Science
What is a black hole?
Quick answer
A black hole is a place in space where gravity is so strong that nothing, not even light, can escape. Most form when a very massive star collapses at the end of its life.
In one sentence
A black hole is a region of space where gravity is so strong that nothing, not even light, can get out.
Explained
Gravity is the pull between things that have mass. The more mass is packed into a small space, the stronger the pull close by. A black hole is mass squeezed into an incredibly tiny space. Its boundary is called the event horizon. Cross it and there is no way back, because to escape you would need to travel faster than light, and nothing can.
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Most black holes begin when a star much more massive than our Sun uses up its fuel. It can no longer push outwards against gravity, so its core collapses. These are called stellar-mass black holes. Much bigger ones, supermassive black holes, sit at the centre of most large galaxies. Ours, called Sagittarius A*, has about four million times the mass of the Sun.
A black hole is not a hole, and it is not a cosmic vacuum cleaner. Far enough away, it pulls on things the same way any object of that mass would. If the Sun were replaced by a black hole with the same mass, Earth would keep orbiting as before, just in the dark. (The Sun is far too small to become a black hole.)
Because no light comes out, we find black holes by their effects. Stars near the centre of our galaxy orbit something invisible and heavy. Gas that falls towards a black hole glows brightly as it heats up. In 2019, a team of astronomers published the first picture of a black hole's shadow, in a galaxy called M87, and in 2022 they showed one of our own galaxy's. Read about the objects that make black holes in stars, and about the force behind them in gravity.
Examples
| Type | Size | Where |
|---|---|---|
| Stellar-mass | a few to dozens of Suns | scattered through galaxies |
| Supermassive | millions or billions of Suns | centres of large galaxies |
| Sagittarius A* | about 4 million Suns | the centre of the Milky Way |
Common mistakes
- "Black holes suck everything in." Only things that get very close are caught.
- "A black hole is empty space." It contains a huge amount of mass.
- "We have seen inside one." Nobody knows what happens at the very centre, because our physics cannot yet describe it.
Quick quiz
Black holes
Check what you know with 4 quick questions.
Question 1 of 4
What can escape from inside a black hole?
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