How do black holes form?

Three main formation pathways, three mass categories, and the incredible photo of M87* by the Event Horizon Telescope: everything black hole physics tells us about the universe in 2026.

What exactly is a black hole?

A black hole is a region of spacetime where gravity is so intense that nothing — not even light — can escape once a critical boundary is crossed. This boundary is called the event horizon.

A black hole is not a material object in the classical sense: it's a region where matter has been compressed to infinite density at a central point called the singularity. Black holes are predicted by Einstein's general relativity (1915) and have been observed indirectly since the 1960s (Cygnus X-1), then directly photographed in 2019 (M87*) and 2022 (Sgr A*).

Singularity

Central point of infinite density where the known laws of physics break down. A theory of quantum gravity remains to be discovered to describe it.

Event horizon

Mathematical surface delimiting the zone from which no signal can escape. Radius = 2GM/c² (Schwarzschild).

Accretion disk

Disk of matter spiraling around the black hole, heated to millions of degrees, emitting intense X-rays and UV.

Ergosphere

Region around a rotating (Kerr) black hole where spacetime itself is dragged into rotation.

What are the 3 categories of black holes?

Black holes are classified by mass:

1. Stellar black holes (3 to 100 M☉) — formed by the collapse of massive stars. Cygnus X-1 (about 21 M☉) is the archetype.
2. Intermediate-mass black holes (100 to 100,000 M☉) — long hypothetical, now detected via gravitational waves (LIGO/Virgo, GW190521 = 142 M☉) and certain globular clusters.
3. Supermassive black holes (millions to billions M☉) — reside at the center of nearly all large galaxies. Sgr A* (4.1 million M☉) at the center of the Milky Way, M87* (6.5 billion M☉) at the heart of M87.
4. Primordial black holes (hypothetical) — formed at the Big Bang, very variable masses.

How is a stellar black hole born?

A stellar black hole is born at the death of a very massive star (at least 20-25 solar masses). The scenario, simplified, unfolds in several steps:

1. The star has successively fused hydrogen, helium, carbon, oxygen, neon, silicon in its core.
2. When the core reaches iron, nuclear fusion ceases to be exothermic: the core no longer produces energy.
3. Radiation pressure collapses, gravity reasserts itself.
4. The core collapses in seconds, reaching nuclear densities.
5. Outer layers rebound and are blown away in a supernova of type II or Ib/c.
6. If the residual mass exceeds about 3 M☉ (Tolman-Oppenheimer-Volkoff limit), no known force can stop the collapse: a black hole forms. Otherwise, you get a neutron star.

For a complete account of the explosion, see: What is a supernova? Types, causes, observations.

Where do supermassive black holes at galaxy centers come from?

This is one of the great enigmas of astrophysics in 2026. How do you form a black hole of one billion solar masses in less than a billion years after the Big Bang? Three main scenarios are studied:

1. Slow accretion from small seeds

Stellar black holes (~100 M☉) growing by accretion. Problem: time available is insufficient for the very early supermassives observed by JWST.

2. Direct collapse (DCBH)

Direct Collapse Black Holes: direct collapse of primordial gas clouds into 10,000-100,000 M☉ black holes, without going through the stellar stage. Scenario favored by JWST discoveries.

3. Repeated mergers

Mergers of intermediate-mass black holes in dense star clusters, then migration toward the galactic center.

The JWST telescope has discovered since 2022 supermassive black holes (1-10 billion M☉) already formed at z > 10 — that is, less than 500 million years after the Big Bang. This discovery favors the direct collapse scenario, and forces a rethink of standard cosmic chronology.

Do primordial black holes exist?

Primordial black holes (PBH) are hypothetical black holes formed not by stellar collapse, but by extreme density fluctuations in the very early universe — in the first fractions of a second after the Big Bang.

Proposed in 1971 by Stephen Hawking, their existence has not been demonstrated but remains actively researched. If they exist, they could have very varied masses (from 10⁻⁵ g to 10⁵ M☉) and constitute a non-negligible fraction of dark matter — a hypothesis revived by some unusual LIGO detections (GW190521).

What happens at the event horizon?

The event horizon is the "point of no return" boundary of the black hole. Its radius (Schwarzschild radius) is proportional to the mass:

• For a 10 M☉ black hole → about 30 km.
• For Sgr A* (4.1 million M☉) → about 12 million km.
• For M87* (6.5 billion M☉) → about 40 billion km (5x the Sun-Pluto distance).

At the horizon, escape velocity reaches the speed of light. For an outside observer, a falling object appears to slow down and freeze at the edge (gravitational time dilation). For the object itself, the crossing is instantaneous — but irreversible.

How do black holes grow over time?

Black holes grow through two mechanisms:

1. Matter accretion (gas, dust, fragmented stars) through an accretion disk that heats and emits intense X-rays and UV. Accretion is limited by the Eddington limit — beyond it, radiation pressure pushes back the incoming gas.
2. Merger with other black holes during coalescences detected by gravitational waves since 2015 (GW150914 was the first, merger of 36 + 29 M☉).

To explain the very early supermassive black holes observed by JWST, some models invoke temporary super-Eddington accretion — a regime where radiation manages to escape without totally braking the accretion.

What are AGN relativistic jets?

Active galactic nuclei (AGN) are supermassive black holes in intense accretion phase. Some of the gas approaching the event horizon is ejected at speeds close to that of light (>99.9% c) as two jets perpendicular to the accretion disk, along the black hole's rotation axis.

These relativistic jets can extend hundreds of thousands of light-years, well beyond the host galaxy. M87* has a visible jet 5,000 light-years long, studied by the Event Horizon Telescope since 2017. Quasars and blazars are AGN whose jet points toward Earth, amplifying their luminosity.

How did Einstein predict their existence?

Albert Einstein did not explicitly predict black holes, but his general relativity theory (1915) contained the mathematical possibility. As early as 1916, German physicist Karl Schwarzschild found the first exact solution to Einstein's equations, which describes the gravitational field of a point mass — a solution containing an event horizon and a central singularity.

Einstein himself remained skeptical for a long time about the physical reality of these objects, which he considered a mathematical curiosity. The term "black hole" was popularized only in 1967 by John Wheeler. It was only in 1971 (with Cygnus X-1) that a first serious candidate was identified.

Black holes are not as black as they are painted. They glow with quantum radiation — and will eventually evaporate. — Stephen Hawking, on Hawking radiation, 1974

Has the M87* black hole been photographed?

Yes. On April 10, 2019, the international Event Horizon Telescope (EHT) collaboration published the first direct image of a black hole: M87*, the supermassive black hole at the center of galaxy M87 in the Virgo cluster, located 55 million light-years away. Its mass is estimated at 6.5 billion solar masses.

The image shows the shadow of the black hole surrounded by a luminous ring: the superheated matter of the accretion disk. The size of the ring and its asymmetry strikingly confirm Einstein's general relativity predictions.

In May 2022, the EHT released the image of Sagittarius A* (Sgr A*), the supermassive black hole at the center of our Milky Way (4.1 million M☉, at 26,000 light-years). It is the only supermassive black hole whose neighboring star orbits have been directly tracked for 30 years (work by Reinhard Genzel and Andrea Ghez, Nobel Prize in Physics 2020).