Tolman-Oppenheimer-Volkoff limit
/ TOV limit /
Just as a white dwarf has a maximum weight it can bear, so does a neutron star — and beyond it, nothing in the universe can stop a complete collapse into a black hole. This ceiling is the Tolman-Oppenheimer-Volkoff limit, usually shortened to the TOV limit. It is the boundary between the two possible endpoints of a dead massive star: stay a neutron star, or become a black hole. It is, quite literally, the heaviest a star can be while still having a surface.
Where the Chandrasekhar limit for white dwarfs is pinned down to 1.4 solar masses, the TOV limit is fuzzier — currently estimated at roughly 2.2 to 2.3 solar masses, but with real uncertainty. The reason is that it depends on how matter behaves when squeezed to nuclear density and beyond, the so-called equation of state, which we still do not fully know. The most massive neutron stars confidently measured sit around 2.0 to 2.1 solar masses, comfortingly close to the theoretical ceiling, which sharpens our estimates of where the wall lies.
The TOV limit is one of the most useful 'forbidden zones' in astrophysics. If a collapsed object is measured to weigh more than about 2.3 solar masses, it almost certainly cannot be a neutron star and must be a black hole. This logic helps astronomers classify compact objects found in X-ray binaries and gravitational-wave mergers. There is even a tantalizing 'mass gap' between the heaviest neutron stars and the lightest black holes that the TOV limit helps define — and gravitational-wave detectors are now probing exactly this borderland.
In the 2017 neutron-star merger GW170817, the two stars combined to roughly 2.7 solar masses — above the TOV limit — so the merged object very likely collapsed into a black hole within moments, exactly as the limit would predict.
Cross the TOV limit and even a neutron star cannot survive — a black hole forms.
Unlike the firm 1.4-solar-mass Chandrasekhar limit, the TOV value is still uncertain (about 2.2-2.3) because it hinges on the unknown equation of state of ultra-dense matter.