The first snapshot of the supermassive black hole at the center of the Milky Way showed a placid “gentle giant,” but astronomers now say our galaxy instead hides more of a sleeping monster.
An international team in May released the first images of the jumbo black hole, called Sagittarius A*, some 4.1 million times heavier than the sun, in simultaneous news conferences worldwide.
Astronomers lucked out to catch Sagittarius A* having a quiet decade, it turns out. And Earth is lucky to be close enough to see it — but not too close. The image showed a red-ringed darkness, a view of superheated particles zipping around the stark “event horizon” surrounding the black hole from which even light can’t escape. Peeks at the black hole provide a check on Albert Einstein’s predictions of how gravity bends both space and time at its most extreme, and offer insight into how its encircling ring behaves at temperatures far higher than anything seen anywhere else.
In reality, say astronomers, we are only here to see it because Earth is far enough away from the Milky Way’s center to have not been fried by one of the black hole’s violent past outbursts.
“It’s not like a friendly, mellow, really restful environment near Sagittarius A*,” said Daryl Haggard, a black hole astronomer at McGill University and member of the international team that released the May image. The black hole erupts with thunderous X-ray flares every few decades after eating a star, according to observations made in the last decade by NASA spacecraft. “It’s actually in this very messy, hot dynamical region of our galaxy,” she said.
Galaxies are the vast islands of stars filling space. Most have their own supermassive black hole at the center, much like the one in our own pinwheel-shaped Milky Way galaxy. Observations of the remnants of past Sagittarius A* blasts show it has likely fried countless stars and planets across time, stripping them of atmospheres, said Sera Markoff, an astrophysicist at the University of Amsterdam. And from the centers of other galaxies, astronomers often observe high-energy jets blasting outward for thousands of light years.
“You can actually see the jets basically engulfing stars, ones like our star system, inside those jets,” said Markoff. The first black hole image scientists ever captured, for example, was from a distant galaxy called M87, which was viewable in 2019 because it is so active. (That galaxy hosts a much heavier black hole, weighing 6.5 billion suns and making Sagittarius A* look “piddly,” added Markoff.)
Fortunately, Earth is some 26,700 light years away from Sagittarius A* (one light year is about 5.9 trillion miles), and our planet formed long after the galaxy’s most active era. This means that our sun circles the galaxy outside the reach of today’s worst blasts, within the “habitable zone” for planets, one of the lucky factors that makes life possible on Earth.
“So, in a sense it is right to say we are fortunate,” said Amedeo Balbi, a professor of astronomy and astrophysics at Tor Vergata University of Rome. In the star-rich center of our galaxy, X-ray flares still fire off randomly when a star falls too close to Sagittarius A* and is shredded, he added, which wrecks the habitability of planets there, blanketed by high-intensity blasts.
Scientists also just lucked out to catch the black hole in a contemplative mood, not flaring, to capture that first picture. The image itself is an astronomical tour-de-force, equivalent to taking a long-distance photo of a doughnut sitting on the moon through obscuring clouds.
“It’s really a beautiful thing to have started with fairly quiet observations where we can really find the black hole shadow,” said Haggard. She now hopes to see X-ray blasts from Sagittarius A* and other nearby galaxies, she added:
“Now those black holes can just go as crazy as they want.”
Thanks to Lillian Barkley for copy editing this article.