The Tolman-Oppenheimer-Volkoff limit is an upper bound to the mass of stars composed of neutron-degenerate matter (neutron stars), analogous to the Chandrasekhar limit, which provides an upper bound to the mass of stars composed of electron-degenerate matter (white dwarf stars). This limit is estimated to be approximately 3 to 5 solar masses. The uncertainty in the value reflects the fact that the equations of state for neutron-degenerate matter aren't known with high accuracy; closed-form solutions don't exist, and numerical simulation is difficult. Above the TOV limit, an object either collapses to form a black hole, or changes composition and is supported by some other form of degeneracy pressure (for example, quark degeneracy if it becomes a quark star). However, the properties of these more exotic forms of degenerate matter are even more poorly known than those of neutron-degenerate matter, which, when coupled with very little experimental evidence, leads most astrophysicists to assume a transition directly from a neutron star to a black hole.