There’s a new addition to astronomers’ portrait gallery of black holes.
Astronomers announced May 12 that they have finally assembled an image of the supermassive black hole at the center of our galaxy.
“This image shows a bright ring surrounding the darkness, the telltale sign of the shadow of the black hole,” astrophysicist Feryal Özel of the University of Arizona in Tucson said at a news conference announcing the result.
The black hole, known as Sagittarius A*, appears as a dark silhouette looming over the glowing material that surrounds it. The image reveals the turbulent, twisting region immediately surrounding the black hole in extreme detail and sheds new light on how black holes feed and grow.
A planet-spanning network of telescopes, known as the Event Horizon Telescope, worked together to create this much-anticipated look at the Milky Way’s giant. Three years ago, the same team released the first-ever image of a supermassive black hole (SN: 4/10/19). That object sits at the center of the galaxy M87, about 55 million light-years from Earth.
But Sagittarius A*, or Sgr A* for short, is “humanity’s black hole,” says astrophysicist Sera Markoff of the University of Amsterdam, and a member of the EHT collaboration.
At 27,000 light-years away, the behemoth is the closest giant black hole to Earth. That proximity means that Sgr A* is the most-studied supermassive black hole in the universe. Yet Sgr A* and others like it remain some of the most mysterious objects ever found.
That’s because, like all black holes, Sgr A* is an object so dense that its gravitational pull won’t let light escape. Black holes are “natural keepers of their own secrets,” says physicist Lena Murchikova of the Institute of Advanced Study in Princeton, N.J., who is not part of the EHT team. Their gravity traps light that falls within a border called the event horizon. EHT’s images of Sgr A* and the M87 black hole skirt up to that inescapable edge.
Sgr A* feeds on hot material pushed off of massive stars at the galactic center. That gas, drawn toward Sgr A* by its gravitational pull, flows into a surrounding disk of glowing material, called an accretion disk. The disk, the stars and an outer bubble of X-ray light “are like an ecosystem,” says astrophysicist Daryl Haggard of McGill University in Montreal, and a member of the EHT collaboration. “They’re completely tied together.”
That accretion disk is where the action is — as the gas moves within immensely strong magnetic fields — so astronomers want to know more about how the disk works.
Like the majority of supermassive black holes, Sgr A* is quiet and faint (SN: 6/5/19 ). The black hole eats only a few morsels fed to it by its accretion disk. Still, “it’s always been a little bit of a puzzle why it’s so, so faint,” says astrophysicist Meg Urry of Yale University, who is not part of the EHT collaboration. M87’s black hole, in comparison, is a monster gorging on nearby material and shooting out enormous powerful jets (SN: 11/10/21). But that doesn’t mean Sgr A* isn’t producing light. Astrophysicists have seen it feebly glowing in radio waves, jittering in infrared and burping in X-rays.
In fact, the accretion disk around Sgr A* seems to constantly flicker and simmer. This variability, the constant flickering, is like a froth on top of ocean waves, Markoff says. “And so we’re seeing this froth that is coming up from all this activity, and we’re trying to understand the waves underneath the froth.”
The big question, she adds, has been if astronomers would be able to see something changing in those waves with EHT.
By combining about 3.5 petabytes of data, or the equivalent of about 100 million TikTok videos, captured in April 2017, researchers could begin to piece together the picture. To tease out an image from the initial massive jumble of data, the EHT team needed years of work, complicated computer simulations and observations in various types of light from other telescopes.
Those “multiwavelength” data from the other telescopes were crucial to assembling the image. “By looking at these things simultaneously and all together, we’re able to come up with a complete picture,” says theorist Gibwa Musoke of the University of Amsterdam.
Sgr A*’s variability, the constant simmering, complicated the analysis because the black hole changes on timescales of just a few minutes, changing as they were imaging it. M87 was easier to analyze because it changed over the course of weeks. “It was like trying to take a clear picture of a running child at night,” astronomer José L. Gómez of Instituto de Astrofísica de Andalucía said at a news conference announcing the result.
Ultimately, a better understanding of what is happening in the disk so close to Sgr A* could help scientists learn how many other similar supermassive black holes work.
The new EHT observations also confirm the mass of Sgr A* at 4 million times that of the sun. If the black hole replaced our sun, the shadow EHT imaged would sit within Mercury’s orbit.
The researchers also used the image of Sgr A* to put general relativity to the test (SN: 2/3/21). Einstein’s steadfast theory of gravity passed: The size of the shadow matched the predictions of general relativity. By testing the theory in extreme conditions — like those around black holes — scientists hope to pinpoint any hidden weaknesses.
Scientists have previously tested general relativity by following the motions of stars that orbit very close to Sgr A* — work that also helped confirm that the object truly is a black hole (SN: 7/26/18). For that discovery, researchers Andrea Ghez and Reinhard Genzel won a share of the Nobel Prize in physics in 2020 (SN: 10/6/20).
The two types of tests of general relativity are complementary, says astrophysicist Tuan Do of UCLA. “With these big physics tests, you don’t want to use just one method.” If one test appears to contradict general relativity, scientists can check for a corresponding discrepancy in the other.
The Event Horizon Telescope, however, tests general relativity much nearer to the black hole’s edge, which could highlight subtle effects of physics beyond general relativity. “The closer you get, the better you are in terms of being able to look for these effects,” says physicist Clifford Will of the University of Florida in Gainesville.
However, some researchers have criticized a similar test of general relativity made using the EHT image of M87’s black hole (SN: 10/1/20). That’s because the test relies on relatively shaky assumptions about the physics of how material swirls around a black hole, says physicist Sam Gralla of the University of Arizona in Tucson. Testing general relativity in this way, “would only make sense if general relativity were the weakest link,” but scientists’ confidence in general relativity is stronger than the assumptions that went into the test, he says.
The observations of Sgr A* provide more evidence that the object is in fact a black hole, says physicist Nicolas Yunes of the University of Illinois Urbana-Champaign. “It’s really exciting to have the first image of a black hole that is in our own Milky Way. It’s fantastic.” It sparks the imagination, like early pictures astronauts took of Earth from the moon, he says.
This won’t be the last eye-catching image of Sgr A* from EHT. Additional observations, made in 2018, 2021 and 2022, are still waiting to be analyzed.
“This is our closest supermassive black hole,” Haggard says. “It is like our closest friend and neighbor. And we’ve been studying it for years as a community. [This image is a] really profound addition to this exciting black hole we’ve all kind of fallen in love with in our careers.”
This story will be updated.