Our Galaxy’s Dark Heart: Astronomers Capture First Ever Image of the Milky Way’s Black
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…