The first image of a black hole

The story

A star so heavy

A black hole is one of the strangest objects in the universe. It is usually born when a star much bigger than the Sun runs out of fuel and collapses in on itself: all its matter is squeezed into a tiny space. Gravity, the force that pulls things together, becomes so strong that nothing, not even light, can escape any more. That is why you cannot see it: it is “black”. Its edge is called the “event horizon”: anything that crosses it can never come back.

The idea goes back to Einstein. In 1915 he published the theory of general relativity, according to which gravity is a curving of space and time. A few months later the German astronomer Karl Schwarzschild, who was at the front during the First World War, found a solution to his equations that described exactly such an object. Einstein himself, however, did not believe it could really exist. The name “black hole” was made famous in 1967 by the American physicist John Wheeler.

Indirect evidence

For decades black holes remained a hypothesis. Then astronomers began to find traces of them: stars orbiting an invisible point, gas falling into a whirlpool and becoming extremely hot, giving off X-rays, and even, in 2015, gravitational waves, tiny vibrations of space caused by the merging of two black holes, picked up by the LIGO instruments. But one thing was missing: seeing one.

A telescope as big as the Earth

Photographing a black hole is an enormous task. Even the biggest ones are very far away and, seen from the Earth, look tiny, like trying to see an orange on the Moon. To get such a sharp image you would need a telescope as big as our planet. Scientists had an idea: to link radio telescopes in places very far apart, and make them all work together as a single giant telescope. This is how the Event Horizon Telescope was born.

In April 2017 eight radio telescopes, in Chile, Hawaii, Arizona, Mexico, Spain and at the South Pole, pointed for a few days at the same target: the centre of a big galaxy called M87, about 55 million light years from the Earth. The telescopes recorded enormous amounts of data, about five petabytes, so much that it could not be sent over the Internet: it was loaded onto hundreds of disks and flown by plane to two computing centres, in the United States and in Germany. For two years more than two hundred scientists from many countries worked to reconstruct the image: among them was a young computer scientist, Katie Bouman, who became famous when a photograph of her next to the disks went around the web, although it was teamwork, to which many people contributed.

The image

On 10 April 2019, at press conferences in Brussels, Washington, Tokyo, Santiago and other cities, the first photograph of a black hole was shown: an orange ring of blurred light around a black patch in the middle. The ring is gas swirling at enormous speed around the black hole and heating up; the dark area is its shadow. The black hole in M87 weighs about six and a half billion times as much as the Sun, and its horizon is bigger than the whole Solar System.

In 2022 the same team published a photograph of another black hole, the one at the centre of our own galaxy, the Milky Way, called Sagittarius A*, about 27,000 light years away and with a mass of about four million Suns. Here too, a bright ring around a dark centre.