Astronomers on Thursday unveiled the first image of a supermassive black hole that roils the center of our galaxy, its gravity so powerful that it bends space and time and forms a glowing ring of light with eternal darkness at the core.
By the standards of other supermassive black holes, the scientists said, the one at the heart of our Milky Way is relatively calm — as quiescent as something that gobbles stars and reaches temperatures measured in the trillions of degrees can possibly be.
Feryal Özel, a University of Arizona astronomer, described the achievement as “the first direct image of the gentle giant in the center of our galaxy.”
“We find a bright ring surrounding the black hole shadow,” she said. “It seems that black holes like doughnuts.”
The image was captured by a global consortium of astronomical observatories, known as the Event Horizon Telescope. Three years ago the project produced the first image of a black hole, in the galaxy Messier 87.
The black hole at the center of the Milky Way is referred to as Sagittarius A*, or Sgr A*, which is pronounced “sadge-ay-star.” It is more than a thousand times smaller than the black hole in Messier 87. But cosmically speaking, Sgr A* is the one closest to home.
The unveiling of the image Thursday at the National Press Club in downtown Washington was part of simultaneous media events on multiple continents. The image was kept under wraps pending the unveiling at precisely 9:07 a.m. Eastern time.
The achievement, supported by the National Science Foundation, relied on contributions from more than 300 scientists at 80 institutions, including eight telescopes. One of the telescopes is at the South Pole. The data collected took years to process and analyze.
Observations of the central region of the galaxy are hampered by intervening dust and ionized particles. The Earth’s turbulent atmosphere further blurs the picture. The black hole itself is not visible by definition, but it is encircled by a swirl of photons that can be detected by the huge radio dishes. The black hole is not a static entity: It is “gurgling,” Ozel said. The appearance changes regularly, challenging the scientists to produce a singular image that fit what their telescopes had observed.
And the coronavirus pandemic added its own challenges.
“The pandemic slowed us down but it couldn’t stop us,” Vincent Fish, a research scientist at the MIT Haystack Observatory, said at the news conference.
The work ultimately proved thrilling.
“What’s more cool than seeing the black hole at the center of our Milky Way?” said team member Katherine Bouman, a computational imaging scientist at Caltech.
There are potentially more cool things coming from the team, such as a movie of a supermassive black hole rather than a mere image. The new observations will keep theorists at work as well as they try to understand the nature of gravity in extreme environments, such as black holes.
Ozel expressed mild disappointment that the image did not generate a “crack” in Albert Einstein’s theory of general relativity, which describes gravity as a warping of space and time.
“This is an extraordinary validation of general relativity,” said Michael Johnson, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.
What’s at the very core of a black hole is a question that the scientists did not attempt to answer. “It is unknowable,” Fish said after the news conference.
“They are the most mysterious objects in the universe, and they hold the keys to large-scale structure in the observable cosmos,” Sheperd Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics and founding director of the Event Horizon Telescope, said in an interview in advance of Thursday’s briefing.
The Milky Way’s central black hole has until now been inferred from its effect on stars and dust in its vicinity, rather than directly observed. It is very far away — about 27,000 light-years — and despite its “supermassive” designation, is not very large in the grand scheme of things, making direct observation with telescopes extremely difficult.
That challenge led to the creation of the Event Horizon Telescope, which is not one telescope but a gaggle of them. The project uses an observational technique known as Very Long Baseline Interferometry, which requires careful calibration to allow multiple radio dishes spread across the planet to function as if they were a single, Earth-size instrument. The consortium claims that this technique allows resolution of distant objects that would be the equivalent of being able to spot a ping-pong ball on the moon.
Black holes come in two scales: “stellar-mass,” which form when stars collapse, and “supermassive,” the monsters that can weigh millions or even billions of times more than our sun and are what the Event Horizon Telescope is designed to detect.
“The black hole is attracting a lot of gas to it. Its gravitational pull is so strong that the matter around it can’t resist. But it’s pulling it into an extremely tiny space,” Doeleman said. “Imagine sucking an elephant through a straw.”
A black hole’s event horizon is the boundary of no return — the point at which an infalling piece of matter vanishes into an inescapable gravity well. Bizarre and mysterious as a black hole might be, Earthlings should understand that it poses no threat to our world and is essentially just a part of the galactic furniture.
As theorists teased out the implications of Einstein’s equations they realized that an object with sufficient mass would create a gravity well so severe that even light could not escape. But the idea of such black holes remained largely in the theoretical realm until the late 20th century. Gravitational waves from colliding black holes were discovered in 2016.
Decades ago, astronomers realized there was something in the heart of the Milky Way galaxy emitting tremendous amounts of radiation. It was the brightest object near the constellation Sagittarius. Was it produced by a black hole? That became the consensus.
Astrophysicists Andrea Ghez and Reinhard Genzel were awarded the Nobel Prize in physics in 2020 for discovering that stars in the Milky Way’s galactic center were moving in a pattern consistent with orbits around a supermassive black hole.
Astrophysicists think black holes are common at the center of galaxies — and are in some way intrinsic to galactic evolution — although the chicken-and-egg question remains unresolved. One possibility is that black holes are the seed of a galaxy. The other is that black holes form more gradually as stars fall into the central gravity well of the galaxy.
The 2019 image of the supermassive black hole in the galaxy Messier 87 has a mass roughly 6.5 billion times that of our sun and is producing a powerful jet of material that spews into deep intergalactic space. Most black holes aren’t so huge and flamboyant.
In that regard, the thing at the center of the Milky Way — however mind-bending and light-bending it may be — is ordinary at the cosmic level, Johnson said.
“Sgr A* is exciting because it’s common,” he said.