Success: Astronomers have taken the first image of the giant black hole in the center of the Milky Way, the galaxy of which the Sun and Earth are a part. An international team of astronomers from Radboud University and the University of Amsterdam presented the image to the world on Thursday afternoon. The image was taken with eight radio telescopes located around the world, from Arizona to the South Pole.
Using the same telescopes, astronomers also took the first ever photo of a black hole three years ago. It was world news at the time. The black hole in the image was M87 * at the center of the elliptical galaxy Messier 87, 55 million light-years away. The black hole in the heart of the Milky Way, Sagittarius A * (Sgr A *), is harder to catch because it is more variable than M87 *. The picture now taken of Sagittarius A * (sgr A *) is the second picture of a black hole. It is reminiscent of M87 * from 2019 and corresponds well to how Einstein’s theory of relativity predicts what the black hole should look like.
Black holes are not holes. They are objects in space whose gravity is so strong that anything that comes too close disappears into them forever. Not even light can escape from it.
Four groups of black holes
Sgr A *, is about 27,000 light-years from Earth and has a mass about four million times the mass of the Sun. M87 * is about six and a half billion times more massive than the Sun. Both fall into the heaviest of the four groups distinguished by astronomers. Presumably, each galaxy has such a massive black hole in the center.
In the late 1990s, the first concrete steps were taken to be able to photograph black holes. “Black holes are the ultimate places to test theories of gravity, such as Einstein’s general theory of relativity,” said Jesse Daas, a PhD student at Radboud University and involved in the project. “Around black holes, one can see if the theories that seem to be true here also work under extremely wild conditions.”
Since even light cannot escape, it is not possible to observe a black hole itself. The cosmic giants proclaim their presence by influencing their surroundings. They attract stars and other material with their gravity. As a result, they are surrounded by a rotating disk of hot matter, which can be seen.
Sgr A * and M87 * are the two largest black holes seen from Earth. This makes them ideal candidates for photography.
To distinguish between M87 * and SgrA *, astronomers need a telescope with an area the size of the entire Earth. The larger the telescope, the higher the resolution.
To create an Earth-sized telescope, astronomers made smart use of the Earth’s rotation. In 2015, they connected eight telescopes around the world. When these telescopes look at the same object at the same time, they behave together like a large telescope due to the rotation of the earth.
In 2017, astronomers focused the telescopes on M87 * and Sgr A *. They collected more than a petabyte of data (or about a million gigabytes).
SgrA * was harder to photograph than M87 *. “The black hole in the Milky Way is about 1,600 times brighter than the M87 *,” says Sera Markoff. She is a professor of astronomy at the University of Amsterdam and involved in the project. “And the brighter the black hole, the smaller it is. The radius of the event horizon for Sgr A *, the boundary around a black hole, beyond which even light can no longer escape, is estimated to be about six million kilometers.” This means that fabric quickly revolves around it; it takes about four to thirty minutes. As a result, Sgr A * is variable, and it’s harder to compose a photo based on that. “A tour of the M87 takes several weeks.
Now, five years after collecting the data, we have managed to extract a photo. Astronomers developed sophisticated tools to deal with the gas movement around the black hole. About three hundred scientists around the world worked on it.
The picture shows the glowing gas ring with a dark area, the so-called shadow, in the middle. The black hole is in that shadow.
“I’m not as emotional now as when we showed the first picture of the black hole from 2019, but this one feels much more satisfying,” says Heino Falcke. He is a professor of astronomy at Radboud University and involved in the project. “We’ve been pondering the data for five years.”