An international astronomy team on Thursday gave the world its first glimpse of the supermassive black hole at the center of our Milky Way galaxy — a glowing ring of light with three bright spots and a mass that is 4 million times that of the Sun.
The image of the previously unseeable black hole was captured by the Event Horizon Telescope, an international organization of more than 200 researchers and engineers linking eight major radio observatories on four continents into one virtual, Earth-sized telescope with a profound degree of angular resolution.
Scientists say the colorized image is "the first direct visual evidence" of the black hole known as Sagittarius A* at the heart of our galaxy about 27,000 light years from Earth — one light year equals 9.5 trillion kilometers (5.9 trillion miles). Their work was published in a special issue of The Astrophysical Journal Letters.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity," said EHT's project scientist Geoffrey Bower, a research scientist with the Academia Sinica Institute of Astronomy and Astrophysics, or ASIAA.
"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," he said.
In 2019, the same organization saw and photographed a supermassive black hole for the first time. That black hole has a mass 6.5 billion times greater than the Sun. It was found at the heart of Messier 87, a massive galaxy in the Virgo galaxy cluster, about 55 million light years away.
The centers of all galaxies are thought to have supermassive black holes, places that are difficult to photograph because they are completely dark and light and matter cannot escape.
The one in our galaxy "appears to us to have about the same size in the sky as a donut on the Moon," EHT said, and was observed by capturing images of it for hours at a time on multiple nights "similar to using a long exposure time on a camera."
Compare and contrast
The black holes in our galaxy and in Messier 87 look similar even though their masses are different and the galaxies differ. "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," said Sera Markoff, a theoretical astrophysics professor at the University of Amsterdam who co-chairs EHT's Science Council.
Albert Einstein’s 1915 theory of general relativity, a major tenet of modern physics, explains the force of gravity as the result of massive objects changing or curving the geometry of space-time. It expanded on his theory of special relativity a decade earlier that argued space and time are inextricably linked.
Even though the black hole in our galaxy is much closer to Earth, getting an image of it was much harder to do than gaining a photograph of the one in Messier 87, according to Chi-kwan Chan, an EHT scientist and associate research professor at the University of Arizona.
That is because the gas in the vicinity of the black holes moves at the same speed — almost as fast as light — but in the much smaller one in our galaxy, it takes only minutes to orbit as opposed to days or weeks in the one in Messier 87, he said. As a result, the brightness and pattern of the gas around the black hole in our galaxy changed rapidly while EHT observed it, he added, and was a bit like “trying to take a clear picture of a puppy quickly chasing its tail.”
Having images of two black holes from the different galaxies also gives scientists an exciting opportunity to compare and contrast, according to EHT.
“We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before," said Keiichi Asada, and EHT scientist and associate research fellow at ASIAA.