Sagittarius A* is 4 million times as massive as the sun and some 26000 light-years from Earth
Animations zoom in on the black hole at the centre of our galaxy. Video: ESO/L. Calçada, N. Risinger (skysurvey.org), DSS, VISTA, VVV Survey/D. Minniti DSS, Nogueras-Lara et al., Schoedel, NACO, GRAVITY Collaboration, EHT Collaboration
By guest author Aylin Woodward from the Wall Street Journal
At the heart of our galaxy, about 26,000 light-years from Earth, lies a black hole 4 million times as massive as the sun. For the first time, scientists have captured an image of this cosmic body at the Milky Way’s center—a region of space so dense that nothing, not even light, can break free of its gravitational pull.
The image, released Thursday during simultaneous press conferences hosted by various institutions in seven countries, reveals the black hole, named Sagittarius A*, as a dark center silhouetted against a bright orange-yellow ring. Black holes by their very nature are unseeable, but their boundaries cast a shadow against the bright backdrop of hot gas and dust that encircles them. That shadow is what’s visible to certain telescopes on Earth.
“Seeing is believing, and this moment of finally seeing something that was always just a fantasy or an idea or model, it’s just such an amazing, captivating moment,” said Heino Falcke, a radio astronomy professor at Radboud University Nijmegen in the Netherlands.
Observations from the Event Horizon Telescope, or EHT, helped scientists get the image. Operated by an international collaboration of researchers, the EHT consists of 11 ground-based radio telescopes scattered across the globe from Hawaii to Greenland that are linked into a single, Earth-sized array. That array can pick up radio waves given off by the gas and dust moving near a black hole’s event horizon—the theoretical boundary beyond which light and matter can’t escape.
As the Earth rotates, each telescope gives scientists a glimpse of the same target from different angles. Then, supercomputers in Europe and the U.S. mathematically combine data from those observations into a single image.
“Imaging Sagittarius A* took an entire planet,” said Dr. Vincent Fish, a research scientist at MIT Haystack Observatory who helped announce the image at a Thursday press conference in Washington.
From its inception more than a decade ago, the EHT had two targets in mind: Sagittarius A* in the Milky Way’s center, and another supermassive black hole known as Messier 87, or M87, located at the center of a galaxy 55 million light-years away. Those were the only two black holes near enough and massive enough to be observed from Earth, said Feryal Özel, a University of Arizona astrophysicist who is part of the EHT collaboration and helped reveal the image Thursday, May 12, 2022.
“Back as early as 2000, we had identified these two black holes as the ideal targets for an imaging experiment,” Dr. Özel said. “They are both big enough in the sky that a global array could resolve the features that we were interested in.”
The EHT revealed its image of M87—the first-ever image of a black hole—in April 2019. Both that image and the image revealed Thursday used data collected in 2017, Dr. Özel said.
Geraint Lewis, an astrophysics professor at the Sydney Institute of Astronomy who wasn’t involved in the collaboration, said he wasn’t surprised that the EHT captured the image of M87 before that of Sagittarius A*, even though the latter was originally the collaboration’s primary target.
“It is quite active and so bright,” Dr. Lewis said of M87, which is 6.5 billion times as massive as the sun. “Sagittarius A* is relatively quiescent, and so M87 made a better target.”
M87 is 1,500 times bigger than Sagittarius A*; the latter’s smaller size is one of the reasons EHT chose to image it second, Dr. Özel said.
The smaller mass of Sagittarius A* means gas in the black hole’s vicinity takes less time to swirl around it than gas nearby the larger M87. “Things are changing around M87 on an order of days, but around Sagittarius A*, it’s minutes to hours,” Dr. Özel said. The researchers were worried that utilizing observations of a target changing that quickly might complicate the image-generation process, so they honed their methodology with M87 first.
Dr. Özel said EHT researchers hope to understand how the environment around these two black holes changes from year to year and between observations. While bad weather and the Covid-19 pandemic scuttled good data collection between 2019 and 2021, the collaboration has data from 2018 that its researchers are currently analyzing.
She said whether those efforts result in a video of a black hole remains to be seen, though according to Dr. Falcke—who is also involved with the EHT—that is the collaboration’s main goal in the future.
The Sagittarius A* image marks only the second direct evidence of the existence of black holes. Prior to 2019, scientists could only collect indirect evidence of a black hole by measuring the impact of its gravity on nearby objects, or detecting the gravitational waves emanated by it.
The EHT plans to add more telescopes to their global network, Dr. Falcke said. These additions will help improve the quality of the collaboration’s images of M87 and Sagittarius A*—the more telescopes observing the same object, the better the resulting image, Dr. Özel said.
But to catch a glimpse of other black holes that are either less massive or more distant than these two, “we’re going to need to put some radio telescopes in orbits around the Earth,” she said. Moving observations into space means researchers would be able to put larger distances between individual telescopes, and ultimately create an array that can resolve objects that are smaller and farther away.
“The size of our virtual telescope is actually limited by the size of the Earth,” Dr. Falcke said.