Scientists have captured the first image of the monster black hole lurking at the heart of our galaxy.
The black hole, known as Sagittarius A* or Sgr A*, lies 27,000 light-years from the Sun.
All our previous knowledge about the supermassive black hole were based on indirect measurements of stars looping around the galactic centre, snared in its gravitational grip.
"Until now we didn't have the direct picture confirming Sgr A* was indeed a black hole," said Professor Feryal Özel of the University of Arizona.
The historic results were presented overnight in six papers published in the Astrophysical Journal Letters.
Getting an image of our cosmic monster was "not an easy journey", Professor Özel said.
"It was extra challenging because … Sgr A* burbled and gurgled as we looked at it."
But the image confirms predictions that have been made by decades of work and proves yet again that Einstein's theory of general relativity was right.
"Because the size of a black hole shadow is proportional to its mass, our image tells us that the mass of Sgr A* is 4 million times greater than that of our Sun," said Sara Issaoun, an astrophysicist at Harvard & Smithsonian Center for Astrophysics and Radboud University in the Netherlands.
"It is really incredible that this prediction from Einstein's theory of general relativity matches the mass measured by the Nobel Prize-winning studies of stellar orbits in our galactic centre."
But that's not to say there weren't some surprises.
Two images, two very different black holes
The first snapshots of Sgr A* were captured by the Event Horizon Telescope (EHT), a planetary wide network of telescopes stretching from Antarctica to Europe.
Capturing images of black holes is extremely challenging because the gravitational pull at the centre of a black hole is so strong that not even light can escape.
But the EHT can see the light beamed out by super-hot gas swirling around the edge of the black hole's disc.
In 2019, the EHT released the first-ever images of the swirling mass of superheated dust and gas surrounding a supermassive black hole at the centre of a galaxy 53 million light-years away called M87.
But the two black holes are very different.
At around 4 million times the mass of our Sun, Sgr A* is much smaller than M87, which weighs in at around 6.5 billion times the mass of our Sun.
And it is much more dynamic than M87, changing on the scale of hours, rather than months.
That makes it very challenging to take an image of, said Katie Bouman of Caltech.
"Its appearance can change from minute to minute."
But the power of the telescope and the team's careful analysis meant the image was incredibly precise — even though it looks like a bit of a fuzzy smudge.
"It looks blurry on the screen because you're only seeing a few pixels — but it's actually one of the sharpest images you've ever seen."
So, what are we seeing?
The image shows a face-on view of a cosmic monster, said Michael Johnson, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics.
"Even though we are peering through the galaxy, it's actually tilted towards us," Dr Johnson said.
To figure this out, scientists ran copious computer simulations of what Sgr A* should look like depending on different spin inclinations, then compared them to the real deal.
It might seem strange that a supermassive black hole would rotate in a different plane to its galaxy, but it makes sense when you think about how black holes feed, said James Miller-Jones, an astrophysicist at Curtin University and the International Centre for Radio Astronomy Research.
The middle of the Milky Way is dominated by a huge bulge of stars surrounding the black hole.
And if a star strays too close to Sgr A* and gets ripped apart and dragged in, or some of the gas blown off by those stars falls in, that matter transfers some angular momentum to the black hole.
"If you have an ice skater, and they're twirling around slowly, and someone throws them a metal object and they catch it, that will give them some extra momentum in that direction," Professor Miller-Jones said.
"It's the same with gas — as it falls into the black hole, it gets it to spin a little bit faster in that direction.
"And because this [gas] is coming in from all directions, the black hole's spin is gradually changed by small amounts."
Black holes also grow by merging with other black holes, which can happen when galaxies collide.
Such mergers would result in a black hole with a spin that combined the spins of the two initial black holes, Professor Miller-Jones added.
There were surprises in store
While the images confirm many predictions, there were also some surprises.
Despite the brightly spiralling gas and dust in the image, Sgr A* was not actually "eating" as much matter as the team had expected, Professor Özel said.
"It turned out to be a gentler, more cooperative black hole than we'd hoped for during our decade of simulating its environment," she said.
Sgr A* traps only a few times the energy of our Sun, Dr Johnson said.
"Sgr A* is the view of a standard black hole. It's quiet. Sgr A* is exciting because it is common."
The images don't match the simulations previously done by the team, said Jess Dempsey, an Australian astrophysicist who is part of the EHT team.
"The simulations that our amazing teams have done predict more variability than what we see in the data," said Dr Dempsey, former director of the East Asia Observatory in Hawaii, one of the telescopes in the EHT.
"We had a very good match with M87, we don't see that here. So something else is going on.
"We are starting to see suggestions in the rotation and how it eats and accretes its mass into itself that we don't understand yet," added Dr Dempsey, who is now the director of ASTRON in the Netherlands.
She said the work was just the beginning of our understanding of what is going on with the black hole lurking in the middle of our galaxy
"This is our first, really quick snapshot of something deep and complex," she said.
