Maximum mass of a nonrotating neutron star
In plain words
Above a certain mass a neutron star must collapse into a black hole. The heaviest known neutron stars weigh a little over two Suns, but the true limit is not known.
Precise statement
Determine $M_{\mathrm{TOV}}$, the maximum gravitational mass of a cold nonrotating neutron star (the Tolman-Oppenheimer-Volkoff limit). Lower bounds come from PSR J0740+6620 at about $2.08\ \text{solar masses}$ and PSR J0952-0607 at $2.35 \pm 0.17\ \text{solar masses}$; arguments about the GW170817 merger remnant suggest $M_{\mathrm{TOV}}$ below about $2.3\ \text{solar masses}$. Answer: $M_{\mathrm{TOV}}$ with uncertainty below $0.1\ \text{solar mass}$.
What would settle it
A radio-timed pulsar mass above 2.3 solar masses or a measured collapse time of a merger remnant, combined with equation-of-state inference.
Status in the literature
The 2.35 solar-mass value (Romani et al., 2022) relies on modeling of an irradiated companion star and is less secure than radio-timing masses.