More than 27,000 light-years away, at the heart of our galaxy, lies a supermassive black hole, one that is more than four million times the mass of our sun. It has never been seen.
Astronomers using the Event Horizon Telescope (EHT) have taken the first image of the Milky Way’s supermassive black hole, named Sagittarius A*.
Astronomers have long speculated about a black hole at our galaxy’s centre. Observations of stars near the core showed them orbiting something invisible, which suggested a black hole. Today’s image release is a confirmation.
The results were published today in a special issue of Astrophysical Journal Letters.
Black holes are relatively small, invisible, extremely dense regions of space with a gravitational field where anything that crosses their threshold — known as the event horizon — gets pulled in, never to return. This includes light, which is why they are so notoriously difficult to detect, unless they’re interacting with a nearby star.
However, in 2019, the EHT — an organization of more than 200 astronomers from around the world, including Canada — released a historic first photograph of a black hole at the centre of another galaxy, Messier 87 (M87), that showed the shadow of the black hole with its surrounding gas illuminated.
Like M87, the newly published image shows the hot material surrounding the shadow of Sagittarius A* (pronounced Sagittarius A-star).
The image was made using eight radio telescopes in six sites around the world: Chile, Mexico, Spain, Hawaii, Arizona and even the South Pole. Used together, they act as a giant, Earth-sized telescope capable of obtaining much more detailed images. The data for both M87 and Sagittarius A* were collected in 2017.
More than pretty pictures, the data collected may shed light not only on the formation of supermassive black holes but also the role they played in the early universe and the role they continue to play at the heart of galaxies, most of which are home to these puzzling structures.
That’s the hope, anyway, of astronomers and astrophysicists around the world.
Ue-Li Pen, an astrophysicist and collaborating scientist on the Event Horizon Telescope project, compared these new images and the data collected to the difference between reconstructing a dinosaur from what we understand to actually seeing one.
“I think the analogy to dinosaurs, it’s not that different,” said Pen, who is also a professor at the Canadian Institute for Theoretical Astrophysics at the University of Toronto. “[It’s] a little bit like seeing a live dinosaur in your neighbour’s backyard: we can’t actually touch it yet. But it’s so much closer and so much newer — and so much more alive than everything before that.”
The challenges of getting these images
Creating imaging of the two black holes presented different challenges. In the case of M87, its black hole is one of the most massive known. It’s six billion times more massive than our sun and 1,500 times more massive than Sagittarius A*. It also lies 2,000 times farther away than Sagittarius A*.
With Sagittarius A*, while closer, it is along the galactic plane, which means the telescopes needed to peer through thick gas, dust and plasma. As well, its black hole is smaller, measuring 4.3 million times the mass of the sun.
“For Sagittarius A* it is … almost 2,000 times smaller [than M87], and that means light zips around it in 2,000 times less time,” said Avery Broderick, an associate professor at Waterloo University, associate faculty member at the Perimeter Institute and a member of the EHT team. “That’s about 15 minutes. So every 15 minutes a day, it starts presenting us with a new face.”
He compares looking through all the plasma, dust and gas to looking through an ice-covered window.
“That’s pretty much what radio astronomy through the galactic plane looks like — the flotsam and jetsam of the galaxies is playing the role of the ice and blurs and variegates the images,” he said.
“So solving that problem, we can play some games to remove the blurring. But addressing how to deal with the kind of rippling effect that you see, that was one of the things we had to figure out how to do efficiently, so that we could produce an image that showed what the source looked like.”
‘Unlike anything else’
Black holes have a fascinating history. They were first predicted by theoretical physicist Albert Einstein in his theory of general relativity in 1915. Even though his own calculations predicted them, Einstein didn’t believe that nature could crush something with the mass of a star into such a compact space. However, astronomer and physicist Karl Schwarzschild, who was fighting in the trenches on the German front during the First World War, managed to prove that they could indeed exist.
So this new image is yet another testament to Einstein’s theory of general relativity, Broderick said.
“The fact that the shadows are the right size, I think, is an incredible confirmation that Einstein’s theory of gravity, Einstein’s general relativity is still batting 1,000,” he said. “It’s still answering every question put to it accurately, which, sometimes, is frustrating, because we’re always looking for something, something new and different.”
Janna Levin, who was not involved in the EHT announcement, is a renowned astronomer who has made it her life’s work to learn as much as possible about black holes.
“Stars can be different in lots of ways. Black holes cannot. Black holes are almost like a fundamental particle in the same way an electron is like a fundamental particle,” said Levin, who is a professor of physics and astronomy at Barnard College of Columbia University in New York and the author of Black Hole Survival Guide.
“If I have a black hole that has a certain charge, electrical charge, like static electricity and a certain mass, and it’s spinning a certain way, it is absolutely indistinguishable from any other black hole. With those three numbers — three numbers, that’s it — you could not tell if that black hole was made out of antimatter, out of dark matter, out of encyclopedias, out of ghosts, out of stars, out of photons.
“They reveal nothing about themselves.”
Our fascination with black holes
Regardless if we’d seen them or not, black holes have long captured the public’s imagination. In 1979, Disney made what is today regarded as a notoriously bad movie called The Black Hole. And these gravitational monsters continued to appear in movies, most recently the 2014 drama Interstellar.
It’s thanks to that movie that we got our first real idea of what a black hole might look like. Nobel Prize-winning astrophysicist Kip Thorne ran calculations on a supercomputer to try to present a realistic vision of what a black hole might look like. The result was a shadow of a black hole with light bending around it due to its massive gravity, called gravitational lensing. When the image of M87 was released, it almost matched that perfectly.
Both Broderick and Pen believe that better understanding black holes will, in turn, help us better understand gravity as a whole.
“Why does it matter in a broader sense? Well, I kind of get philosophical on that point,” Broderick said. “We live now in this information age, I’m talking to you, over Zoom, and my computer’s on the Wi-Fi, communicating with some other computer that gets beamed around the world to you. And that all operates because of Maxwell’s equations, James Clerk Maxwell wrote down some description of electromagnetism.
“I don’t know any more than Maxwell could have predicted cell phones, what the gravity age will provide. We might all be flying around in Jetson-like cars because of it, or you might be warping across the universe.”