On Thursday morning, an international team of astrophysicists and other researchers released the world’s first image of the supermassive black hole at the center of our galaxy, 27,000 light-years from Earth.
The event marks “the dawn of a new era of black hole physics,” the team announced on Twitter.
More than 4 million times the mass of our sun, the black hole, named Sagittarius A* (or Sgr A*, pronounced “sadge ay star,” for short) is so dense and massive that no light can escape its gravity. The visible light in the new image comes from bright stars, gas, and other space matter swarming around the black hole itself.
The image is the first visual evidence of Sgr A*’s existence; the black hole was previously only indirectly confirmed by observations of stars in orbit around it. The image comes from the Event Horizon Telescope’s (EHT) international consortium. A group of more than 300 scientists helped with the effort to capture it, “compiling information from radio observatories around the world,” Bill Chappell reports for NPR. (The EHT, named for the event horizon, or “point of no return” from a black hole, consists of eight radio telescopes scattered across the globe.)
In 2019, the EHT team compiled an image of Pōwehi, a supermassive black hole 55 million light-years away in the galaxy Messier 87. Similar in appearance to Sgr A* but much more massive, Pōwehi is 6.5 billion times the mass of our sun.
The much smaller Sgr A* proved difficult for scientists to visually capture: It’s a “dynamic environment” full of burbling and gurgling, Feryal Özel, a professor of astronomy and physics at the University of Arizona, told NPR. And the necessity to look through both the Earth’s atmosphere, gas clouds, and interstellar dust near the Milky Way’s center added to the challenge, Özel said. “It took several years to refine our image and confirm what we had, but we prevailed," Özel told NPR. And, despite its meager distance from Earth compared to Pōwehi, Sgr A* is far enough away to appear “to us to have about the same size in the sky as a donut on the Moon,” the EHT team stated in a press release.
Hot gas whirling around Sgr A* at nearly the speed of light made capturing an image of this black hole considerably more difficult than Pōwehi, the EHT team said in the press release. “The brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT was observing it,” EHT scientist Chi-Kwan Chan said in the press release.
Images of Pōwehi captured by astronomers across the EHT were visually similar to one another, allowing scientists to compile a sharp composite image of Messier 87’s black hole. “That was not the case for Sgr A*,” the EHT team said in the press release. But despite these initial inconsistencies, the scientists were eventually able to compile an image of Sgr A*.
Now, having images of such differently sized black holes provides a better look into the behavior of swirling gas around supermassive black holes, a phenomenon that’s not yet fully understood, but thought to play a key role in how galaxies, like our own, came to be. "Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works," EHT scientist Keiichi Asada said in the press release.