How Black Holes Form and Why They Are the Universe's Most Extreme Objects

A black hole is a region of space where gravity is so strong that nothing, not even light, can escape once it crosses a boundary called the event horizon, according to NASA. That single property — a one-way surface in space — is what makes these objects, rather than their visible size, the most extreme environments known in physics.

How a black hole forms

The best-understood pathway is stellar collapse. NASA explains that stars with more than about 20 times the Sun's mass eventually exhaust their nuclear fuel; the core then collapses inward while the outer layers are blown off in a supernova explosion. If the crushed core that remains still holds more than roughly three solar masses, NASA states that no known force can stop that core from collapsing into a black hole.

ESA describes the same mechanism: once a stellar core above that mass threshold is compressed into a small enough volume, its own gravity overwhelms every other physical force, and the object keeps shrinking until light can no longer leave it. Black holes can keep growing afterward by pulling in surrounding gas, a process called accretion, according to NASA. ESA notes that accretion alone struggles to explain how some supermassive black holes grew so large so early in the universe's history, which is one reason scientists study intermediate-mass black holes as a possible missing link.

A separate, less settled idea concerns the earliest moments of the universe. Some theories propose that primordial black holes could have formed directly from extreme conditions shortly after the Big Bang, without any star involved. ESA says its Euclid mission may help test whether such objects exist and whether they could account for some of the universe's dark matter.

Four classes, defined by mass

NASA and ESA group black holes into categories based primarily on how much mass they contain, since mass determines size, strength of gravity, and formation route.

What makes them extreme

The event horizon is the feature that sets black holes apart. NASA describes it as the point where the escape velocity needed to leave the object exceeds the speed of light, so nothing — including light itself — can cross back outward. Because no light escapes, black holes themselves emit no light; what astronomers detect is the glow of matter just outside this boundary.

That glow comes from the accretion disk, material pulled from nearby stars or gas clouds that spirals inward and heats up as it approaches the event horizon, according to NASA's description of black hole anatomy. Around actively feeding supermassive black holes, this region — called the corona — reaches temperatures of about a billion degrees and produces high-energy X-rays, with particles moving at velocities approaching the speed of light.

At the very center, general relativity predicts a singularity, a point where matter is compressed to infinite density, according to NASA. It is described as the final destination for anything that falls past the event horizon, though physicists note this is where current theories of gravity break down rather than giving a fully tested description of what actually happens there.

Seeing a black hole directly

Because black holes emit no light of their own, astronomers cannot photograph them directly — they image the shadow a black hole casts on the glowing gas around it. The Event Horizon Telescope (EHT), a global network of radio observatories linked through a technique called very-long-baseline interferometry, produced the first such image of the supermassive black hole M87* in 2019 and of Sagittarius A*, the Milky Way's own central black hole, in 2022, according to the European Southern Observatory (ESO), a partner in the EHT collaboration.

ESO states that Sagittarius A* has a mass of roughly 4 million times that of the Sun and lies about 27,000 light-years from Earth, yet from our vantage point ESO describes it as appearing no larger than a doughnut would look if placed on the surface of the Moon. NASA separately reports that M87*, imaged by the EHT in 2019, holds more than 6 billion solar masses, with an event horizon that NASA says would reach well beyond the planets of the solar system.

ESO notes that such images let scientists directly probe the behavior of spacetime at a black hole's boundary, a way of testing general relativity in the most extreme gravitational conditions available for observation.