At the center of the Milky Way is a massive and mysterious presence that exerts a powerful influence on the stars around it – and on the imagination of astronomers.
Now scientists have the first photograph of the formidable force at the center of our galaxy: Sagittarius A*, a supermassive black hole with the mass of 4 million suns.
The image, revealed on Thursday, was captured by a network of eight radio observatories in six locations around the world. Together they form the practical equivalent of an Earth-sized telescope designed to see some of the most mysterious and baffling objects in the universe.
Taking a picture of a black hole is a singular feat, as its hallmark is that nothing within its gravitational range can escape – including light.
But astronomers can see the ring-shaped boundary known as the event horizon, and beyond that, the golden, transparent ring of superheated gas and curved light that skirts the edge of the black hole’s point of no return.
“What’s cooler than seeing the black hole at the center of our Milky Way?” said Katie Bouman, professor of computational imaging at Caltech and a member of the international telescope team.
The results were published today in the Astrophysical Journal Letters.
Black holes are the densest objects in the universe. When a giant star explodes in a dramatic, final supernova, its collapse creates a tiny clot of matter so dense that its gravitational pull warps the fabric of space and time around it.
Scientists have long suspected that supermassive black holes are at the center of all galaxies, including our own. However, despite their colossal size, they are an elusive presence in the universe, observable only by their influence on the objects around them.
Capturing an image of an object from which no light can escape is the monumental challenge the Event Horizon Telescope consortium set out to tackle in 2009. The effort involves the collaborative work of more than 300 scientists and engineers at 80 institutions worldwide.
It took a decade to produce the first photograph of a black hole, the one at the center of the galaxy Messier 87, about 55 million light-years away (the black hole is also known as M87*). Its event horizon is nearly 25 billion miles wide, with a mass of approximately 6.5 billion suns.
Although Sagittarius A* – or Sgr A* for short – is a mere 27,000 light-years from Earth, it is less than 0.1% the mass of M87*. Had it not been conveniently located in our own galaxy, it would have been nearly impossible to photograph it. Bouman compared this to being in Los Angeles and taking a picture of a grain of salt in New York.
“It’s a kinder, more cooperative black hole than we expected,” said Feryal Özel, an astronomer at the University of Arizona and a founding member of the telescope consortium. “We love our black hole.”
In fact, the images provide the strongest evidence to date for Einstein’s theory of general relativity. With Sgr A* in particular, the size and shape of the ring around the event horizon is remarkably consistent with what scientists predicted based on Einstein’s theory.
“They’re so different in so many ways, but the same theory of gravity explains” the shape of both images, Bouman said. “And that is a great result. It’s actually really exciting that they look so much alike.”
A popular classroom model of a black hole offers a useful way to visualize this cosmic phenomenon. Imagine the fabric of spacetime as a tight-fitting sheet of plastic and the Earth as a tennis ball falling at its center. The ball will create a slight curve in the movie, just as our relatively modest-sized planet does with spacetime.
A steel ball, however, will bend the film much more. If the ball is heavy enough, the film will sag so much that any other objects will roll inescapably towards the heavier one. That’s what black holes do with time and space.
“Black holes are not the big cosmic vacuum cleaners that Hollywood likes to portray them to,” said Bouman.
The smaller and less efficient Sgr A* is more likely a better representative of the typical black hole in the universe than the ultramassive M87*, said Bouman.
UCLA astronomer Andrea Ghez received the 2020 Nobel Prize for the discovery of Sgr A*. The image the EHT produced was “remarkably similar” to the supermassive black hole that she and her colleagues theorized to be at the center of this galaxy.
“There’s a prediction that you should see this concentration of light around the black hole, outside the event horizon, and that you can actually see that is remarkable,” Ghez said. “It’s really exciting.”
Photographing a black hole with a single telescope would require a lens 13 million meters wide – in other words, a telescope the size of Earth itself.
In place of this logistical impossibility, the Event Horizon Telescope collects data via eight radio observatories in Greenland, Antarctica and six other locations in between, synchronized with atomic clocks. As the Earth rotates, observatories observe their target from various angles.
The glamorous photo of Sgr A* was distilled from 5 petabytes of data, which equates to 100 million TikToks, said EHT member Vincent Fish of the MIT Haystack Observatory. The published image is an average of multiple images extracted from this data.
Two decades ago, “I thought we would never see pictures like this. It would be very difficult,” said Daniel Stern, an astrophysicist who studies black holes at NASA’s Jet Propulsion Laboratory in La Cañada Flintridge.
“It looked better than I expected,” he said. “This corresponds to decades-old theories of what we thought black holes would be.”
Because this black hole is so much smaller, the ring around it looks a lot busier. Gases that take weeks to orbit M87* can circulate Sgr A* in just a few minutes. Given the rapid changes in emissions, it is possible that the telescope will be able to capture moving images of activity around the event horizon over the next few years, Bouman said — potentially in multiple dimensions.
“What if we could actually map where the gas is over time in three dimensions around the black hole?” said Bouman. “This is something that makes me very excited.”