Three years ago, scientists used an Earth-sized telescope to produce the first-ever image of a black hole. Now they’ve done it again – this time, closer to home, and from a very different invisible juggernaut.
The Event Horizon Telescope (EHT) has now produced images of two supermassive black holes: the one at the center of a galaxy called M87 and the one at the center of our own galaxy, the Milky Way. In the process, the hundreds of scientists involved in the effort got their first glimpses of two objects with surprisingly little in common.
“The two images look very similar to us when we look at them in the sky,” said Feryal Özel, an astrophysicist at the University of Arizona and a member of the scientific council of the EHT, during a press conference held on Thursday. by the National Science Foundation. (May 12) to unveil the new image of Sagittarius A* in the Milky Way. “But the two black holes couldn’t have been more different from each other in virtually every way.”
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In particular, the two black holes differ in the difficulty of imaging matter moving around its boundary, or event horizon. (This material, which glows when consumed, is what illuminates the fuzzy rings that the EHT has worked to produce, since black holes themselves emit no light.)
Noticeably, the EHT attacked the less difficult target first. This is the monster that hides in M87, also known as M87*. This black hole is farther from Earth, of course, but it’s also much larger, and matter is moving around its event horizon at a more leisurely rate.
“Matter swirls around M87* for several days,” Vincent Fish, an astrophysicist at the Massachusetts Institute of Technology’s Haystack Observatory and EHT science board member, said at the press conference. “He stays still for his photograph.”
The Milky Way’s supermassive black hole, dubbed Sagittarius A*, is less cooperative. “It only takes a few minutes for the material to move near the Sagittarius A* horizon because it’s much smaller,” Fish added. “It changes quickly, so we had to collect our data quickly.”
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And the Sagittarius A* challenge was evident when the scientists also analyzed the data collected by the EHT. Katie Bouman, a computer scientist at the California Institute of Technology and co-lead of the EHT Imaging Task Force, said at the press conference that when the independent imaging teams trained to analyze the M87* results compared their first takes on a picture, they were more or less the same.
This is not the case for the second black hole. “The Sagittarius A* imagery was a bit more complicated story than the M87* imagery,” Bouman said. This time, the imaging groups were reluctant to produce an initial image because there was much less consensus among team members.
Although the material surrounding Sagittarius A* is moving around the event horizon at an inconvenient speed, our supermassive black hole nonetheless offers a much more docile environment near its surface than M87*. The turbulence in this region is determined by the black hole’s appetite, which varies despite the popular conception that all black holes gorge themselves on the suction of a cosmic vacuum cleaner.
“If Sagittarius A* were a person, it would consume a single grain of rice every million years,” said Michael Johnson, an astrophysicist at the Harvard/Smithsonian Center for Astrophysics and member of the EHT science board, during the the press conference. “Only a trickle of matter actually makes it to the black hole.”
Growing at this slow pace, Sagittarius A* is quite a picky eater. “Because of this, its environment is relatively mild,” Özel said. “When we say that, of course, the temperatures and magnetic field strengths are still quite high, and the movement of gas around it is still turbulent.”
But while Sagittarius A* was the hardest target for EHT scientists to study, it was also a significant challenge to overcome, the scientists said. “Sagittarius A* gives us a glimpse into the much more standard state of black holes: silent and at rest,” Johnson said. “M87 was exciting because it was extraordinary. Sagittarius A* is exciting because it’s common.”
Yet despite all the differences, at a glance, the new portrait of Sagittarius A* could even be mistaken for the iconic image released in 2019.
“When we look at the core of each black hole, we find a bright ring surrounding the shadow of the black hole,” Özel said. “These two images resemble each other because they are the consequence of the fundamental forces of gravity.”
It’s physics, although it’s always tempting to play around with similarity. “Spacetime, the fabric of the universe, wraps around black holes in exactly the same way, regardless of their mass or whatever surrounds them,” Özel said. “It seems that black holes like donuts.”