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On Thursday, May 12, astronomers unveiled the first image of the supermassive black hole at the center of our own galaxy. The image was the work of 300 researchers from 80 different institutes who worked for more than five years with the Event Horizon Telescope , a worldwide network of eight radio telescopes linked together to create a single "virtual" instrument the size of the Earth. The photo reminds me of looking down a well — a very deep well where imagination has no limit.
We've long known of the Milky Way's bulky black hole because astronomers have tracked stars orbiting a massive, invisible object at the galaxy's center. Based on their orbital tracks, something with a mass four million times greater than the sun holds them in place, suggesting that the object — called Sagittarius A* or simply "sadge-ay-star" — could only be a black hole.
Today's photos provide the first direct visual evidence of the object. It was no small achievement: seeing this unholy doughnut is equivalent to spotting an actual doughnut on the moon.
Remarkably, it looks very much like the model of a black hole astronomers have derived based on observation and good, old physics, with a central "shadow" surrounded by a glowing ring of light. The light represents radiation released from material heated to trillions of degrees as it spirals down the hole on a one-way ticket to oblivion. Thanks, Einstein! His Theory of General Relatively predicted the existence of these bizarre objects long before their discovery.
The gravitational pull of a black hole is so tenacious that nothing, not even light, can escape its pull, the reason it appears black. Most black holes form in the aftermath of a supernova explosion, when the core of a failing supergiant implodes, compressing itself into an object so dense it literally disappears from sight.
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Single black holes are understandable. But astronomers still don't know exactly how the supermassive variety forms, and we detect them in the cores of almost every galaxy we see. One path might be through collisions of stars in compact star clusters that would lead to the formation of multiple smaller black holes. Over time, these would drift to the center of the galaxy and merge to form a single supermassive black hole.
Sgr A* sits at the very center of the Milky Way 27,000 light-years away, but it's invisible in regular optical telescopes because it's hidden behind clouds of light-blocking cosmic dust. Telescopes that can see in other "colors" of light such as infrared and radio light render it visible.
The EHT achievement follows the collaboration’s 2019 release of the first image of a black hole, called M87* (M87 star), at the center of the more distant Messier 87 galaxy 54 million light-years away. The Milky Way's black hole was much more difficult to pin down:
“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 to 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 (‘CK’) Chan.
What's up next? Upgrades and expansion of the telescope network are planned that will allow scientists to share even clearer images as well as movies of black holes in the near future. Movies? I can't wait!
