Other meanings of black hole
Astrophysics
A stellar black hole is a black hole formed by the gravitational collapse of a massive star (typically more than 8–10 solar masses) at the end of its life. These objects have masses ranging from about 3 to several tens of solar masses, and they are the most common type of black hole in the universe, with dozens of confirmed examples in the Milky Way alone.
Stellar black holes form when a massive star exhausts its nuclear fuel and undergoes core collapse, leading to a supernova explosion. If the remnant core exceeds the Tolman–Oppenheimer–Volkoff limit (about 2–3 solar masses), it collapses into a black hole1. The resulting black hole is characterized by its mass, spin, and electric charge, though charge is negligible in practice. The event horizon, the boundary from which nothing can escape, has a radius (Schwarzschild radius) proportional to the mass: about 3 km per solar mass2.
Stellar black holes are typically detected through their interaction with a companion star in a binary system. As matter accretes onto the black hole, it heats up and emits X-rays, making them visible to X-ray telescopes such as Chandra and XMM-Newton3. The first confirmed stellar black hole, Cygnus X-1, was discovered in 1971 using this method. Gravitational wave observatories like LIGO and Virgo have also detected mergers of stellar black holes, providing a new way to study them.
Cygnus X-1, with a mass of about 21 solar masses, is the most famous stellar black hole. Others include V404 Cygni, LMC X-3, and GRO J1655-40. In 2022, astronomers discovered a stellar black hole in the Milky Way named Gaia BH1, located about 1,600 light-years away, making it the closest known black hole to Earth. These objects are often found in X-ray binaries, but some may be isolated.
Stellar black holes can also form through mechanisms other than direct collapse, such as the merger of two neutron stars, which can produce a black hole if the combined mass exceeds the limit. Additionally, some stellar black holes may be 'quiet' and not accreting matter, making them detectable only through gravitational microlensing or dynamical effects on nearby stars. The first isolated stellar black hole, OGLE-2011-BLG-0462, was discovered in 2022 using microlensing. Another intriguing case is the black hole in the system HR 6819, initially thought to be a triple system with a black hole, but later reinterpreted as a binary with two stars, highlighting the challenges in identification.
Stellar black holes are crucial for testing general relativity, especially in strong gravitational fields. Observations of their accretion disks and the orbits of companion stars provide measurements of mass and spin, which constrain stellar evolution models. They are also potential sources of gravitational waves, and their mergers are used to probe the population of black holes and the history of star formation.
Stellar black holes are the most common type of black hole, with masses between about 3 and 100 solar masses, and are key to understanding stellar evolution and gravity.
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