The world has been gifted the second photograph ever taken of a supermassive black hole — and this time, it’s a black hole that’s relatively close to home. Today, scientists collaborating on the massive Event Horizon Telescope (EHT) project released an image of Sagittarius A*, the gargantuan black hole spinning away at the center of our own galaxy, the Milky Way.
This celestial present comes from the same project which made quite the splash back in 2019 when they released the very first image taken of a black hole. That now iconic fuzzy orange photo showcased a supermassive black hole at the heart of a mammoth galaxy called Messier 87, or M87, which is located 55 million light-years from Earth. The groundbreaking result helped scientists verify the circular shape of these objects as well as further confirm Albert Einstein’s theory of general relativity, which predicted the existence of black holes.
Now, the team is back with another photo of a black hole, this one right in our own backyard. Located 26,000 light-years from Earth, Sagittarius A*, or Sgr A*, is thought to be roughly 4 million times the mass of our Sun. Scientists have inferred its existence at the center of our galaxy for decades based on how objects move around the black hole. But this is the first time we have a direct image of its dark central area, or “shadow,” even more proof of life of the nexus of our cosmic neighborhood. “Until now, we didn’t have the direct picture confirming that Sgr A* was indeed a black hole,” Feryal Özel, an astronomer at the University of Arizona and member of EHT, said during a press conference announcing the news. She added, “This image shows a bright ring surrounding the darkness — the telltale sign of the shadow of the black hole.”
With each new supermassive black hole picture we get, scientists learn a little bit more about these enigmatic objects. “These supermassive black holes, we don’t really know how they formed or grew so large,” Meredith Clark Powell, a black hole researcher at Stanford University, tells The Verge. “So it’s a very active area of research.”
Truthfully, one cannot capture an image of a black hole directly. Black holes, by their very nature, can’t be “seen,” as these objects are so massive that nothing can escape their gravitational pull — including light. Instead, we can capture the silhouette of a black hole. If a supermassive black hole is surrounded by a swirling disc of gas and dust, that material will glow bright since the gas and dust is sped up and heated by the powerful gravitational pull of the nearby hole. What EHT is actually capturing is the black hole’s shadow against the backdrop of that glowing gas and dust.
Snapping pictures of these black hole shadows is no simple task, though. To capture an image like this of Sagittarius A*, a single telescope would have to be the size of planet Earth to get the job done, according to the Event Horizon Telescope group. Since building such an apparatus isn’t exactly realistic, scientists came up with a workaround. The EHT is a wide-ranging array of radio dishes spread out across five different continents. The radio telescopes all work together to observe the same object, behaving as if they are one giant planet-sized telescope. It’s then up to the EHT scientists to piece together the data the telescopes gathered to create a single image.
EHT used the same technique to capture M87; eight EHT radio dishes spent a week observing that black hole in April 2017, which then resulted in months and months of work to compile the data into the image that was eventually released. At the same time, EHT also observed Sagittarius A*, but crafting its image proved to be much more challenging and time-consuming. “Taking a picture with the EHT is like listening to a song being played on a piano that has a lot of missing keys,” Katie Bouman, an astronomer at Caltech, said during the press conference.
Though it’s closer to Earth than M87’s black hole, Sgr A* is smaller and less active, and the material surrounding the object is much dimmer, making it harder to observe. On top of that, the material that does surround Sgr A* exhibits weird flaring when the particles surrounding the black hole are accelerated to much higher energies. While it makes for an interesting light show, it changes the composition of the black hole every few hours, making it tricky to observe over time. The material that swirls around Sgr A* close to the event horizon — the point of no return for the particles falling into the black hole — moves so incredibly fast that the object appears to change in real time. “That means as we were collecting data during the Earth’s rotation, the material was swirling around Sgr A* so quickly that Sgr A*’s appearance could change from minute to minute,” Bouman said.
In addition to all of that, Sgr A* is in our own galaxy, which makes it harder to see from Earth. Observing this black hole means peering through the galactic plane of the Milky Way — and all of the gaseous material in between us and the black hole. That provides a lot of interference that the scientists had to work around. “The result is an image that until we finished our analyses, we were never sure we could get,” Vincent Fish, an astronomer at the MIT Haystack Observatory and an EHT collaborator, said during the press conference.
When the EHT scientists observed Sgr A*, they collected roughly 3.5 petabytes of data. “That’s equivalent to about 100 million TikTok videos,” Fish said. “It’s way too much data to stream over the internet.”
The team had to transfer the information by shipping hundreds of hard drives to correlation centers in Westford, Massachusetts, and Bonn, Germany, according to Fish. There, supercomputers compiled the signals together. After that, the data underwent an intense calibration process, as the scientists attempted to construct the best image they could make of the black hole’s silhouette and plasma. Part of this calibration process had to move online, too, when the COVID-19 pandemic hit.
The resulting image is the one that was unveiled today, though it may look a bit blurry to the average observer. That’s simply due to the limitations of our instruments here on Earth. “Every telescope has something we call the diffraction limit,” Michael Johnson, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics, said during the press conference. “It’s the finest features that it can see, and that’s basically the level that we’re seeing here.”
But, with this fuzzy image, the scientists have already learned a lot. For one thing, they’ve determined that Sgr A* isn’t a particularly hungry black hole. Only a small portion of the material surrounding the object actually makes it inside. “If Sgr A* were a person it would consume a single grain of rice every million years,” Johnson said. Sgr A* also doesn’t convert much of its gravitational energy into light. The gravitational pull of some black holes, like the one in the middle of M87, can actually speed up the surrounding plasma, causing the material to shoot outward as jets of light. That’s not the case for Sgr A*, which is a much more quiet kind of black hole. And that kind of black hole may be the standard. “Sgr A* is giving us a view into the much more standard state of black holes, quiet and quiescent,” Johnson said. “M87 was exciting because it was extraordinary. Sgr A* is exciting because it’s common.”
Now, with two black hole images on its resume, the EHT collaboration has big plans for the future. EHT is adding even more telescopes with the goal of creating the next-generation Event Horizon Telescope (ngEHT), Johnson said. This will potentially allow scientists to process a moving image of a black hole, showing how it evolves over time.
“This improvement will help us move from these still images to capturing the first high resolution movies of black holes, letting us witness them in action and continuing this quest toward the boundary of the unknown,” Johnson said.