Success: Astronomers have captured the first image of the gigantic black hole at the center of the Milky Way, the galaxy of which the Sun and Earth are part. An international team including astronomers from Radboud University and the University of Amsterdam presented the photo to the world on Thursday afternoon. The image was taken with eight radio telescopes located around the world, from Arizona to the South Pole.

With the same telescopes, astronomers also made the first ever photo of a black hole, three years ago. That was world news at the time. The black hole in that photo was M87* at the center of the elliptical galaxy Messier 87, 55 million light-years away. The black hole at the heart of the Milky Way, Sagittarius A* (Sgr A*), is more difficult to capture because it is more variable than M87*. The picture taken now of Sagittarius A* (sgr A*) is the second picture of a black hole.

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.

The two photos of the black holes side by side: M87* (left) and Sgr. A*.
Photo ESO

Four groups of black holes

Sgr A*, is about 27,000 light-years from Earth and has a mass about four million times that 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 every galaxy has such a massive black hole at its 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 for gravity, such as Einstein’s general theory of relativity,” says Jesse Daas, PhD student at Radboud University and involved in the project. “Around black holes you can see whether the theories that seem to be correct here also work under extremely wild conditions.”

Since even light cannot escape, it is not possible to observe a black hole itself. The cosmic behemoths proclaim their presence by influencing their environment. They attract stars and other material with their gravity. As a result, they are surrounded by a spinning disk of hot matter, which can be seen.

Sgr A* and M87* are the two largest black holes viewed from Earth. That makes them ideal candidates to photograph.

To distinguish M87* and SgrA*, astronomers need a telescope with an area the size of the entire Earth. The larger the telescope, the higher the resolution.

Earth rotation

To create a telescope the size of the Earth, astronomers made clever use of the Earth’s rotation. In 2015, they linked eight telescopes around the world. When those telescopes look at the same object at the same time, they act together like one large telescope due to the rotation of the earth.

In 2017, the astronomers focused the telescopes on M87* and Sgr A*. They collected more than a petabyte of data (or about a million gigabytes).

SgrA* was more difficult to photograph than M87*. “The black hole in the Milky Way is roughly 1,600 times brighter than M87*,” says Sera Markoff. She is 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 of Sgr A*, the boundary around a black hole beyond which even light can no longer escape, is estimated to be about six million kilometers. “That means matter orbits rapidly around it; it takes about four to thirty minutes. As a result, Sgr A* is variable and it is more difficult to compose a photo from it.” A round trip at M87 takes several weeks.

Sophisticated Tools

Now, five years after collecting the data, we have succeeded in extracting a photo. The astronomers developed sophisticated tools to deal with the gas movement around the black hole. About three hundred scientists worldwide worked on it.

The photo shows the glowing gas ring with a dark area, the so-called shadow, in the middle. The black hole is in that shadow.

The photo taken now resembles that of M87* from 2019 and corresponds well with how Einstein’s theory of relativity predicts what a black hole should look like.

“I’m not as emotional now as when we showed the first picture of the black hole from 2019, but this feels much more satisfying,” says Heino Falcke. He is professor of astronomy at Radboud University and involved in the project. “We’ve been brooding on the data for five years.”

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