What is a black hole?

A place even light cannot escape

Every body has an 'escape velocity': to leave Earth you need 11 km per second. The heavier and denser the body, the higher it is. If matter is squeezed so tightly that the escape velocity exceeds the speed of light (300,000 km/s), nothing — not even light — can get out. That is a black hole.

How are black holes born? When a star 20 times heavier than the Sun runs out of fuel, it collapses under its own weight: the core shrinks to the size of a city in a second, while the outer layers explode as a supernova. At the centres of galaxies sit 'supermassive' black holes of millions of solar masses — ours is Sagittarius A*, 4 million Suns.

You cannot see a black hole itself, but you can see its effects: gas around it spirals in, heats up and glows as a bright disc; stars orbit an 'invisible' point. In 2019 the Event Horizon Telescope — a network of telescopes the size of Earth — took the first picture of the 'shadow' of the black hole at the centre of galaxy M87.

Should we be afraid? No: the nearest known black hole is 1,500 light-years away, and the Sun will never become one — it is far too light. Black holes are not 'cosmic vacuum cleaners': if the Sun were replaced by a black hole of the same mass, Earth would keep orbiting exactly as it does now — it would just be dark and cold.