Some 27,000 light-years away lies a massive astrophysical object, some four million times the mass of our sun, surrounded by swirling super-hot gases. The existence of this supermassive black hole called Sagittarius A* has been theorized for decades when astronomers observed nearby stars orbiting something invisible, compact and very massive at the center of the Milky Way. But they have never seen what it is, so far.
At a press conference today, astronomers released the first image of this supermassive black hole: an orange and yellow donut with a dark center. “We finally got a glimpse of our own black hole,” says Angelo Ricarte, an EHT collaborator and an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, in an interview. “A better understanding of this black hole at the center of our galaxy will help us understand our cosmic origin story.”
The announcement represents the work of more than 300 researchers at 80 institutions, including the Smithsonian Astrophysical Observatory, around the world who turned a network of telescopes into a planet-sized observatory known as the Event Horizon Telescope (EHT).
“Our telescope has to be almost as big as Earth,” Vincent Fish, an astronomer at the MIT Haystack Observatory and a collaborator on the EHT, said at the event. To do that, the team linked more than half a dozen telescopes around the world using a technique called interferometry. “By correlating their signals and studying the resulting data, we can reconstruct images of the source. The more telescopes, the better.”
The image provides further insight into the mysteries of black holes and further confirms Einstein’s theory of relativity. Despite its name, the supermassive black hole at the center of our galaxy is quite small in the night sky. From Earth, “it’s like looking for a tennis ball on the Moon,” says University of Central Florida cosmologist James Cooney, who is not affiliated with the recent announcement.
Observations by the EHT telescope network generated 3.5 petabytes of data, the equivalent of 100 million TikTok videos, which were combined using a sophisticated computer algorithm. The data was so large that it would have taken years to transmit over the Internet, so the team sent hard drives to different locations around the world for analysis.
Since black holes gobble up everything around them, including light, these images are not what we might think of as traditional photographs. Instead, they are images of the black hole’s shadow. As the superheated material spins around the black hole, it glows and lights up. The supermassive black hole casts a shadow on this glowing, engulfed gas.
EHT scientists used a similar technique to create the first images of the event horizon of an M87* black hole, published in 2019. But the supermassive black hole at the center of the Milky Way is quite different; it is much smaller and the gas swirls around it much faster. Capturing the image was “a bit like trying to get a clear picture of a puppy quickly chasing its tail,” said EHT scientist Chi-kwan Chan of the University of Arizona.
The team faced other challenges, such as observing the black hole through Earth’s atmosphere and the turbulent gas in our galaxy. EHT researchers collected tens of thousands of images and analyzed them. “By averaging these images together, we can emphasize the common features that appear in most of them, and a bright ring clearly appears here,” said Katie Bouman of Caltech’s EHT, who credits the power of computational imaging in helping to overcome those obstacles.
The images of Sagittarius A* along with earlier images of the black hole M87* give scientists more data to study black holes. M87* is much farther from Earth and more than 1,000 times larger than Sagittarius A*, giving scientists an opportunity to compare the two. They can take a closer look at how gases around black holes behave and gain insight into how gravity behaves in extreme environments, such as those near black holes.
The observations also give scientists the opportunity to test Einstein’s theory of relativity. Both images of the two black holes look alike because they are a consequence of gravity. According to Einstein, space-time, or the fabric of our universe, transforms around black holes in exactly the same way, regardless of their mass, and he can predict the size of the shadow cast. The images of two black holes, despite the differences in size, appear almost identical.
“This is what we expected to find given the predictions of Einstein’s general theory of relativity,” said Feryal Özel, an EHT scientist at the University of Arizona, at the news conference. “It is precisely because of this fact that we can use these new observations, the image of [Sagittarius A*] to perform one of the strongest tests of general relativity to date.”
The image of the black hole at the center of our galaxy is not the last observation planned by the EHT team. The researchers began a major observing campaign in March 2022, which included more telescopes added to the array, with the goal of generating higher resolution images of black hole event horizons, even movies. And because the EHT is limited in size by our own planet, some astronomers hope the project will head into space for even more impressive images of black holes.