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Offline ianhutt

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Why does a collapsing star heat up?
« on: 14/09/2008 11:02:33 »
ian hutt  asked the Naked Scientists:

I was newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] and the answer to another listeners question which has prompted mine. The sun collapsed under the force of gravity which caused it to heat up and this was illustrated by the example of the bicycle pump which also heats under pressure.

However My understanding is that heat is a form of energy and is determined by how active the molecules and atoms in a substance are. I would have expected that under the very great pressure exerted in a body as large as the sun the capacity of atoms and molecules to move would be very severely restricted and that in turn this would restrict its capacity to heat up. Can you explain this apparent anomaly.

The Naked Scientist always is stimulating and thought provoking- Keep up the excellent work!

Regards and best wishes

Ian Hutt (Little Chalfont, Bucks)

What do you think?


 

Offline graham.d

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Why does a collapsing star heat up?
« Reply #1 on: 14/09/2008 16:10:12 »
The temperature is indeed related to the movement of the particles that would make up the sun, and you raise a good point. Although there is great contraint on the distance particles can move due to the intense gravitational field, this does not stop the particles moving with a high velocity. Their "mean free path" may be short, that is the average distance they can travel without impacting something, but the energy on impact is not lost but simply passed on (or enhanced) by the impact. Our sun maintains its size because the gravitation forces trying to collapse it are balanced by the radiation from fusion reactions near its core. The process of what happens to a star through its life depends on its size. A good description of what happens to a star the size of our sun is here:

http://wind.cc.whecn.edu/~marquard/astronomy/sunlike.htm

At times it can be considered like a contained gas, but not always. If the energy output is the same then a shrinkage in size would mean a higher temperature, but the energy output does change as other effects come into play.
 

Offline DoctorBeaver

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Why does a collapsing star heat up?
« Reply #2 on: 14/09/2008 16:31:56 »
graham - would it have anything to do with the electrons being squeezed into tighter orbits?
 

Offline lightarrow

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Why does a collapsing star heat up?
« Reply #3 on: 14/09/2008 17:01:05 »
As a simplistic answer It should be gravitational potential energy converting into kinetic energy.
 

Offline DoctorBeaver

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Why does a collapsing star heat up?
« Reply #4 on: 14/09/2008 18:28:29 »
As a simplistic answer It should be gravitational potential energy converting into kinetic energy.

Believe it or not, I thought that may be the answer. But what about my point concerning electron orbits?
 

Offline graham.d

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Why does a collapsing star heat up?
« Reply #5 on: 14/09/2008 20:51:07 »
I'm not sure about electron orbits. I think there is a lot of changing of electron orbits - hence light emissions. I think the Pauli exclusion principle still applies. The gravitational pressure and heat goes way beyond the effects on an atomic level and the source of energy is nuclear fusion. I am guessing you know this aleady :-)

Lightarrow is right. Perhaps I was being overcomplicated. In the case of the ultimate fate of our sun, it will expand to a red giant before collapsing to a white dwarf. It is a bit simplistic to just use the gas laws though, which I thought was being suggested. It is primarily radiation pressure from the core that stops our sun gravitationally collapsing and not thermal exitation. The sun is not just a uniform sphere of hot gas.
 

lyner

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Why does a collapsing star heat up?
« Reply #6 on: 14/09/2008 21:42:21 »
Pauli still applies - it just means that you won't get line spectra (transitions between well defined energy levels) you get band spectra because the energy levels are spread out because they can't all occupy one. But that is a phenomenon in all condensed materials - line spectra only occur for low pressure gases.
The limit is 'black body radiation' where all transitions are possible.

As lightarrow says, the radiation pressure at the centre of a massive object is what counts. A photon which starts at the centre of a star takes years and years to 'get out'. Every time a photon bashes into something on the way out you have a contribution to the pressure which effectively keeps the star 'pumped  up'.
 

Offline graham.d

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Why does a collapsing star heat up?
« Reply #7 on: 15/09/2008 12:16:47 »

As lightarrow says, the radiation pressure at the centre of a massive object is what counts.

I don't wish to be picky, but I thought I said that :-)
 

Offline lightarrow

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Why does a collapsing star heat up?
« Reply #8 on: 15/09/2008 13:06:02 »
Ok, however, to be picky, radiation pressure is not what most contribute to counteract the gravitational collapsing; what contribute most is the particles kinetic energy.
 

Offline graham.d

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Why does a collapsing star heat up?
« Reply #9 on: 15/09/2008 15:21:33 »
The kinetic energy balance to the gravity is primarily during star formation. I am no expert here, but from what I understood during the majority of life of a star (like our sun), it is the photon flux that is the primary balancing agent.
 

lyner

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Why does a collapsing star heat up?
« Reply #10 on: 16/09/2008 13:28:55 »
Ok, however, to be picky, radiation pressure is not what most contribute to counteract the gravitational collapsing; what contribute most is the particles kinetic energy.
Wikkers seems to think otherwise - but they have been known to be wrong.
http://en.wikipedia.org/wiki/Stellar_evolution
It strikes me that the gamma photons from the fusion processes could have a lot of momentum and do the job of support quite well. But, then, the nuclei would be moving quite fast, on average, too and there would be more of them per m cube. You'd need to do the actual sums.
It is always annoying to find a Google search produces more TNS hits than anything else! It would seem to be a way of rewriting history / textbooks.
« Last Edit: 16/09/2008 13:33:25 by sophiecentaur »
 

Offline lightarrow

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Why does a collapsing star heat up?
« Reply #11 on: 16/09/2008 18:21:10 »
Ok, however, to be picky, radiation pressure is not what most contribute to counteract the gravitational collapsing; what contribute most is the particles kinetic energy.
Wikkers seems to think otherwise - but they have been known to be wrong.
http://en.wikipedia.org/wiki/Stellar_evolution
It strikes me that the gamma photons from the fusion processes could have a lot of momentum and do the job of support quite well. But, then, the nuclei would be moving quite fast, on average, too and there would be more of them per m cube. You'd need to do the actual sums.
It is always annoying to find a Google search produces more TNS hits than anything else! It would seem to be a way of rewriting history / textbooks.
The pressure at the core of the Sun is ~ 1016 Pa; the radiation pressure, assuming a temperature of ~ 1.5*107 K there, is ~ 1013 Pa.
(Said from an expert; please don't ask me to make the computations  [xx(])
 

lyner

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Why does a collapsing star heat up?
« Reply #12 on: 16/09/2008 21:41:49 »
Fair enough. A factor of one thousand is fairly conclusive!
 

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Why does a collapsing star heat up?
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