What is Tolman-Oppenheimer-Volkoff limit?

It's an upper bound to the mass of cold, nonrotating neutron stars, analogous to the Chandrasekhar limit for white dwarf stars. Observations of GW170817, the first gravitational wave event due to merging neutron stars (which are thought to have collapsed into a black hole within a few seconds after merging), suggest that the limit is close to 2.17 solar masses.

A neutron star in a binary pair (PSR J2215+5135) has been measured to have a mass close to or slightly above this limit, 2.27+0.17−0.15 M☉.Earlier theoretical work placed the limit at approximately 1.5 to 3.0 solar masses, corresponding to an original stellar mass of 15 to 20 solar masses. In the case of a rigidly spinning neutron star, the mass limit is thought to increase by up to 18%.

An approximation of the limit was first given by Robert Oppenheimer and the Canadian physicist George Volkoff in 1939. Because Oppenheimer and Volkoff based their estimate on the work of the American physicist Richard Tolman, the quantity is sometimes called the Tolman-Oppenheimer-Volkoff limit.

If this is the first time that you have heard about this term, then do not worry because you are not alone. This is a term that is used when non-rotating stars are being measured. This is applied when the star is not as massive as the usual limit.

There is neutron to neutron interactions that will occur that are meant to prevent the collapse of the star. If in case that the star is massive then there is a big chance that the start will start to form a black hole. There are some stars that change its overall composition to become another type of star.