Supermassive black hole seen at the center of our galaxy
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.
The black hole, seen from Earth near the constellation Sagittarius, has a mass equal to more than 4 million suns. The new image shows it with three bright spots along a ring that, to the surprise of the scientists, tilts face-on toward the Earth.
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 Ozel, 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 more than a thousand times smaller than the one in Messier 87. But cosmically speaking, it is the one closest to home. The unveiling of the image at the National Press Club in 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. The data collected took years to process and analyze. The black hole itself is not static but changes in appearance on short time scales, challenging the scientists to produce a singular image that fit what their telescopes had observed. And the 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.
“They are the most mysterious objects in the universe, and they hold the keys to large-scale structure in the observable cosmos,” said Sheperd Doeleman, an astronomer at the Harvard-Smithsonian Center for Astrophysics and the founding director of the Event Horizon Telescope, 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-sized 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 which 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.
Albert Einstein’s 1915 general theory of relativity postulated that gravity is the result of massive objects bending the fabric of spacetime. 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.
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. The luminous astronomical object became known as Sagittarius A*.
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 believe black holes are common at the core 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.