About 27,000 light-years away sits a massive astrophysical object, some four million times the mass of our sun, surrounded by swirling super-hot gasses. The existence of this supermassive blackhole called Sagittarius A* has been theorized for decades as 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—until now.
In a press conference today, astronomers unveiled the first image of this supermassive black hole -—an orange and yellow donut with a dark center. “We finally got a glimpse at our very own black hole,” says Angelo Ricarte, an EHT collaborator and astrophysicist at the Center for Astrophysics, Harvard & Smithsonian, 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 that 300 researchers at 80 institutions, including the Smithsonian Astrophysical Observatory, across the globe 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 the Earth,” said Vincent Fish, an astronomer at the MIT Haystack Observatory and EHT collaborator, at the event. To do that, the team connected more than half a dozen telescopes around the globe 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 gives greater insight into the mysteries of black holes and further confirms Einstein’s long-standing 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.
The observations of 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 massive it would have taken years to transmit via the internet, so the team shipped hard drives to different locations around the world for analysis.
Since black holes gobble up everything around them including light, these images aren’t what we might think of as traditional photographs. Instead, they are images of the shadow of the black hole. As the superheated material swirls around the black hole, it glows and illuminates. The supermassive black hole casts a shadow on this glowing, gobbled-up gas.
EHT scientists used a similar technique to create the first images of a black hole M87*’s event horizon, released in 2019. But the supermassive black hole at the center of the Milky Way is quite different; it’s much smaller, and the gas swirls around it far more rapidly. Capturing the image was “a bit like trying to take a clear picture of a puppy quickly chasing its tail,” said EHT scientist Chi-kwan Chan from the University of Arizona.
The team was faced with other challenges, like observing the black hole through Earth’s atmosphere and the turbulent gas of our galaxy. Researchers at the EHT collected tens of thousands of images and analyzed them. “By averaging these images together, we're able to emphasize the common features appearing in most of them, and here a bright ring clearly pops out,” said EHT’s Katie Bouman, of Caltech, who credits the power of computational imaging to help overcome those obstacles.
The images of Sagittarius A* along with previous images of the black hole M87* give scientists more data to study black holes. M87* is much farther away from Earth and more than 1,000 times bigger than Sagittarius A*, which gives scientists the opportunities to compare the two. They can take a closer look at how gasses behave around black holes and gain insight into how gravity behaves in extreme environments like ones near around black holes.
The observations also give scientists the opportunity to test Einstein’s theory of relativity. Both images of the two black holes look similar because they are the consequence of gravity. According to Einstein, spacetime—or the fabric of our universe—morphs around black holes the exact same way, regardless of their mass, and can predict the size of the shadow cast. The images of two black holes, despite the differences in size, look nearly identical.
“This is what we had hoped to find given the predictions of Einstein's theory of general relativity, said Feryal Özel, an EHT scientist from the University of Arizona, at the press conference. “It is exactly because of this fact that we are able to 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 isn’t the last observation planned by the EHT team. Researchers began a major observation campaign in March 2022, which included more telescopes added to the array, aimed at 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 are hoping the project heads to space for even more impressive images of black holes.