Astronomers always talk about astrophysical objects being in virial equilibrium (e.g., a stellar system, or a disk of gas within a galaxy, etc.). But I never hear about thermodynamical equilibrium.

What are some examples of objects that are not in virial equilibrium?

One example question: suppose that what you thought was a disk of rotating gas within a galaxy (as revealed by spectroscopy) is actually a bipolar outflow of gas. Would the bipolar outflow of gas be in virial equilibrium?

Is a radio jet (length of hundreds of kpc) from a supermassive black hole in virial equilibrium?


Not clear what the initial part of your question means. Objects can be in virial equilibrium without being in thermal equilibrium.

A clear exception to the virial theorem would be any system that is gravitationally unbound. So you couldn't apply it to a supernova explosion or a dissolving cluster of stars.

The examples you quote are not self-gravitating, bound systems, so no, the virial theorem would not apply to them.

  • $\begingroup$ Thanks. Can you please refer me to an intuitive explanation/book that says how to determine whether a system is "self-gravitating"? I know a mathematical condition for a system to be bound is that its kinetic energy < potential energy. What is the analogous mathematical condition for self-gravitating? Clearly a gas disk is both self-gravitating and bound. But if a radio jet out of a SMBH is confined by a magnetic field then it's not supported by self-gravity? Even a little bit? How could I calculate what fraction of "self-support" in a bipolar galactic gas outflow comes from "self-gravity"? $\endgroup$ – quantumflash Oct 15 '16 at 19:28
  • $\begingroup$ If something is acted upon by forces other than gravity and kinetic gas pressure, then there are other terms in the virial. $\endgroup$ – ProfRob Oct 15 '16 at 20:33

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