Scientists had long inferred that, at the heart of our very own Milky Way Galaxy, there lurked a voracious, invisible monster, skulking in the shadows of warped space and time, consuming all that came its way.
They’d made the deduction through observations of how the stars and the galactic dust — how even the light itself — moved at the center of the galaxy.
Right there, they reasoned, right near the edge of the constellation Sagittarius, there must lie an unseen, incredibly compact, incredibly massive object, four million times the mass of our own sun, so much mass in such a small space that its gravity prevents even light from escaping.
They called it Sagittarius A*, a supermassive black hole of the type that scientists believe occupy the centers of most galaxies.
But, though they had many reasons to predict its existence and many clues as to its nature, until recently, they had never laid eyes on it, so to speak.
That changed Thursday, when astronomers, walking a fine line on the border of giddiness, announced that, after five years of work, they had the first direct visual evidence of the black hole at the center of the Milky Way.
Not only did that announcement — and that image — confirm that Sgr A* (pronounced “sadge-ay-star”) was, in fact, a black hole, but the imaging opened up valuable data and techniques that will enhance astronomers’ understanding of the structure of black holes and help them to search for others of its ilk elsewhere in the cosmos.
And — in passing — the imaging helped to support Einstein’s theory of relativity — the very scientist and theory that led to the postulated existence of black holes, long before they had ever been observed.
The image was created with the Event Horizon Telescope (EHT), a worldwide network of radio telescopes working together — in this case, eight telescopes in five different countries, including two in Hawaii and one in Antarctica — effectively creating one virtual Earth-sized telescope .
“Until now, we didn’t have the direct picture confirming that Sgr A* was indeed a black hole,” said Feryal Özel, professor of astronomy and physics at University of Arizona.
“Today, the Event Horizon Telescope is delighted to share with you the first direct image of the gentle giant in the center of our galaxy, Sagittarius A*.”
The data was collected over several nights in 2017, 3.5 petabytes of it. So much data that researchers were unable to send it via the internet. Hundreds of hard drives had to be physically shipped from each of the eight telescopes to the correlation centers in Westford, Mass., and Bonn, Germany, where supercomputers combined the signals.
In the image released Thursday, Sgr A* appears as a dark spot surrounded by a bright ring.
The ring is light escaping from the hot gas swirling around the black hole. The black disc at center is the region where light has crossed the event horizon — the point where the black hole’s gravity prevents anything — even light — from escaping.
In structure, Sgr A* appears very similar to the very first image of a black hole, a supermassive star in the galaxy Messier 87, which researchers released in 2019.
But that black hole is 1,500 times more massive than the Sgr A*, and 2,000 times further away. It’s highlighted by a gas jet that extends some 5,000 light years — almost a galactic beacon for astronomers.
By contrast, Sgr A* is more of a galactic wallflower. Though it’s much closer to Earth, it’s also much smaller and fainter and much less voracious. And observing it means astronomers and their telescopes have to pierce the galactic dust and clutter of the Milky Way.
“What made it extra challenging was the dynamic environment of Sgr A*, a source that burbled and gurgled as we looked at it,” Özel said.
The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sgr A* and M87*.
But where gas takes days or weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes.
“This means the brightness and pattern of the gas around Sgr A* were changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail,” said EHT scientist Chi-kwan Chan , of the Steward Observatory at the University of Arizona.
But, using massive amounts of data and different perspectives from the eight telescopes — the process is called interferometry, for the astronomically inclined — the shape of Sgr A* began to emerge.
Collating all that data allowed scientists to start making accurate measurements of the structure of Sgr A*, specifically the diameter of the bright ring around the black hole. Essentially, the diameter of the event horizon.
“The orbits of stars have given us an exquisite measurement of the mass of Sgr A* — about four million times the mass of the sun,” said Michael Johnson, an astrophysicist at the Center for Astrophysics | Harvard & Smithsonian.
“And Einstein’s theory of general relativity then predicts exactly what size shadow the black hole should cast. This theoretical feature — the shadow — is the image of the event horizon. It’s our line of sight into the black hole.”
When scientists compared their measurement from their direct EHT observations with that predicted by Einstein’s theory, the found they were the same.
“We found that these two completely different views of the black hole, one relying on the motion of stars, and one relying on the bending and capture of light were perfectly aligned,” Johnson said.
“This is an extraordinary validation of general relativity.”
The image of Sgr A* also now gives astronomers two datapoints for studying supermassive black holes, one at either end of the spectrum for size and voracity. The next step, they say, is to use the data they have to create movies, to watch how material moves around such massive gravitational pulls.
“EHT has turned the center of our galaxy into a cosmic laboratory,” Johnson said.
“We are peering into a new environment, the curved space-time near a supermassive black hole and it is teeming with activity — always burbling with turbulent energy and occasionally erupting into bright flares of emission.”
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