How to take a picture of a black hole

IT LOOKS LIKE a misshapen, golden doughnut. But the image above, released by astronomers on May 12th, is an important one. It is the first picture of Sagittarius A* (Sgr A*), the huge black hole at the centre of the Milky Way, the galaxy that contains Earth’s solar system. It is only the second image of a black hole ever taken, and the data analysis necessary to assemble it took more than 300 researchers over five years. How did they take a picture of a black hole?

Technically speaking, they did not. Black holes themselves cannot be seen: their gravitational fields are so strong that nothing can escape them—including light. That is why their edges are called event horizons, because, much like with normal horizons, seeing beyond them is impossible. But it is possible to see the gas which surrounds the hole, which is attracted by its mass and heated by its gravitational pull. Even that is easier said than done, as from Earth they appear rather tiny. The first picture of a black hole was completed in 2019. M87*, 55m light years from Earth in the Messier 87 galaxy, has a mass 6.6bn times greater than the Sun’s—but from Earth, it appears the same size as a coin on the surface of the Moon. Sgr A* is nearer—just 27,000 light years away—but much smaller. From Earth it looks like a doughnut on the Moon.