[Kryptid (OP)]

It's apparently possible for white dwarf stars composed of helium to exist, so presumably helium and all heavier elements can become degenerate matter. What about hydrogen, though? Is it possible to compress hydrogen to the point where it becomes degenerate matter, or will it invariably undergo nuclear fusion before reaching that point?

[evan_au]

It is thought that the core of Jupiter may contain a large quantity of metallic hydrogen, where the electrons are no longer in shells in isolated atoms, but forms a continuous band across all the atoms. This is a condition of electron degeneracy.

The temperature and pressure in Jupiter's core is not high enough to initiate Deuterium fusion (this is thought to require a mass around 13 times higher than Jupiter).

See: https://en.wikipedia.org/wiki/Degenerate_matter

[Bill S]

It is thought that the core of Jupiter may contain a large quantity of metallic hydrogen, where the electrons are no longer in shells in isolated atoms, but forms a continuous band across all the atoms

Presumably, this is still identified as hydrogen because of the ratio of electrons to nuclei?

[evan_au]

Presumably, this is still identified as hydrogen because of the ratio of electrons to nuclei?

I would call it hydrogen because the nuclei are made up of single protons (with some small fraction having a bonus neutron).

But I agree - to make it electrically neutral, the ratio of protons to electrons would be 1:1.

[Kryptid (OP)]

Metallic hydrogen is not degenerate in the same sense that white dwarf matter is, presumably? White dwarf matter is many, many times more dense than metallic hydrogen. Is there a fundamental difference? Or does it just take that much more pressure to turn helium into degenerate matter than it does hydrogen?

[evan_au]

Metallic hydrogen is not degenerate in the same sense that white dwarf matter is, presumably?

There are many types of degeneracy:
- electron degeneracy
- proton degeneracy
- neutron degeneracy
- (potentially) quark degeneracy

It is characterised by a divergence from ideal gas laws - quantum effects (Pauli Exclusion Principle) provides increased stiffness.

Metals (including hydrogen) can show electron degeneracy.
See: https://en.wikipedia.org/wiki/Degenerate_matter#Electron_degeneracy