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SpaceDifficulty 2-5

Black holes

A black hole is a huge amount of matter packed into so small a space that, inside the boundary called the event horizon, nothing — not even light — can escape. Black holes form when the cores of very massive stars collapse, and supermassive ones sit at the centres of most large galaxies. Although they give out no light themselves, they have been detected through orbiting stars, X-rays, gravitational waves and, since 2019, images of their shadows.

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Measuring Black Holes

In an X-ray binary the black hole's mass comes from the orbit of its visible companion, and that analysis depends on knowing the system's distance. In 2021 radio astrometry put Cygnus X-1 farther away than earlier estimates, which raised its black hole's mass to 21.2 ± 2.2 solar masses. That is far above the roughly 3 solar masses at which neutrons can no longer support a collapsed core.

The Event Horizon Telescope links radio dishes across the globe into an Earth-sized interferometer observing at 1.3 mm. For M87* it resolved a ring 42 ± 3 microarcseconds across, and comparing it with general-relativistic simulations gave a mass of (6.5 ± 0.7) × 10⁹ solar masses. The event horizon itself is about 2.5 times smaller than the shadow it casts.

Gravitational waves give a separate route: the 2015 signal swept from 35 to 250 Hz and matched the predicted waveform for two black holes of about 36 and 29 solar masses merging into one of about 62, with about 3 solar masses radiated as gravitational waves.

Info: A worked check: 36 + 29 = 65 solar masses before and about 62 after; the difference left as gravitational-wave energy (E = mc²).
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What it is

Every object has an escape velocity, the speed needed to get away from its gravity; for Earth it is 11.2 km/s. Squeeze the same mass into a smaller space and the escape velocity rises. If it exceeds the speed of light, nothing can get out: that is a black hole. The boundary at which escape becomes impossible is the event horizon. It is not a solid surface, and a black hole is not a hole but an extreme concentration of matter.

RS=2GMc2R_S = \dfrac{2GM}{c^2}

Schwarzschild radius of a non-rotating black hole of mass M; G is the gravitational constant and c the speed of light.

Kinds of black hole

  • Stellar-mass black holes, of about 5 to 20 solar masses, form when the core of a very massive star collapses and is too heavy — more than about 3 solar masses — for neutrons to support.
  • Supermassive black holes, millions to billions of times the Sun's mass, sit at the centres of most large galaxies, including our own.
  • Intermediate-mass black holes, in between, were long proposed; possible evidence has only appeared in recent years.

Worked example

Using Rₛ = 2GM/c² with G = 6.67 × 10⁻¹¹ N m² kg⁻², M = 4 × 10⁶ × 1.99 × 10³⁰ kg (Sagittarius A*) and c = 3.00 × 10⁸ m/s gives Rₛ ≈ 1.18 × 10¹⁰ m, about 12 million kilometres (our own calculation). For one solar mass the same formula gives about 3 km, and because Rₛ is proportional to M, doubling the mass doubles the radius.

Common misconception: A common misconception is that black holes roam about sucking everything in like cosmic vacuum cleaners. From far enough away, a black hole pulls just like any other object of the same mass. Only close to the event horizon does its gravity become extreme.

Where it connects

Black holes are one possible end of a massive star's life (Stellar evolution), are described by curved space-time (Space-time), anchor the centres of galaxies (Galaxies) and are studied with radio and gravitational-wave observatories (Telescopes).

How we know they exist

  • X-ray binaries: Cygnus X-1, found in 1964 by rocket-borne detectors, contains a black hole of about 21 solar masses.
  • Stellar orbits: 30 years of tracking stars round the Milky Way's centre revealed about 4 million solar masses in a tiny region (2020 Nobel Prize in Physics).
  • Gravitational waves: in 2015 LIGO detected ripples in space-time from two black holes, of about 36 and 29 solar masses, merging.
  • Shadow images: the Event Horizon Telescope, an Earth-sized network of radio telescopes, imaged M87* (2019) and Sagittarius A* (2022).

Assumptions and limits

  • The simple formula Rₛ = 2GM/c² describes a non-rotating black hole; real black holes spin.
  • The Event Horizon Telescope mass for M87* comes from comparing the image with computer simulations of hot gas around a black hole.
  • General relativity predicts a point of infinite density at the centre; whether that is physical, or a sign the theory breaks down, is unknown.

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Sources and methodology

  • Inside a black hole's event horizon the speed needed to escape is greater than the speed of light, so nothing that crosses the horizon — not even light — can get back out. (awaiting scientific review)
  • A black hole is not really a hole: it is a huge concentration of matter packed into a very tiny space. (awaiting scientific review)
  • The radius of a black hole's event horizon (the Schwarzschild radius) is R = 2GM/c², the radius at which the escape velocity from a mass M equals the speed of light. (awaiting scientific review)
  • To become a black hole, an object with the Sun's mass would have to be squeezed into a sphere about 3 kilometres in radius. (awaiting scientific review)
  • A rocket must reach 11.2 km/s to escape Earth's gravity. (awaiting scientific review)
  • Black holes do not suck in matter like cosmic vacuum cleaners: from far enough away, their gravitational effects are just like those of any other object of the same mass. (awaiting scientific review)
  • Stellar-mass black holes have about 5 to 20 times the Sun's mass and supermassive black holes millions to billions of times; evidence for an in-between class of intermediate-mass black holes has only begun to appear in recent years. (awaiting scientific review)
  • When the collapsing core of a massive star has more than about 3 times the Sun's mass, even neutrons cannot stop the collapse and a stellar-mass black hole forms. (awaiting scientific review)
  • A 2021 study refined the distance to the X-ray binary Cygnus X-1 with radio astrometry and, combined with optical data, found its black hole has 21.2 ± 2.2 times the Sun's mass. (awaiting scientific review)
  • Cygnus X-1, described as the first black hole ever detected, was discovered in 1964 by Geiger counters carried on a sub-orbital rocket. (awaiting scientific review)
  • In April 2019 the Event Horizon Telescope published the first image of a black hole's shadow, of the supermassive black hole in the galaxy M87, and derived its mass as (6.5 ± 0.7) × 10⁹ solar masses. (awaiting scientific review)
  • The black hole in M87 lies about 55 million light-years from Earth, and its event horizon is about 2.5 times smaller than the shadow it casts. (awaiting scientific review)
  • In May 2022 the Event Horizon Telescope published the first image of Sagittarius A*, the black hole at the centre of the Milky Way, which is four million times more massive than the Sun and about 27,000 light-years away. (awaiting scientific review)
  • By measuring the orbits of stars close to the Milky Way's centre over 30 years, teams led by Reinhard Genzel and Andrea Ghez concluded that the object there is most likely a supermassive black hole; the work shared the 2020 Nobel Prize in Physics. (awaiting scientific review)
  • On 14 September 2015 the two LIGO detectors made the first direct detection of gravitational waves, from the merger of two black holes of about 36 and 29 solar masses into one of about 62. (awaiting scientific review)
  • In the 2015 merger detected by LIGO, about 3 times the mass of the Sun was converted into gravitational waves in a fraction of a second. (awaiting scientific review)
  • Comparing the Event Horizon Telescope images of Sagittarius A* and M87* shows consistency with general relativity across black holes whose masses differ by more than three orders of magnitude. (awaiting scientific review)
  • General relativity predicts that the centre of a black hole is a point where matter is crushed to infinite density; whether this singularity is physical or only mathematical is unknown, and it may mark the limits of the theory. (awaiting scientific review)

Claims marked “awaiting scientific review” cite the sources listed but have not yet been signed off by a scientific reviewer.

Content status: published 1 October 2026.

  • Scientific review: this version has not yet been signed off by a scientific reviewer.
  • The Advanced explanation has not yet been reviewed for age suitability.