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The formula of enthalpy change is

change in H = U + P*(change in V)

In space since there is no air, thus atmospheric pressure = 0 , P= 0 , Pv = 0 ,

So will H = U ?

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  • $\begingroup$ Yes. Then again, every compound has a vapor pressure, so in the equilibrium (given $P=0$) they all will up and evaporate, leaving you with a pretty dull environment. $\endgroup$ – Ivan Neretin Jun 6 '18 at 13:20
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Well, in principle yes, for example if you had a blob of liquid floating in empty space and wanted to consider the whole blob as a thermodynamic system.

But it may be worth mentioning that wouldn't be a typical use of thermodynamics in astrophysics. Usually you'd consider your thermodynamic system to be some small part of a larger system and the larger system would be a thermal and volume reservoir, so T and P would both be nonzero.

For example, it's common to consider the thermodynamics of stars, but what you do is consider a small packet of gas to be your system, and the rest of the star to be surroundings that establish a constant T and P for your packet.

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