Three years ago, scientists used an Earth-sized telescope to create the first-ever image of a black hole. Now they’ve done it again – this time closer to home and from a whole different unseen behemoth.
The Event Horizon Telescope (EHT) has now produced images of two supermassive black holes: one at the center of a galaxy called M87 and the other at the center of our own galaxy, the Milky Way. In doing so, the hundreds of scientists involved in the endeavor have gotten their first glimpses of two objects that have surprisingly little in common.
“The two images appear 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 EHT’s Science Council, during a National Science Foundation news briefing on Thursday (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 in virtually every other way.”
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In particular, the two black holes differ in how difficult it is to image material moving around their boundary or event horizon. (This material, which glows as it’s consumed, is what makes the fuzzy rings that the EHT worked to create glow, since black holes themselves don’t emit light.)
Prudently, the EHT tackled the less demanding target first. This is the monster hiding in M87, also known as M87*. This black hole is, of course, farther from Earth, but it’s also much larger, and material moves more leisurely around its event horizon.
“Material swirls around M87* over the course of many days,” said Vincent Fish, an astrophysicist at the Massachusetts Institute of Technology’s Haystack Observatory and a member of EHT’s Science Council, during the press conference. “It sits still for its photo.”
The Milky Way’s supermassive black hole, dubbed Sagittarius A*, is less cooperative. “It takes only a few minutes for material to move near the horizon of Sagittarius A* because it’s much smaller,” Fish added. “It’s changing fast, so we had to collect our data quickly.”
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And Sagittarius A*’s challenge became apparent when scientists analyzed the data the EHT also collected. Katie Bouman, a computer scientist at the California Institute of Technology and co-chair of the EHT Imaging Working Group, said during the press conference when the independent imaging teams formed to analyze the M87* results compared their first shots of an image, they were more or less the same.
Not so with the second black hole. “Imagery of Sagittarius A* was a bit more messy story than imagery of M87*,” said Bouman. This time the imaging groups were reluctant to create an initial image because there was so much less consensus among team members.
Although the matter surrounding Sagittarius A* moves impractically fast around the event horizon, our supermassive black hole still presents a much tamer environment near its surface than M87*. The turbulence in this region is driven by the black hole’s appetites, which vary despite the popular notion that all black holes suck like a cosmic vacuum cleaner.
“If Sagittarius A* were human, he would eat a single grain of rice every million years,” said Michael Johnson, an astrophysicist at the Harvard/Smithsonian Center for Astrophysics and a member of the EHT Science Council, during the press conference. “Only a trickle of material actually makes it to the black hole.”
Growing at this slow pace, Sagittarius A* is quite a petite eater. “That’s why his environment is relatively gentle,” said Özel. “Of course, if we say so, the temperatures and magnetic field strengths are still quite high, and the movement of the gas around it is still turbulent.”
But while Sagittarius A* was the harder target to study for EHT scientists, it was also an important challenge to address, the scientists said. “Sagittarius A* gives us a glimpse into the much more common state of black holes: calm and still,” Johnson said. “M87 was exciting because it was extraordinary. Sagittarius A* is exciting because it’s widespread.”
But despite all the differences, the new portrait of Sagittarius A* could even be confused with the iconic image from 2019 at first glance.
“If we look into the heart of any black hole, we find a bright ring surrounding the black hole’s shadow,” Özel said. “These two images look similar because they are the result of fundamental gravity.”
That’s physics, although it’s still tempting to poke fun at the resemblance. “Spacetime, the fabric of the universe, wraps around black holes in exactly the same way, regardless of their mass or what surrounds them,” Özel said. “It seems that black holes like donuts.”