black holes keep their secrets tight. They lock up forever whatever happens. Light itself cannot escape the hungry pull of a black hole.
So it seems that a black hole should be invisible – and it’s impossible to take a picture of it. Such great fanfare accompanied the release of the first image of a black hole in 2019. Then, in spring 2022, astronomers unveiled another photo of a black hole – this time of the one at the center of our own Milky Way.
The image shows an orange donut-shaped blob that looks remarkably similar to the earlier image of the black hole at the center of the galaxy Messier 87. But the Milky Way’s black hole, Sagittarius A*, is actually much smaller than the first, and was also more difficult to see, having to look through our galaxy’s hazy disk. So even though the observations of our own black hole were made at the same time as those of M87, it took three more years to create the image. This required international collaboration of hundreds of astronomers, engineers and computer scientists, and the development of sophisticated computer algorithms to assemble the image from the raw data.
Of course, these “photos” don’t directly show a black hole, defined as the region of space within a point-of-no-return barrier known as the event horizon. They are actually picking up bits of the flat pancake of hot plasma swirling around the black hole at high speed in what is known as the accretion disk. The plasma consists of high-energy charged particles. As the plasma spirals around the black hole, its accelerating particles emit radio waves. The fuzzy orange ring seen in the images is an elaborate reconstruction of these radio waves, captured by eight telescopes scattered around the 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 and provides unprecedented details about Sagittarius A*. The image also represents “one of the most important visual pieces of evidence for general relativity,” our currently 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 accumulate in massive clusters across the Universe.
From the galactic core
Sagittarius A* is 1,600 times smaller than Messier 87’s black hole imaged in 2019 and is also about 2,100 times closer to Earth. This means that the two black holes appear roughly the same size in the sky. Geoffrey Bower, an EHT project scientist at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, says the resolution required to see Sagittarius A* from Earth is the same as would be required to take an image of a Orange moon recorded on the surface of the earth.
The center of our galaxy is 26,000 light-years from us, so the radio waves collected to create this image were emitted around the time one of the earliest known permanent human settlements was established. 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 the intervening galactic gas and dust. If the wavelength was much shorter, like visible light, the radio waves would have been scattered by the dust. If the wavelength were much longer, the waves would have been bent by charged plasma clouds, distorting the image.
Finally, after the 26,000-year migration, the radio waves were picked up and recorded at radio observatories spread across our planet. The large geographic separation between the observatories was crucial – it allowed the research consortium to detect extremely subtle differences in the radio waves collected at each site through a process called interferometry. These small differences are used to derive the tiny differences in the distance each radio wave has traveled from its source. Using computer algorithms, the scientists were able to decipher the differences in the path lengths of the radio waves to reconstruct the shape of the object that emitted them.
The researchers put all of this into a false-color image, with orange representing high-intensity radio waves and black representing low-intensity. “But each telescope only picks up a tiny fraction of the radio signal,” explains Fulvio Melia, an astrophysicist at the University of Arizona who has written about our galaxy’s supermassive black hole. Because we’re missing much of the signal, “instead of a crystal clear photo, you’re seeing something that’s a little foggy…a little fuzzy.”
The image helps reveal more about the black hole’s event horizon — the point at which anything can get closest to 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 conclude that the accretion disk is tilted more than 40 degrees from the disk of the Milky Way, allowing us to see the round face of the flat accretion disk, rather than the thin sliver of its edge.
But even if the black hole’s accretion disk were aligned edge-to-earth, the gravity around the black hole distorts space so much that light emitted from the back of the black hole would be bent over to come toward us ring-like 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, the ring would be more squashed and elongated.
Markoff believes this new ability to peer into the heart of our galaxy will help fill in gaps in our understanding of how galaxies evolved and the large-scale structure of the Universe. A dense, massive object like a black hole at the center of a galaxy affects the movement of nearby stars and dust, and this affects how the galaxy changes over time. Properties of the black hole, such as which direction it spins, depending on its history of collisions – perhaps with stars or other black holes. “A lot of people… look up at the sky and think everything is static, right? But it is not. It’s a big ecosystem of stuff that’s evolving,” says Markoff.
The fact that the image has so far corresponded so closely to the scientists’ expectations makes it an important confirmation of current physical theories. “That’s a prediction we’ve had for two decades,” says Bower, “that we would see a ring of this magnitude. But seeing is believing.”
This article originally appeared in Interesting Magazine, an independent journalistic endeavor by Annual Reviews. Sign up for the newsletter.
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