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Author Topic: What is the entropy of supernova gas?  (Read 1362 times)

Offline Atomic-S

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What is the entropy of supernova gas?
« on: 09/05/2015 01:51:47 »
According to a certain reference, the current entropy S of an ideal gas is

S = nR ln V + S0

V being its current volume and S0  being its entropy at some starting volume V0.  n being the number of moles present, and R being the gas constant. So, is it correct to conclude that as the gases spewed out by a supernova continue out into space, their entropy is increasing?


 

Offline chiralSPO

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Re: What is the entropy of supernova gas?
« Reply #1 on: 09/05/2015 02:31:03 »
I think it is safe to assume that the entropy of an expanding supernova gas cloud is increasing.

However, the assumption that the gas is ideal may not hold entirely well over the course of the expansion. Initially the ejecta from a supernova is really, really hot plasma, which cools as it expands (and radiates). Eventually it should cool enough to condense into a gas (is condense the right term??) which should reduce the entropy of the matter itself, though if you count the photons that are emitted, I think the entropy still increases overall. The material will also eventually condense/coalesce into liquids and solids (and planets etc.) that has less entropy than the gas...
 

Offline evan_au

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Re: What is the entropy of supernova gas?
« Reply #2 on: 09/05/2015 10:10:03 »
Entropy increases over time, in a closed system. However, a supernova is not a closed system.

For a few weeks, the supernova puts out more light than an entire galaxy, and this light outruns the gas produced by the supernova. The gas radiates X-Rays, light and heat, cooling down.

To make a supernova into a closed system, you would need to surround it by (very strong) mirrors, which keep the heat energy in the gas.
 

Offline Thebox

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Re: What is the entropy of supernova gas?
« Reply #3 on: 09/05/2015 10:33:37 »
According to a certain reference, the current entropy S of an ideal gas is

S = nR ln V + S0

V being its current volume and S0  being its entropy at some starting volume V0.  n being the number of moles present, and R being the gas constant. So, is it correct to conclude that as the gases spewed out by a supernova continue out into space, their entropy is increasing?

 The entropy of an expanding gas into a space lessens, it is only when there is an enclosed system that entropy can increase.  It is simpler to imagine a standard glass and a bottle with a lid.   Pour water into  glass and it overflows, it as an escape route, where in a closed bottle that was full, if we add more water we expand the bottle by pressure.
Or you can imagine metal expansion heat gain and heat loss. More gain than loss and the metal expands. 
 

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Re: What is the entropy of supernova gas?
« Reply #3 on: 09/05/2015 10:33:37 »

 

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