Topics · Space Codex
What is a black hole?
A black hole forms when matter is crushed into a region where spacetime curves so steeply that nothing, including light, can climb out past the event horizon.
Gravity so strong light cannot escape
At a glance
Key points
Stellar black holes are born from collapsing massive stars; supermassive ones sit in galaxy centres, including Sagittarius A* in the Milky Way.
- Stable orbits exist outside the horizon: black holes are not cosmic vacuum cleaners
- Gas spiralling in heats until it glows in X-rays
- LIGO, Virgo and KAGRA hear black holes merge as ripples in spacetime
How they form
When a star of roughly 20 or more times the Sun's mass runs out of fuel, its core collapses and can leave a black hole of a few to tens of solar masses. Supermassive black holes, from hundreds of thousands to billions of Suns, sit at the centres of large galaxies; how they grew so big so early in cosmic history is still debated. Intermediate-mass black holes in between are expected, but only a handful of candidates exist.
A sense of scale
The event horizon grows in step with mass, at about 3 km of radius per solar mass. A black hole with the Sun's mass would be about 6 km across. Sagittarius A*, at 4.3 million Suns, is about 25 million km across, which would fit well inside Mercury's orbit. M87*, at 6.5 billion Suns, is just under 40 billion km across, several times wider than Neptune's orbit.
Common misconceptions
Black holes do not suck in everything nearby. Far from the horizon, an object orbits a black hole exactly as it would orbit a star of the same mass: if the Sun were swapped for a one-solar-mass black hole, the planets would keep their orbits, though Earth would freeze. Nor do the Event Horizon Telescope pictures show the hole itself. They show glowing gas bent into a ring around a dark shadow about 2.6 times wider than the horizon.
How we know they exist
No single observation carries the case; several independent ones agree. In X-ray binaries, gas torn from a companion star heats to millions of degrees as it spirals in. Decades of tracking stars such as S2 weighed Sagittarius A*. LIGO first detected gravitational waves from two merging black holes on 14 September 2015. And the Event Horizon Telescope imaged the shadows of M87* in 2019 and Sagittarius A* in 2022.
Related entries
- Black holeEvent horizon · gravity light cannot escape
- Sagittarius A*The black hole at the centre of the Milky Way
- M87*The first black hole ever imaged
- Gravitational wavesRipples in spacetime
Sources
- LIGO: GW150914, the first detection (opens in a new tab)
- ESO: eso1907: first image of a black hole (opens in a new tab)
- ESO: eso2208: first image of Sgr A* (opens in a new tab)
- LIGO: O4b catalog (GWTC-5.0) (opens in a new tab)
- James, von Tunzelmann, Franklin & Thorne 2015, CQG 32:065001 (arXiv) (opens in a new tab)
- NASA Science: Black hole types (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.