The smallest known black hole is only about twenty kilometres wide
In 2020 astronomers reported a black hole of just 3.3 solar masses, only 19.5 kilometres across, hiding in a binary called 2MASS J05215658+4359220. Stellar black holes, the collapsed remains of heavy stars, usually weigh between five and several dozen Suns, and they can be almost impossible to tell apart from neutron stars.
When a massive star exhausts its fuel, collapse is unavoidable, and its fate depends on the mass of the collapsing core. Below the Chandrasekhar limit it becomes a white dwarf. Above that but under the Tolman-Oppenheimer-Volkoff limit, it stops as a neutron star. Beyond the TOV limit nothing halts the crush and a black hole forms. The thresholds shift with a star's chemistry and spin.
Where exactly a neutron star tips over remains uncertain. A 1939 estimate put the limit at 0.7 solar masses; a 1996 estimate raised it to between 1.5 and 3. The heaviest neutron star observed, PSR J0740+6620, found in September 2019, weighs about 2.14 Suns. General relativity itself allows black holes of any mass, but lighter ones need ever greater density, and no known stellar process makes them below a few solar masses; any tiny ones would probably be primordial.
Once formed, a black hole is remarkably simple. The no-hair theorem says it has just three basic properties: mass, electric charge and spin, the spin inherited from the star that produced it. Records keep climbing. Until 2016 the heaviest known stellar black hole weighed about 15.65 solar masses, but in September 2015 gravitational waves revealed a spinning one of about 62 Suns born when two smaller black holes merged. Much bigger kinds also exist: intermediate-mass black holes in globular clusters and supermassive ones at galaxy centres.
Stellar black holes betray themselves when paired with a companion star. Gas flowing onto the black hole heats to several hundred million degrees and shines in X-rays, while the companion is visible through ordinary telescopes. Neutron stars release similar energy, so the key clue is mass: every identified neutron star is below 3 solar masses, and no compact object above that behaves like one.
Source: Stellar black hole
