A black hole is a region of space where matter is packed so tightly that its gravity lets nothing escape, not even light. Its boundary is called the event horizon: anything that crosses it, a gas cloud, a star or a beam of light, can never come back out. Inside, as far as Einstein’s general relativity can tell, everything is crushed towards a point of infinite density.
From an equation to the sky
The idea began on paper. In 1916, only months after Albert Einstein published general relativity, the German physicist Karl Schwarzschild found an exact solution of its equations describing the space around a single mass. It contained a boundary, now called the Schwarzschild radius, inside which nothing could escape. For a mass like the Sun’s, that radius is only about 3 kilometres; for the Earth, under a centimetre. For decades most physicists thought nature would never squeeze matter that far.
The name “black hole” appeared in print in the 1960s and was popularised by the American physicist John Wheeler from 1967. By then, astronomers had a candidate. Cygnus X-1, a powerful X-ray source discovered in 1964, turned out to be a star orbiting an invisible companion far too heavy to be a neutron star. By the end of 1973 most astronomers accepted that it was a black hole.
How they form
Most black holes form when very massive stars die. When the core of such a star runs out of fuel, nothing can hold it up and it collapses under its own weight, usually in a supernova. These stellar black holes typically have between a few and several tens of times the Sun’s mass. The Milky Way may contain about 100 million of them, most of them invisible because they have nothing nearby to feed on. The nearest known, Gaia BH1, is about 1 560 light-years away.
Far larger supermassive black holes, millions or billions of times the Sun’s mass, sit at the centres of galaxies.
Seeing the invisible
A black hole gives off no light of its own, but its surroundings do. Gas falling towards one heats up and glows, and stars orbiting one reveal its mass. In 1974 Stephen Hawking showed that, because of quantum effects, black holes should also emit a faint radiation and very slowly evaporate.
On 14 September 2015 the LIGO detectors caught gravitational waves, ripples in space itself, from two black holes of about 36 and 29 solar masses merging 1.4 billion light-years away. In April 2019 the Event Horizon Telescope released the first image of a black hole’s shadow, in the galaxy M87, followed in 2022 by the black hole at the centre of our own galaxy. In 2020 Roger Penrose, Reinhard Genzel and Andrea Ghez shared the Nobel Prize in Physics for work on black holes.