&gt; They are unable to decay into protons plus electrons because there isn&#x27;t enough &quot;room&quot; for the two particles to exist in the space formerly taken up by one particle. See: Electron degeneracy pressure.<p>Just being pedantic here but in a neutron star, it&#x27;s neutron degeneracy pressure that&#x27;s at play. Electron degeneracy pressure keeps a white (and later brown) dwarf from collapsing further, but if there&#x27;s enough mass it is overcome and the next thing that kicks in is neutron degeneracy pressure, which for whatever reason is a lot stronger, and holds up another solar mass or two before it passes the limit where collapsing into a black hole is inevitable.<p>The principles for both types of degeneracy are similar (it&#x27;s the Pauli exclusion principle at work basically) but the particles are different.