The maximum mass that a neutron star can have without it being overwhelmed by its own gravity. Calculations put this between 1.6 and 2 solar masses, although the exact figure is uncertain. A neutron star with a mass greater than this is expected to collapse further into a black hole. The limit is named after the American physicist (Julius) Robert Oppenheimer (1904–67) and the Russian-born Canadian George Michael Volkoff (1914–2000).