Chandrasekhar limit
A collapsar is a star which has undergone gravitational collapse. When a star no longer has enough fuel for significant fusion reactions there are three possible outcomes, depending on the star's mass. If it is less than 1.4 solar masses, known as the Chandrasekhar limit, it will stabilize and shrink, becoming a white dwarf. Above the Chandrasekhar limit, but below the Tolman–Oppenheimer–Volkoff limit (approximately 3 solar masses), it will become a neutron star. Above the Tolman-Oppenheimer-Volkoff limit, it will become a black hole. However, it is theorized that the high density of neutron star cores allow for quark matter, and as a result, a star that is larger than even the Tolman-Oppenheimer-Volkoff limit, yet still not a black hole.