Atoms in gas cloud                Stars
 ----------------------------------------------------
       10^(-10) m        radius      10^9 m

       0.01 m          separation    10^(16) m

       100 m/s           speed       30 km/s
 ----------------------------------------------------

   You have two lacrosse balls, each of which is 3 cm in radius.
Suppose you use those balls to represent hydrogen atoms...

   1. How far apart should the balls be?




 


   2.  How long does it take each one to move its own diameter?







   3.  How long does it take each one to cross the distance to its neighbor?






  A rough calculation of the time it takes objects to collide with each
other is given by
                                      3
                          (separation) 
          time  =  -------------------------------
                    (speed) * (radius) * (radius)


   4.  Roughly how long will it take two hydrogen atoms in a cloud
       of gas to collide with each other?