For the first time, humanity has stared into the dark heart of unfathomable chaos at the center of the Milky Way and brought its shadowy form into focus. The object staring back at us, Sagittarius A*, is a monstrous black hole that binds our home galaxy together.
On Thursday, scientists with the Event Horizon Telescope (EHT) Collaboration revealed the first direct visual evidence of Sagittarius A*, or Sgr A*, in coordinated worldwide press conferences. Composed of over 300 researchers, the collaboration made headlines three years ago for unveiling the first image of any black hole and has been attempting to image Sgr A* since 2009.
Today, the world bears witness to the fruits of their labor. And it’s every bit as groundbreaking as expected.
This dazzling light, swirling orange around a shadowy circle, traveled more than 26,000 years to reach us. It is of luminescence birthed at the edge of Sgr A* when Earth’s northern ice sheets reached as far as Manhattan, cave bears still roamed Europe and Homo sapiens settlements were being built from mammoth bones.
“I wish I could tell you that the second time is as good as the first, when imaging black holes. But that wouldn’t be true. It is actually better,” said Feryal Özel, an astrophysicist at the University of Arizona and part of the EHT Collaboration.
Özel’s sentiment comes from the fact that EHT’s image of SgrA* isn’t just a spectacular sight. It’s concrete proof that humanity has, in fact, managed to take pictures of the elusive engines powering our universe. SgrA* has a doughnut-like structure akin to the team’s previous black hole picture, therefore confirming these glowing rings aren’t the product of coincidence or environmental noise.
They represent black holes.
The saga of Sagittarius A*
It was 1974 when astronomers initially discovered evidence of Sgr A*, thanks to a very bright radio signal emanating from the heart of the Milky Way. But at the time, it wasn’t clear whether the cue came from a black hole. It was only suspected.
Over the next four decades, however, further observations revealed stars circling the radio source in extreme orbits and at extreme speed — both expected to occur around black holes. And by 2018, there was even more comprehensive confirmation that Sgr A* is absolutely a supermassive black hole, and one with a mass of over 4 million suns. Two of the scientists who studied Sgr A* were awarded the 2020 Nobel Prize in Physics.
Yet we still couldn’t actually see the black hole. Until now, that is.
The EHT’s incredible image is the long-sought visual confirmation of Sgr A*’s true nature, allowing us to finally lay eyes on the motor behind the Milky Way’s swirls and refining our capability to study the universe’s colossal chasms and their exotic physics. “This is a big — no, it is a huge — moment for everyone in the Event Horizon Telescope Collaboration,” said J. Anton Zensus, director at the Max-Planck-Institute for Radio Astronomy in Germany.
A detailed outline of the findings were published Thursday in a series of papers appearing in the journal The Astrophysical Journal Letters.
Image of the invisible
The gravitational effects of a black hole are so mighty the chasm basically punches a hole in spacetime. But black holes aren’t exactly “black holes.” They’re more like unseeable rifts in the cosmos.
Basically, when a big enough star dies, it collapses to a single point with an immense gravitational pull called a singularity. This pull is so unimaginably strong that when gas, dust or light falls in, the particles can never escape. Nothing can escape, which makes black holes practically invisible.
In fact, since black holes were first theorized by Einstein in the early 20th century, astronomers were only convinced these voids existed because of pure mathematics. But there’s a caveat. While we can’t exactly “see” a black hole, we can visualize the surrounding region where those forever-doomed particles are about to descend toward its center.
In other words, just outside the dark of the mighty void, gas and dust are being superheated to trillions of degrees Celsius and releasing light across the electromagnetic spectrum. To us, that light appears as X-rays and radio waves. Both of those signals can be detected from Earth, and that’s how we can see the unseeable.
To capture those priceless black hole fingerprints, however, you kind of need a telescope that’s the size of our entire planet.
But because that’s obviously not feasible, EHT found a fascinating way to get around the prerequisite. It virtually linked 11 ground-based radio telescopes together, all positioned around Earth. Over time, these devices looked for the super-hot, particle-derived black hole signatures, or rather, the boundary between our universe and a black hole’s unknown, “invisible” innards.
This region is actually the namesake of EHT: the event horizon.
The Event Horizon Telescope sees the event horizon by syncing up observations from their many radio telescopes scattered across the world. It gathers light from the area just outside the horizon using a technique known as “very-long baseline interferometry,” or VLBI.
In a nutshell, VLBI requires two individual telescopes to focus on the same spot in space at the same time. For instance, a telescope in Chile and a telescope in the South Pole might look toward an event horizon. Then, because the scopes are subject to some extremely accurate time-keeping, results from each telescope can be combined to a final composite. In a way, that creates a virtual telescope as big as the distance between the two sites. And bigger telescopes, generally, mean higher resolution.
Radio astronomers have used this method for decades, but extend the concept to 11 telescopes across the world, and you’ve got yourself a telescope the size of our planet. Perfect for imaging a black hole.
EHT’s multiple telescopes teamed up at once and observed the black hole over a period of several hours. As Katie Bouman, a computational imaging researcher and member of EHT puts it, “our radio telescope shakes hands.” Then, those results were combined, all the data was run through an algorithm and — bang! — we have our picture of a black hole.
“Taking a picture with the EHT is a bit like listening to a song being played on a piano that has a lot of missing keys,” Bouman said. “Since we don’t know when the missing keys should be hit, there’s an endless number of possible tunes that could be playing. Nonetheless, with enough functioning keys, our brains can often fill in the gaps to recognize the song correctly.”
Back in 2019, this is also how scientists created the world’s first black hole photo. But EHT’s new black hole subject posed a few extra hurdles.
M87* vs. SgrA*
The muse of EHT’s first image — a blurry-looking, orange and yellow ring of light stamped against the colorless cosmic void — is M87*, a supermassive black hole that lies at the heart of the Messier 87 galaxy about 55 million light-years from Earth. It has a mass 6.5 billion times more than that of our sun.
But the EHT was always hoping to catch a glimpse of Sgr A* too, especially because our home galaxy’s black hole is what scientists think most black holes across the universe would look like.
“While M87* was one of the biggest black holes in the universe, and it launches the jet that pierces its entire galaxy, SgrA* is giving us a view into the much more standard state of black holes — quiet, and quiescent,” said Michael Johnson, an astrophysicist at the Harvard Smithsonian Center of Astrophysics.
However, SgrA* was much harder to image than M87 simply because we don’t have a great angle it, and EHT’s telescopes had to see through bothersome gas and dust which further obscures the void from view. When studying M87*, these issues weren’t really present.
Think of it this way. In the cinema of the cosmos, we’d been sitting in an empty theater with reclining seats, observing Messier 87’s black hole on our planet-wide screen. For Sgr A* we were surrounded by other patrons constantly getting up to pee and interrupting the show.
The other problem was the film we were trying to watch. The region around a black hole is quite dynamic, or in flux, because of extreme gravitational mechanics. Because Sgr A* is much closer to Earth and has a smaller event horizon than M87*, the light it beams out to our telescopes sort of changes much faster. It’s more variable. And this variability poses a problem to the EHT because the Earth-sized telescope must observe the black hole over several hours. Sgr A* is changing over several minutes.
“This is a bit like changing the the key of the song as we are playing it on our broken piano,” Bouman said.
It’s “like trying to take a picture of a waterfall with a long shutter speed; the subject is changing too quickly to get a sharp image,” notes James Miller-Jones, an astronomer at Curtin University in Western Australia. To see Sgr A* requires a lot more work from the algorithm that pieces together the final image.
But, alas, they did it.
The collaborators collected tens of thousands of different images with different methods — including some mock simulations of the black hole based on hard data — to get as much information as possible on SgrA*. Then, they grouped these pictures by likeness into four categories, and finally, averaged everything together.
“Through literally years of exhaustive tests on both real and simulated data, we’re now confident that there is compelling evidence that the true underlying structure is a ring,” Bouman said.
So, with the two major problems overcome, we’ve doubled our stash of black hole photographs to a grand total of two — and, opened up a portal to the unthinking infinity at the center of the Milky Way. Now what?
It’s all Relativity
Seeing a black hole gives us a chance to test one of the fundamental theories of the universe: Einstein’s General Relativity.
In a nutshell, the theory gives us a way to understand gravity via the warping of space and time, or spacetime. This is the ocean-like fabric of the universe. Massive objects bend spacetime a lot and black holes… well, they almost break it. So, by studying them, astronomers can put Einstein’s theory to the test in some of the most extreme environments we know of.
With two black holes of different masses, like M87* and Sgr A*, we can put the theory to the test yet again. One of the key predictions of GR is that black holes are described by three features: their mass, their spin and their charge. Now that we’ve seen two, does the theory hold? Well, of course it does!
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” said Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei.
In July 2021, the EHT revealed it had turned its many eyes toward the black hole at the center of the Centaurus A galaxy and studied its astrophysical jets, which stretch out into the cosmos. The jets, produced by many black holes, are essentially runaway freight trains of plasma hurled from the edges of the event horizon. The incredibly high resolution of the EHT allowed astronomers to peer inside these jets for the first time, revealing their characteristics.
Unsurprisingly, Einstein’s theory of General Relativity held up here, too.
And it’s not just trying to swell Einstein’s genius ever further. Supermassive black holes seem to lurk at the center of most galaxies. “The growth of supermassive black holes is closely connected with the evolution of their host galaxies,” said Miller-Jones. The more we learn about Sgr A*, the more we learn about the Milky Way as a whole.
“There’s so much more to do,” said Anton Zensus. “We now want to go and make movies. We want to study magnetic fields. We want to look at the jets in galaxies. And yes, we want to tackle gravitational theory again.”
In the coming years our knowledge should skyrocket. Observations by the EHT will be complemented by, for instance, NASA’s recently launched James Webb Space Telescope. Once it’s up and running, it will focus in on Sgr A* and detect the faint light from the stars surrounding the black hole. It’s entirely possible that Webb might spot a star being eaten by Sgr A* or detect some wild collisions close to the event horizon. It’s likely astronomers will discover things they’ve never dreamed of.
For today, at least, they can bask in the orange glow of Sgr A*, captured by an Earth-sized telescope, and imagine the possibilities.