A black hole is a place in space where gravity pulls so strongly that nothing, not even light, can escape. Because no light comes out, black holes are invisible, but we can detect them by their effects on nearby stars and gas.
How do black holes form?
Most black holes form when a very massive star, many times heavier than our Sun, runs out of fuel. Its core collapses under its own gravity, squeezing a huge amount of mass into a tiny space. The outer layers may blast away in a supernova explosion.
Parts of a black hole
- Event horizon: the boundary of no return. Anything that crosses it cannot escape.
- Singularity: the centre, where scientists believe mass is crushed to a point of infinite density. Our current laws of physics cannot fully describe it.
- Accretion disk: a swirling ring of gas and dust that heats up and glows brightly as it spirals in.
Types of black holes
- Stellar black holes: a few to tens of times the mass of the Sun.
- Supermassive black holes: millions to billions of times the Sun’s mass, found at the centres of most large galaxies. Our Milky Way has one called Sagittarius A*.
The first photograph
In 2019, the Event Horizon Telescope, a network of radio telescopes around the world working as one Earth-sized telescope, released the first image of a black hole’s shadow, in the galaxy M87. In 2022 it released an image of Sagittarius A*.
Would Earth be sucked into a black hole?
No. A black hole only has extreme gravity close to it. If the Sun were replaced by a black hole of the same mass, Earth would keep orbiting just as it does now, though it would be very cold and dark.
Famous connections
Albert Einstein’s general theory of relativity predicted black holes, and Stephen Hawking showed that black holes may slowly lose energy through what is now called Hawking radiation.
