A black hole is not a thing in the ordinary sense; it is a region of spacetime where gravity has become so extreme that nothing, not even light, can escape once it crosses the event horizon. Understanding what a black hole is requires understanding how it forms. And the answer turns out to depend entirely on mass. Different masses produce black holes through different processes, on different timescales, in different corners of the universe. Here is how black holes form — through several distinct physical processes, each leaving a different signature on the black hole’s mass and environment.
Stellar Collapse: The Main Factory
The most common path to a black hole begins with a massive star. Stars above roughly 20 times the mass of the Sun are the most likely to end their lives as black holes. The story of their formation is the story of nuclear fuel running out.
The energy released as radiation creates outward pressure that counteracts the inward pull of gravity, a balance called hydrostatic equilibrium. When the hydrogen runs out, the core contracts and heats up enough to fuse helium into carbon and oxygen. Then those into neon, oxygen, silicon. Each successive fuel burns faster. The silicon-burning phase lasts only about a day.
Iron is the end of the nuclear energy road; fusing iron consumes energy rather than releasing it. The core, now a sphere of iron roughly the size of Earth, has lost its pressure support. In less than a second, it collapses under its own gravity, falling inward at a quarter of the speed of light.
If the collapsing core exceeds about three solar masses (the Tolman-Oppenheimer-Volkoff limit for neutron stars), no known force can stop the collapse. Neutron degeneracy pressure, which holds up neutron stars, is overwhelmed. The core collapses to a singularity. The outer layers of the star, reflecting off the collapsing core, are blasted away as a supernova. The black hole left behind contains typically 5 to 20 solar masses, concentrated in a point of infinite density according to general relativity, though quantum effects are expected to modify this picture at the smallest scales.
Very massive stars above about 40 solar masses may collapse so completely that almost no explosion occurs (a “failed supernova”), leaving behind a black hole with little or no associated optical display.
Core Collapse Without a Bright Explosion
The distinction between a successful supernova and a failed supernova matters observationally. A failed supernova would appear as a star that simply vanishes. In 2009, a red supergiant called N6946-BH1 in the galaxy NGC 6946 brightened briefly then disappeared from view — a possible failed supernova and one of the best black hole formation event candidates observed directly. Follow-up observations with Hubble confirmed the star was gone, replaced by faint infrared emission consistent with a black hole accreting from leftover gas.
Detecting black hole formation directly is difficult precisely because the most massive collapses are often the quietest. Much of what we know about stellar-mass black hole formation comes from studying their populations through gravitational wave events and X-ray binary systems.
Neutron Star Mergers
A second mechanism produces black holes on much shorter timescales: the merger of two neutron stars. Neutron stars in a binary system lose orbital energy through gravitational wave emission and slowly spiral together. When they merge, the combined mass may exceed the maximum neutron star mass. If it does, the merged object collapses into a black hole, often accompanied by a kilonova — a burst of heavy element synthesis and a short gamma-ray burst.
The 2017 event GW170817, detected by LIGO and Virgo and followed by electromagnetic observations across the spectrum, was the first confirmed neutron star merger. The combined mass was about 2.7 solar masses — close to the maximum neutron star mass. Whether the remnant became a neutron star or a black hole is still debated, but the event demonstrated that neutron star mergers are a real path to black hole formation.
Direct Collapse in the Early Universe
Supermassive black holes (the million-to-billion solar mass objects at the centers of galaxies) pose a formation mystery that stellar collapse alone cannot explain. The largest known supermassive black holes reached masses of over a billion solar masses within the first billion years after the Big Bang. There is not enough time for a stellar-mass black hole formed from a star to grow that large through accretion alone, even at maximum (Eddington-limited) rates.
One proposed solution is direct collapse: in the early universe, before the interstellar medium was enriched with heavy elements, dense regions of primordial hydrogen and helium gas could have collapsed directly into massive black holes of 10,000 to 100,000 solar masses, skipping the star formation step entirely. This requires conditions where the gas cannot fragment into stars — unusually strong ultraviolet radiation from a nearby galaxy could suppress molecular hydrogen formation and keep the gas hot enough to collapse as a whole rather than fragment.
Observations by the James Webb Space Telescope have found supermassive black holes in galaxies at redshifts above 7 (less than 700 million years after the Big Bang) that are too massive to have grown from stellar seeds. These observations have sharpened the case for some form of massive-seed formation, whether through direct collapse or through dense star cluster mergers that formed intermediate-mass black hole seeds.
Black Hole Mergers
Black holes can grow by merging with other black holes. The gravitational wave detections by LIGO, Virgo, and KAGRA since 2015 have revealed a population of stellar-mass black hole mergers. GW190521, detected in 2020, involved two black holes of approximately 85 and 66 solar masses, producing a merger remnant of about 142 solar masses — firmly in the intermediate-mass range. This was the first direct evidence for an intermediate-mass black hole formed through merger.
Repeated mergers in dense stellar environments such as globular clusters or galactic centers could build up black hole mass over billions of years, providing a pathway from stellar-mass seeds to intermediate-mass black holes, and potentially contributing to supermassive black hole growth.
Primordial Black Holes: A Formation Route from the Big Bang
One additional formation mechanism predates all stars: density fluctuations in the early universe. During the first fractions of a second after the Big Bang, regions that were slightly denser than average could have collapsed under their own gravity to form primordial black holes. Unlike stellar black holes, these would not be constrained to masses above a few solar masses — they could form at any mass, from sub-atomic scales up to thousands of solar masses.
Primordial black holes remain hypothetical. No confirmed primordial black hole has been identified. Observational constraints from gravitational lensing, gravitational wave background measurements, and cosmic microwave background data have ruled out primordial black holes as the dominant component of dark matter, but have not ruled them out entirely. They remain an active area of research.
The Common Thread: Gravity Without Support
The observation of black holes spanning eight orders of magnitude in mass — from a few solar masses to billions — shows that how black holes form is not a single process but a family of pathways written across cosmic history. Every black hole formation mechanism, regardless of the mass scale or environment, involves the same basic process: gravity overwhelms whatever forces were providing support. In stellar collapse, it is the radiation pressure from nuclear burning. In neutron star mergers, it is neutron degeneracy pressure. In direct collapse, it is gas pressure and fragmentation. The event horizon that defines a black hole forms at the moment escape velocity exceeds the speed of light — and once it forms, nothing from within can communicate its existence to the outside universe except through its gravitational effects. The question of how black holes form is not just astrophysical — it connects to nuclear physics, general relativity, and the evolution of galaxies. Gravitational wave detectors have opened a new observational window into how black holes form and merge across cosmic time. The diversity of black hole masses observed suggests that how black holes form depends heavily on their environment and the era of cosmic history in which they arose.
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This article is part of our framework exploring Stars and Planets — how stars forge the elements of life and shape planetary systems.