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

<p>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.</p>

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

CC BY-SA 3.0
2016-10-15T19:28:39.523Z
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"?
quantumflash
CC BY-SA 3.0
2016-10-15T20:33:49.867Z
If something is acted upon by forces other than gravity and kinetic gas pressure, then there are other terms in the virial.
ProfRob
CC BY-SA 3.0
2016-10-15T00:02:30.957Z
2016-10-15T00:02:30.957Z
ProfRob
<p>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 <strong>thermodynamical</strong> equilibrium. </p>

<p>What are some examples of objects that are not in virial equilibrium?</p>

<p>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? </p>

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

<gravity><galactic-dynamics><observational-astronomy><interstellar-medium><stellar-dynamics>
Examples of astrophysical objects NOT in virial equilibrium?
quantumflash
# Examples of astrophysical objects NOT in virial equilibrium?

(Asked by: username_0 on 2016-10-14)

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?

**Tags:** gravity, galactic-dynamics, observational-astronomy, interstellar-medium, stellar-dynamics


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## 1 answers

### Answer by username_1 on 2016-10-15. Score: 4 | Accepted answer

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.

> **Comments:**
> **username_0**: 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"?  
> **username_1**: If something is acted upon by forces other than gravity and kinetic gas pressure, then there are other terms in the virial.  

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