Interstellar Cloud collapse

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  • #26
CAF123
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Not quite. In the initial stages of the collapse is slow and the gas cloud is transparent. Sans radiation, the collapse would cause the temperature to rise. That it can radiate means the temperature remains fairly constant and fairly low (but higher than the background 2.7 kelvin).
So maybe I should recast the condition as dW = -dQ. As the cloud is gravitationally collapsing, this will tend to increase its temperature. But it radiates as it does so, and hence there is no net effect on the temperature. (isothermal phase)


As the collapse continues, the density and temperature rise, particularly in the center. The cloud becomes opaque, starting in the center but the opaque region grows with time. The opaque cloud does not interact much at all with the environment, not even radiationally.
Okay, so in the adiabatic phase, the gravitational collapse does increase the temperature.
 
  • #27
D H
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So maybe I should recast the condition as dW = -dQ. As the cloud is gravitationally collapsing, this will tend to increase its temperature. But it radiates as it does so, and hence there is no net effect on the temperature. (isothermal phase)
There is no work here. The problem is that the simplistic form of the first law of thermodynamics you learned ignores thermal radiation.
 
  • #28
CAF123
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There is no work here. The problem is that the simplistic form of the first law of thermodynamics you learned ignores thermal radiation.

Why is there not work done by gravity as the cloud collapses?
 
  • #29
D H
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Because it's self gravitation. Work in thermodynamics is that done on the system by the external environment. (Alternatively, it's work done by the system on the external environment, depending on which sign convention one uses.) Gravitation is not doing work on the external environment. The only thing the external environment does is act as a place into which the collapsing cloud can emit electromagnetic energy.
 

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