The Event Horizon Collaboration, the group behind the first-ever image of a black hole, just achieved another huge accomplishment: an image of Sagittarius A*, the black hole at the center of the Milky Way. This is a breakthrough in our understanding of our home galaxy and the astrophysics of its center.
The Event Horizon Telescope (EHT) is a consortium of radio telescopes around the world. By combining the observations of these telescopes, the team is able to see black holes, regions of spacetime with such intense gravity that not even light can escape them. The image released today shows the shadow of Sagittarius A* (pronounced “A star”) as well as the high-energy region around it.
The collaboration of hundreds of scientists is specifically investigating the structure and immediate environments of two supermassive black holes called Messier 87 and Sagittarius A*. M87 is 54 million light-years from Earth and was imaged by the collaboration in 2019, a historic feat in astrophysics. Sagittarius A* is the 4-million-solar-mass black hole at the center of our own Milky Way just 27,000 light-years away and is the subject of the research announced today.
The Milky Way’s center was first located 104 years ago; just over a decade later, the radio waves emanating from the galaxy’s core were discovered, and it took a half century for Sagittarius A* to be identified. But today’s image is a gravitational shift toward confirming that Sagittarius A* is indeed a supermassive black hole, as we’ve assumed, as opposed to some other bright radio source.
The EHT relies on a technique called Very Long Baseline Interferometry, which uses the differences in the time it takes light from a source to reach each telescope in a group to make precise observations of those sources. Black holes like the one at the center of the Milky Way are very strong sources of radio waves, making them fantastic targets for radio telescopes like the ones that make up the EHT.
The gas around the black hole swirls at ridiculous speeds, making the object a tricky subject to image. Getting this composite image was like getting a “clear picture of a running child at night,” said José L. Gómez, the VLBI Group Leader and an astrophysicist at the Instituto de Astrofísica de Andalucía in Spain, at the press conference. “You can imagine how crazy it drove us for many years.”
“Our conclusion indicates that Einstein’s theory is still holding strong… now we have the most compelling evidence to date that the supermassive compact object at the center of our galaxy is a black hole,” said Mariafelicia de Laurentis, an astrophysicist at University of Naples “Federico II” in Italy, during a press conference Thursday morning. “These environments offer us the unique opportunity to data-mine where and how Einstein’s theory breaks down, and if it does, it will transform our understanding of gravity and the properties of space and time.”
If general relativity had broken down, the image would have looked slightly different. According to Maciek Wielgus, an astrophysicist at the Max Planck Institute for Radio Astronomy, the ring structure we see may have been double in size, or its shape may have been different. But as evidenced by the landmark image, the century-old theory stands firm. “Personally, I’m not betting against Einstein,” Wielgus told Gizmodo in a video call. “It seems very difficult to win this bet.”
Wielgus added that Sagittarius A*’s orientation was a bit of a surprise. When we look toward the Milky Way center, we are seeing through dust, gas, and millions of stars. Rather than the black hole’s axis lining up with the disc, it’s pointing directly at us. That’s why if Sagittarius A* has a massive jet of material spewing out of it (like M87 does), we cannot see it. It would be spewing directly toward us, rather than at some angle that makes it apparent.
Another takeaway was the accretion rate—the appetite of the black hole. The collaboration found that Sag A* is basically starving. “If you had the same diet as Sagittarius A* scaled to your mass, you’d eat one grain of rice every million years,” said Sara Issaoun, the NASA Einstein Fellow at the Harvard & Smithsonian Center for Astrophysics.
“Black holes are either fasting or feasting,” said Priya Natarajan, an astrophysicist at Yale University, in a phone call with Gizmodo. Sagittarius A* happens to be fasting (though it’s still accreting material), and as the recent data revealed, its consumption is hardly constant. “One of the things that emerged today was that the time scale of matter flowing in onto these black holes is widely variant,” Natarajan said. “These fluctuations in the image capture that variation.”
Natarajan is working on the Next Generation EHT, which will increase the number of telescopes in the EHT array and seek out black holes farther afield that may be accessible to us. Eventually, she said, dynamic movies of black holes and the intense physics around them could be observed in detail.
Wielgus said that observing our black hole at higher frequencies could improve the resolution of these images, which are admittedly a bit fuzzy. At higher frequencies, the images would be 1.5 times sharper and would be less disturbed by the vast amount of matter between us and Sagittarius A*, but that could be years in the making. It’s only been three years since our first image of a black hole, so EHT is moving at a clip.
“My worry is if [general relativity] breaks down only beyond the event horizon, it is hidden from us,” Wielgus said. Meaning that if some key to a unified theory does exist beyond the event horizons of black holes, we’ll never know it. For now, at least, you can bask in the awesome glow of our galactic core and ponder what it all means.