Gammie group at Illinois Physics contributes to first-ever image of supermassive black hole at Milky Way's galactic center

A team of University of Illinois Urbana-Champaign researchers led by Physics and Astronomy Professor Charles Gammie is part of a large international collaboration that unveiled the first image of the supermassive black hole at the center of the Milky Way. This result provides evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which researchers think reside at the center of most galaxies.

A global team of researchers involved in the Event Horizon Telescope collaboration produced the image using observations from a worldwide network of radio telescopes. The results are published in six papers included in a special issue of The Astrophysical Journal Letters.

Scientists had previously seen stars orbiting around something invisible, compact and very massive at the center of the Milky Way. This observation strongly suggested that the object—known as Sagittarius A star—is a black hole, and the new image provides the first direct visual evidence.

Although we cannot see the black hole, the researchers said glowing gas around it reveals a telltale signature—a dark central region, called a shadow, surrounded by a bright ringlike structure. The new view captures light bent by the powerful gravity of the black hole, which is 4 million times more massive than our sun.

“We were stunned by how well the ring size agreed with predictions from Einstein’s theory of general relativity,” said EHT scientist Geoffrey Bower, of Academia Sinica, Taipei. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”

Because the black hole is about 27,000 light-years away, it appears to observers on Earth to be about the same size as a doughnut on the moon. To image the black hole, the team created the EHT, which links together eight existing radio observatories across the planet to form a single Earth-sized virtual telescope. The EHT observed the Sgr A star on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.

The new image follows the EHT collaboration’s 2019 release of the first image of a black hole, called the M87 star, at the center of the more distant Messier 87 galaxy.

The two black holes look remarkably similar, even though our galaxy’s black hole’s radius and mass are more than a thousand times smaller than those of the M87 star. “We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes, they look amazingly similar,” said Sera Markoff, the co-chair of the EHT science council and a professor at the University of Amsterdam. “This tells us that general relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”

This achievement was considerably more difficult than for the M87 star, even though the Sgr A star is much closer to us. “The gas in the vicinity of the black holes moves at the same speed—nearly as fast as light—around both Sgr A star and M87 star,” said EHT scientist Chi-kwan “CK” Chan, of the University of Arizona. “But where gas takes days to weeks to orbit the larger M87 star, in the much smaller Sgr A star, it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A star was changing rapidly as the EHT Collaboration was observing it—a bit like trying to take a clear picture of a puppy quickly chasing its tail.”