Just a quick (and hopefully simple) question on the distribution of stars in the Milky Way and in globular clusters. I was wondering why it IS NOT possible to describe the distributions of stars in both these systems by analogy to the distribution of air molecules in a room.

Are we talking of the more random motion of air molecules as well as the relatively large number of collisions between air molecules...something which we wouldn't see in a galaxy. I'm assuming also that air molecules do not have the same central gravtational potential...and therefore do not move relative to some fixed point. That is to say, that it is much more random? Any advise on this point would be greatly appreciated.


1 Answer 1


Particles in a gas approximate to point-like objects that interact roughly elastically through short-range forces when they collide, but are otherwise non-interacting.

Stars interact gravitationally over long ranges, occasionally with each other, but always with the overall gravitational potential of the system.

Sometimes people do talk thermodynamically about star clusters. You can discuss the "temperature" of stars when you are referring to the velocity dispersion in a cluster. The concept of heating or cooling a cluster also has some merit.

  • $\begingroup$ Why is it that gravity may be neglected in the interaction of gas molecules but not in the case of stars? $\endgroup$ Oct 6, 2015 at 12:36
  • 1
    $\begingroup$ @ConradTurner Because the gravitational force between air molecules in a room is of order $10^{40}$ times smaller than the short range electromagnetic forces at typical separations. $\endgroup$
    – ProfRob
    Oct 6, 2015 at 13:22

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