What the image of the Milky Way's black hole really shows

Enlarge / To create the first image of the Milky Way's black hole, the Scientists have performed numerous simulations of the swirling envelope of plasma that surrounds it. Knowable Magazine (CC-BY-ND)

Black holes keep their secrets close at hand. They forever imprison everything that enters. Light itself cannot escape the hungry pull of a black hole.

It would therefore seem that a black hole is invisible and that it is impossible to photograph it. A great fanfare therefore accompanied the broadcast in 2019 of the first image of a black hole. Then, in the spring of 2022, astronomers unveiled another black hole photo, this time the one at the center of our own Milky Way.

The image shows an orange donut-shaped blob that looks remarkably similar to the previous image of the black hole at the center of the galaxy Messier 87. But the Milky Way's black hole, Sagittarius A*, is actually much smaller than the first and was harder to see, as you had to look through the hazy disk of our galaxy. So even though observations of our own black hole were conducted at the same time as those of M87, it took three more years to create the image. This required an international collaboration of hundreds of astronomers, engineers and computer scientists and the development of sophisticated computer algorithms to reconstruct the image from the raw data.

Enlarge / The new image of the black hole Sagittarius A* confirms and refines previous predictions about its size and orientation. The black hole's mass determines its size, or what scientists call its gravitational diameter. The point at which no light can escape from the black hole, called the event horizon, is determined by this mass and the spin of the black hole. The hot plasma moves around the massive object in the accretion disk, emitting radio waves. These radio waves are bent and distorted by gravity (by the "gravitational lensing" effect) to produce the image of the orange outer c...