What is a black hole? Formation, types and observed black holes

A simple definition

A black hole is a region of space where matter is so concentrated that nothing can get out, not even light. Its boundary is called the event horizon. It is not a solid surface: it is a point of no return, beyond which every path leads inward.

Any mass can become a black hole if it is squeezed enough. Earth would have to fit in a marble about 1.8 cm across; the Sun, in a ball 6 km across.

How black holes form

Most known black holes are dead stars. A massive star burns its fuel for a few million years; when its core no longer produces enough energy to hold up its own weight, it collapses in a fraction of a second. Depending on the star's mass and composition, the collapse leaves a neutron star or a black hole, with or without a supernova.

The threshold is not sharp: models suggest a starting mass of roughly 20 to 25 solar masses or more, but rotation, chemical composition and a companion star change the outcome. The lightest known black holes weigh about 3 solar masses, and the boundary with neutron stars is still debated.

Black holes then grow by swallowing gas or merging with each other, which gravitational wave detectors have observed since 2015. How supermassive black holes formed, already present less than a billion years after the Big Bang, remains an open question.

Four families

• Stellar-mass, from a few to about a hundred solar masses: the remains of massive stars, such as Cygnus X-1 or Gaia BH1.
• Intermediate-mass, from about a hundred to a hundred thousand solar masses: long missing, they are now attested, for example by the GW190521 merger, which produced a black hole of about 142 solar masses.
• Supermassive, from a hundred thousand to more than ten billion solar masses: at the centre of almost every large galaxy. The one in the galaxy Holm 15A, measured directly from the motion of its stars, reaches about 40 billion solar masses.
• Primordial: hypothetical, formed just after the Big Bang. None has been detected so far.

Anatomy of a black hole

• The horizon: the boundary of no return, with radius r_s.
• The photon sphere, at 1.5 r_s: light can orbit there. It outlines the shadow seen in the EHT images.
• The accretion disk: gas spiralling inward and heating up until it emits X-rays. For a non-rotating black hole, its innermost stable orbit is at 3 r_s.
• Jets: some black holes launch matter at nearly the speed of light over thousands of light-years. M87 has a famous one.
• The singularity: where general relativity predicts infinite density and stops applying. Penrose showed in 1965 that it is unavoidable in a collapse; a quantum theory of gravity should replace it.

The black holes we know

• Cygnus X-1: about 21 solar masses, 7,200 light-years away. Spotted through its X-rays, it became in the 1970s the first object widely accepted as a black hole.
• Gaia BH1: 9.6 solar masses, about 1,560 light-years away in the constellation Ophiuchus. Found in 2022 with the Gaia satellite, it is the nearest known black hole.
• Gaia BH3: 33 solar masses, about 1,930 light-years away, announced in 2024. It is the most massive stellar black hole known in the Milky Way.
• Sgr A*: about 4.3 million solar masses, at the centre of our galaxy, 27,000 light-years away. Its mass comes from the stars orbiting it, work recognised by the 2020 Nobel Prize (Reinhard Genzel and Andrea Ghez, together with Roger Penrose).
• M87*: 6.5 billion solar masses, 55 million light-years away. The first black hole ever imaged.

Real image

Real image

How do you see something invisible?

A black hole emits nothing, but it gives itself away in four ways: through the motion of nearby stars, the light of the gas it heats, the gravitational waves it emits when merging, and its shadow. To get the images above, the Event Horizon Telescope combined radio telescopes across the planet, observing at a wavelength of 1.3 mm, like a virtual telescope the size of Earth.

And if you are wondering what would happen if you fell in, the answer is on this page.