gas cloud                       globular cluster
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  density  n            1010 atoms/m^3                  3 x 10-47 stars/m^3

radius of particle      10-10 m                         7 x 108 m

  cross section  A      3.1 x 10-20 m^2                 1.5 x 1018 m^2

  velocity  v           300 m/s                         7000 m/s

mean free path  L       3.2 x 109 m                    2 x 1028 m

collision time  t       1.1 x 107 s = 0.3 yr            2.8 x 1024 s = 90 million billion yr

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