The Surprising Reason Why Neutron Stars Don’t All Collapse To Form Black Holes

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Protons and neutrons are similar. Despite being composite particles, made up of three quarks apiece, they behave as single, individual fermions themselves. They, too, obey the Pauli Exclusion Principle, and no two protons or neutrons can occupy the same quantum state. The fact that electrons are fermions is what keeps white dwarf stars from collapsing under their own gravity; the fact that neutrons are fermions prevents neutron stars from collapsing further. The Pauli exclusion principle responsible for atomic structure is responsible for keeping the densest physical objects of all from becoming black holes.

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And yet, when you look at the white dwarf stars we have in the Universe, they cap out at around 1.4 solar masses: the Chandrasekhar mass limit. The quantum degeneracy pressure arising from the fact that no two electrons can occupy the same quantum state is what prevents black holes from forming until that threshold is crossed.

In neutron stars, there should be a similar mass limit: the Tolman-Oppenheimer-Volkoff limit. Initially, it was anticipated that this would be about the same as the Chandrasekhar mass limit, since the underlying physics is the same. Sure, it’s not specifically electrons that are providing the quantum degeneracy pressure, but the principle (and the equations) are pretty much the same. But we now know, from our observations, that there are neutron stars much more massive than 1.4 solar masses, perhaps rising as high as 2.3 or 2.5 times the mass of our Sun.

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And yet, there are reasons for the differences. In neutron stars, the strong nuclear force plays a role, causing a larger effective repulsion than for a simple model of degenerate, cold gases of fermions (which is what’s relevant for electrons). For the past 20+ years, calculations of the theoretical mass limit for neutron stars have varied tremendously: from about 1.5 to 3.0 solar masses. The reason for the uncertainty has been the unknowns surrounding the behavior of extremely dense matter, like the densities you’ll find inside an atomic nucleus, are not well known.

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One thought on “The Surprising Reason Why Neutron Stars Don’t All Collapse To Form Black Holes

  1. Not quite sure why the statement “The Pauli exclusion principle, which only applies to fermions, not bosons, states, explicitly, that in any quantum system, no two fermions can occupy the same quantum state.” is the reason to differentiate between neutron stars and black holes unless the suggestion is that fermions somehow go through a conversion to bosons before they can enter a black hole??

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