On Wednesday, astronomers revealed the first image ever taken of a black hole, bringing a dramatic conclusion to a decades-long effort. The iconic image offers humanity its first glimpse at the gas and debris that swirl around its event horizon, the point beyond which material disappears forever. A favorite object of science fiction has finally been made real on screen.
Their target was a nearby galaxy dubbed M87 and its supermassive black hole, which packs the mass of six and half billion suns. Despite its size, the black hole is so far from Earth – 53 million light-years – that capturing the image took a telescope the size of the planet.
This monumental accomplishment was only possible thanks to the Event Horizon Telescope (EHT). The image data was taken back in 2017 but scientists have spent two years piecing it together. That’s because EHT is made of up eight independent observatories that are scattered across the globe, cooperating together to act as one enormous detector. Shep Doeleman, director of the EHT, announced at today’s press event, “We are delighted to report to you today that we have seen what we thought was unseeable.” Researchers made their grand announcement simultaneously in seven different countries this morning, accompanied by a series of scientific papers published at the same time in
The Astrophysical Journal Letters.
An impossible black hole image
Black holes are so massive and dense, not even light can escape their pull. They’re often referred to as a singularity, or a point source, because they take up zero actual space. But this mysterious singularity is surrounded by the sphere of its event horizon. And anything that travels past it is doomed to fall into the black hole, with no hope of escape. That means the black hole itself is literally dark – it neither reflects nor gives off any light. So there’s nothing to photograph, no matter how advanced the technology. In the Event Horizon Telescope’s image, it simply appears as a central dark blob, or what astronomers often call the black hole’s “shadow.”
Event Horizon Telescope: How does it work?
Feryal Özel is an astrophysicist at the University of Arizona and an EHT collaborator. She explains the shadow as the black hole absorbing the light around it. The light stems from the hot gas that’s swirling around it and gets heated as it falls into the black hole. “So, our telescopes are able to pick up the light as long as it comes not from the immediate vicinity of the black hole, but just outside it,” Özel says. “When the light falls into the event horizon, that part is dark in the image. Whether or not shadow is the perfect word, it imprints this darkness on the surrounding emission.”
Taking an image of a shadow where a black hole should exist might not seem extraordinary, but black holes also aren’t strictly speaking proven to exist — at least not everywhere scientists expect them, like at the centers of most large galaxies. And seeing this shadow confirms that it really is a black hole, Özel says.
Like a whirlpool, the material spiraling around a black hole is mostly flat. Scientists call it an accretion disk. And these accretion disks can stretch across vast distances and give off incredibly bright energy that shines across the cosmos. But capturing these beacons is like photographing a mushroom cloud during an atomic blast, when the real science is happening on the level of atoms at the heart of the explosion. Scientists have long desired to see inside the disk to where the material actually disappears into the black hole. Before EHT, that level of detail had eluded them.
Why the Event Horizon Telescope took so long to image a black hole
Occasionally, this semi-chaotic swirl of accretion disk material collides with itself, launching matter out in jets that extend thousands of light-years and travel at nearly the speed of light. And astronomers have already photographed M87’s jets using more conventional instruments, like the Hubble Space Telescope.
But the exact cause of these extreme speeds remains unclear. Scientists say that magnetic fields are a prime suspect. “We think the spin of the black hole interacting with the magnetic field is what causes the jets, but we don’t have proof,” says Özel.
Imaging the central area of the black hole should give that proof. “If we see jet-like images or anything associated with it I think it increases our confidence that jets are formed very close to the black hole,” Özel adds.
In addition to the jets, studying the swirl of material near M87 also gives astronomers the most accurate weight ever for this monster black hole, which is one of the most massive in the known universe. Astronomers can weigh the black hole at the center of our Milky Way, called Sagittarius A* (pronounced A-star), by watching the motions of individual stars zooming around its perimeter. But M87’s black hole is much farther away, and the scale is likewise fuzzier. That’s led to disagreements about its mass. “There are two discrepant measurements,” Özel says. “Our uncertainty is much less than the difference between those two measurements.”