            Express your answer in seconds and years.

 
                     L           3 x 10^(11) m
             t  =  -------  =   ---------------   =   2.7 x 10^(8)  s
                     v              1100 m/s


                                                  =   9  years