The Tolman Oppenheimer Volkoff limit is a theoretical ceiling for neutron-degenerate matter that has been debated since 1939 when it was first estimated at 0.7 solar masses. Updated models by 1996 placed this limit between 1.5 and 3 solar masses, yet the true value remains elusive. This limit determines the exact tipping point where a star stops being a neutron star and becomes a black hole.
The system 2MASS J05215658+4359220 reported in 2019 holds the title of the smallest-mass black hole currently known to science with a mass of 3.3 solar masses and a diameter of only 19.5 kilometers. The distances to these objects vary widely from the relatively close 350 light-years of A0620-00 to the distant 45,000 light-years of GS 1354-64. This specific system is part of a diverse population of stellar-mass black hole candidates in the Milky Way galaxy.
The lower gap roughly between 2 and 5 solar masses represents a scarcity of observed candidates just above the maximum possible neutron star mass. All known neutron stars weigh less than 3.0 solar masses while any compact system exceeding that limit displays no neutron star properties strongly suggesting a black hole. The discovery of black holes within these forbidden zones challenges current theories and suggests alternative formation mechanisms such as the merger of binary neutron stars.
One of the most famous stellar black holes Cygnus X-1 lies approximately 6,000 to 8,000 light-years away and weighs about 21.2 solar masses. This object is a member of an X-ray binary system where the black hole reveals itself through X-rays while the companion star can be observed with optical telescopes. The distances to these objects vary widely from the relatively close 350 light-years of A0620-00 to the distant 45,000 light-years of GS 1354-64.
The event GW150914 involved two black holes with masses of 36 and 29 solar masses merging to form a black hole of 62 solar masses. The distances to these events are vast ranging from 1.3 billion to 2.9 billion light-years. These discoveries have pushed the boundaries of our understanding revealing black holes that exist in the upper mass gap.