Why is the ideal gas law a good approximation for stars?

[jimmy_jollop]

Any ideas appreciated!

 

[chemisttree]

It's not.

 

[harcel]

It IS, that is, at least in a large fraction of the volume of many stars. And why would it not be? Only in very high densities for example the approximation breaks down...

 

[chemisttree]

Then please explain to me the gravity, expansion of the universe and dark matter terms in the ideal gas equation.

 

[Dadface]

It IS, that is, at least in a large fraction of the volume of many stars. And why would it not be? Only in very high densities for example the approximation breaks down...

The approximation also breaks down when collisions become non elastic for example exciting ionising and fusing?

 

[harcel]

@chemisttree: this really is a question that does not make much sense: the ideal gas equation is an equation of state, relating pressure, density and temperature. Gravity is the attractive force between masses, or if you like the curvature of space-time due to the presence of mass/energy, and the expansion of the universe is just one of the solutions to einsteins field equations which happens to describe the evolution of our universe at large scales. The three entities you name can work together to build galaxies and so on, but one cannot be described in terms of the other.

@Dadface: you're right, it breaks down for non-elastic collisions, or actually in any case where interparticle forces cannot be neglected. But, for example in stars, fusion is going on, but at such a low rate, that even the interior of a star is relatively well approximated by an ideal gas.

 

[chemisttree]

@chemisttree: this really is a question that does not make much sense: the ideal gas equation is an equation of state, relating pressure, density and temperature.

Please describe the density term in the ideal gas equation.

Gravity is the attractive force between masses, or if you like the curvature of space-time due to the presence of mass/energy, and the expansion of the universe is just one of the solutions to einsteins field equations which happens to describe the evolution of our universe at large scales. The three entities you name can work together to build galaxies and so on, but one cannot be described in terms of the other.

Sorry, I misunderstood your response as being applicable to the "volume of many stars" instead of the volume of gas in the outer atmosphere of a star. Thus my confusion regarding gravity, dark matter and expansion.

@Dadface: you're right, it breaks down for non-elastic collisions, or actually in any case where interparticle forces cannot be neglected. But, for example in stars, fusion is going on, but at such a low rate, that even the interior of a star is relatively well approximated by an ideal gas.

You cannot be more wrong than this. Tell me more about ion-ion particle interactions in the ideal gas law... both repulsive and attractive. How about radiation pressure?

 

[Ich]

You cannot be more wrong than this.

There are in fect several ways of being more wrong. For example by claiming that an ideal gas is generally a bad approximation of the matter inside stars.
There's radiation pressure (dominant only in massive stars) and ther'es degeneracy, most important for collapsed stars. Everything else at high density and temperature should be well described by an ideal gas.

 

[alxm]

There are in fect several ways of being more wrong. For example by claiming that an ideal gas is generally a bad approximation of the matter inside stars.
There's radiation pressure (dominant only in massive stars) and ther'es degeneracy, most important for collapsed stars. Everything else at high density and temperature should be well described by an ideal gas.

Hmm, but the ideal gas law is valid that the low pressure-temperature limit.

That said, perhaps the disagreement here is on what constitutes a 'good' approximation.
It's not a good enough approximation for chemical engineering, say. But astrophysics on the other hand is dealing with much larger objects with much lower requirements for accuracy.

 

[Ich]

Hmm, but the ideal gas law is valid that the low pressure-temperature limit.

Not necessarily for a plasma, with strong magnetic fields around. AFAIK the ideal gas approximation works better inside the star than in the low density regions.