Black holes keep their secrets close at hand. They forever imprison everything that enters. Light itself cannot escape the hungry pull of a black hole.
It would therefore seem that a black hole is invisible – and that it is impossible to photograph it. A great fanfare therefore accompanied the broadcast in 2019 of the first image of a black hole. Then, in the spring of 2022, astronomers unveiled another black hole photo – this time of the one at the center of our own Milky Way.
The image shows a donut-shaped orange blob that looks remarkably similar to the previous image of the black hole at the center of the Messier 87 galaxy. But the Milky Way’s black hole, Sagittarius A*, is actually much smaller than the first and was harder to see, as you had to look through the hazy disk of our galaxy. So even though observations of our own black hole were conducted at the same time as those of M87, it took three more years to create the image. This required an international collaboration of hundreds of astronomers, engineers and computer scientists, and the development of sophisticated computer algorithms to reconstruct the image from the raw data.
These “pictures” of course do not directly show a black hole, defined as the region of space inside a point of no return barrier known as the event horizon. They actually record portions of the flat pancake of hot plasma swirling around the black hole at high speed in what is called the accretion disk. Plasma is made up of high-energy charged particles. As the plasma spirals around the black hole, its accelerating particles emit radio waves. The blurry orange ring seen in the images is an elaborate reconstruction of those radio waves captured by eight telescopes scattered around Earth, collectively known as the Event Horizon Telescope (EHT).
The latest image tells the story of the epic journey of radio waves from the center of the Milky Way, providing unprecedented detail about Sagittarius A*. The image is also “one of the most important visual proofs of general relativity”, our current best theory of gravity, says Sera Markoff, an astrophysicist at the University of Amsterdam and a member of the EHT collaboration.
Studying supermassive black holes like Sagittarius A* will help scientists learn more about how galaxies evolve over time and how they come together in vast clusters across the universe.
From the galactic core
Sagittarius A* is 1,600 times smaller than the Messier 87 black hole that was photographed in 2019, and is also around 2,100 times closer to Earth. This means that the two black holes appear to be about the same size in the sky. Geoffrey Bower, EHT project scientist at Academia Sinica’s Institute of Astronomy and Astrophysics in Taiwan, says the resolution required to view Sagittarius A* from Earth is the same as that required to take a photo. of an orange on the surface of the Moon.
The center of our galaxy is 26,000 light-years away, so the radio waves collected to create this image were emitted around the time one of the first known permanent human settlements was built. The journey of radio waves began when they were first emitted by particles in the black hole’s accretion disk. With a wavelength of about 1 mm, the radiation traveled towards Earth relatively undisturbed by intervening galactic gas and dust. If the wavelength was much shorter, like visible light, radio waves would have been scattered by dust. If the wavelength was much longer, the waves would have been bent by clouds of charged plasma, distorting the image.
Finally, after the 26,000 year journey, the radio waves were picked up and recorded in the radio observatories spread across our planet. The large geographic separation between the observatories was key – it allowed the consortium of researchers to detect extremely subtle differences in the radio waves collected at each site through a process called interferometry. These small differences are used to infer the tiny differences in the distance each radio wave travels from its source. Using computer algorithms, scientists were able to decode differences in the path length of radio waves to reconstruct the shape of the object emitting them.
The researchers put all of this into a false-color image, where orange represents high-intensity radio waves and black represents low-intensity. “But each telescope only picks up a tiny fraction of the radio signal,” says Fulvio Melia, a University of Arizona astrophysicist who has written about our galaxy’s supermassive black hole. Because we’re missing a lot of the signal, “instead of seeing a perfectly clear photo, you see something a little hazy…a little fuzzy.”
The image provides insight into the black hole’s event horizon – the closest point at which anything can approach the black hole without being sucked in. Beyond the event horizon, not even light can escape.
From the image, scientists were able to better estimate the size of the event horizon and deduce that the accretion disk is tilted more than 40 degrees from the Milky Way disk, so we see the round face of the flat accretion disk, rather than the thin ribbon at its edge.
But even if the black hole’s accretion disk were oriented relative to Earth, the gravity around the black hole distorts the space around it so much that light emitted from the back of the black hole would be bent to come toward it. us, making a ring image regardless of its orientation. So how do scientists know its orientation? Because the ring is mostly round; if we looked at the accretion disk from the edge, then the ring would be more flattened and oblong.
Markoff believes this new ability to peer into the core of our galaxy will help fill gaps in our understanding of galaxy evolution and the large-scale structure of the universe. A dense, massive object such as a black hole at the center of a galaxy influences the movements of nearby stars and dust, and this influences how the galaxy changes over time. The properties of the black hole, such as the direction it spins, depend on its collision history – with stars or other black holes, perhaps. “A lot of people…look up at the sky and think everything is static, right? But it’s not. It’s a huge ecosystem of things that’s evolving,” says Markoff.
So far, the fact that the image matches scientists’ expectations so precisely makes it an important confirmation of current theories in physics. “It’s been a prediction we’ve had for two decades,” Bower says, “that we would see a ring of this magnitude. But, you know, seeing is believing.
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