[Atomic-S (OP)]

If a compressed bunch of neutrons, such as in a neutron star, cools to a very low temperature, and is then slowly decompressed, what happens?

[syhprum]

It does not happen the universe is not old enough for a Neutron star ever to cool, outside the high pressure environment of a nucleus or a neutron star they decay in about 15 minutes emitting an electron and an anti neutrino   

[evan_au]

It skips the cooling phase, but when binary neutron stars collide, they gouge out huge hunks from their surfaces, and spray the results into space.

It is thought that this decompressed neutron star material breaks down into heavy elements (gold and beyond). The released energy from these heavy nuclei looks like the light curve of a nova explosion.

Before this theory was developed, it was hard to see how these heavy elements could be formed in a supernova explosion.

See: https://en.wikipedia.org/wiki/Neutron_star#Neutron_star_binary_mergers_and_nucleosynthesis

[PmbPhy]

If a compressed bunch of neutrons, such as in a neutron star, cools to a very low temperature, and is then slowly decompressed, what happens?

Neutrons cannot be compressed. What happens in a neutron star is that when the energy of a star can no longer support its ow weight the gravitational force compresses the atoms in a way that the electrons combine with protons yielding neutrons which, when combined with the other neutrons in the atom, form a neutron star. Basically one can think of a neutron star as a giant nucleus of atom composed of all neutrons.

[chris]

Andrew Pontzen talked about neutron stars in some detail on a recent episode of the Naked Scientists podcast. Here's a link to the transcript:

https://www.thenakedscientists.com/articles/questions/what-would-happen-thimble-full-neutron-star-if-you-brought-it-earth?page=1