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 faint silhouette amidst the glowing material that surrounds it. The image reveals the turbulent, twisting region immediately surrounding the black hole in new detail. The findings also were published May 12 in 6 studies in the Astrophysical Journal Letters.
A planet-spanning network of radio 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 for 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 its region 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. In the new work, they’ve seen hints of those changes below the froth, but the full analysis is still ongoing.
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 the researchers were imaging it. “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 in Granada, Spain, said at a news conference announcing the result. M87 was easier to analyze because it changed over the course of weeks.
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
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