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. The collaboration, made up of over 300 researchers and famous for unveiling the first image of any black hole three years ago, has been attempting to image Sgr A* since 2009.
“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.
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. The light was 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.
Astronomers discovered Sgr A* in 1974 after detecting a very bright radio signal from the heart of the Milky Way. At the time, it wasn’t clear if this was a black hole. Over the next four decades, further observations revealed stars circling the radio source in extreme orbits and at extreme speed.
By 2018, there was a more comprehensive confirmation: Sgr A* is a supermassive black hole, with a mass of over 4 million suns. Two of the scientists who studied Sgr A* were awarded the 2020 Nobel Prize in Physics. But we still couldn’t actually see the black hole.
On Thursday, the EHT offered a definitive visual confirmation of Sgr A*’s true nature, allowing us to lay eyes on the engine that turns the Milky Way and refining our capability to study black holes and their exotic physics. The findings were published Thursday in a series of papers appearing in the journal The Astrophysical Journal Letters.
“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.
The image of the invisible
What does it take to see the unseeable? An Earth-sized telescope — or 11 telescopes linked together virtually, to be precise.
The gravitational effects of a black hole are so mighty that it basically punches a hole in spacetime. When a big enough star dies, it collapses to a single point with an immense gravitational pull. It’s so immense that when gas and dust or light falls in, it can never escape. In fact, no particle can, which makes black holes practically invisible.
Since they were first theorized by Einstein in the early 20th century, astronomers were convinced they existed because of math. But then we found them, indirectly, via their interactions with cosmic bodies and light.
While we definitely can’t look at a black hole, we can visualize the region just before a particle is doomed to forever descend toward its center. This boundary between our cosmos and a black hole’s unknown innards is called the event horizon. Just outside the dark of the mighty void, gas and dust are being superheated, releasing light across the electromagnetic spectrum, like X-rays and radio waves, which can be detected from Earth.
The Event Horizon Telescope is designed to see this region by syncing up the observations from ground-based radio telescopes scattered across the world. It gathers light from the area just outside the event horizon using a technique known as “very-long baseline interferometry,” or VLBI. It’s a technique radio astronomers have used for decades.
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, the results from each telescope can be combined. In combining them, you’ve created a virtual telescope as big as the distance between the two sites. And bigger telescopes, generally, mean higher resolution.
Now, extend the concept to 11 telescopes across the world, and you’ve got a telescope the size of our planet. Multiple telescopes team up at once, observing the black hole over a period of several hours. Combine those results and then run the data through an algorithm and — bang! — you get an image of a black hole.
This is exactly how scientists created the first black hole photo in 2019.
The subject of that first image — a blurry, 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 our sun.
But the EHT was always hoping to catch a glimpse of Sgr A*, too. The problem, however, was that we just don’t have a great angle on our home galaxy’s black hole. Our telescopes had to see through bothersome gas and dust that obscured Sgr A* from view — something they didn’t have to contend with when studying M87*.
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 our viewing.
The other problem was the film we were trying to watch. The region around a black hole is quite dynamic because of the gravitational extremes. Because Sgr A* is much closer to Earth and has a smaller event horizon than M87* the light it beams out to our telescopes changes much faster — it’s more variable. This variability poses a problem to the EHT because, remember, the Earth-sized telescope wants to observe the black hole over several hours… and Sgr A* is changing over several minutes.
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, they did it, as today shows. With the two major problems overcome, we’ve doubled our stash of black hole photographs and opened up a portal to the unthinking infinity at the center of the Milky Way. So, 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 (GR).
In a nutshell, the theory gives us a way to understand gravity via the warping of spacetime, the fabric of the universe. Massive objects bend spacetime a lot and black holes… well, they almost break it. 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 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,” says 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 its 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 never even dreamt 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.