There's a monster twirling around in the centre of our galaxy, and its portrait has finally been unveiled.
Overnight, the international Event Horizon Telescope (EHT) crew revealed an image of superheated gas coursing around and falling into Sagittarius A* or Sgr A*, the supermassive black hole in the Milky Way's core.
It's the culmination of five years of simulations and data crunching.
And while it might look a bit like a glazed donut, there's more to the new image than meets the eye.
For one, it tells us the black hole is 4 million times the mass of the Sun — a figure physicists suspected, but is now confirmed.
The black hole's spinning too, but it’s skew-whiff — slightly tilted face-on to us.
But despite this veritable goldmine of information about our galaxy's black hole, there's still plenty we're yet to discover.
What's so special about Sgr A*?
Well, for one, it's our supermassive black hole.
"It's home," said Jessica Dempsey, an Australian astrophysicist and member of the EHT team.
"That's why this one is special to a lot of people. The hunt to understand what is going on at the centre of our galaxy is hundreds of years old."
And while it may not be the biggest black hole, Sgr A*'s proximity means it's our best bet for understanding how it and its counterparts behave.
"As our instruments on the ground and in space improve our understanding, the Milky Way black hole is going to go a long way to unpacking general relativity, and how that works with quantum mechanics," said Dr Dempsey, former deputy director of the East-Asia Observatory in Hawaii.
Understanding more about the Milky Way’s hefty heart can give clues as to how our galaxy formed.
"And maybe what we can learn from Sgr A* we can start to look for … in other galaxies," she said.
An energy-inefficient giant
One of the biggest ongoing questions in black hole physics is exactly how they collect, ingest and expel material orbiting them at near light speed in a process known as "accretion".
This process is fundamental to the formation and growth of planets, stars and black holes of all sizes, throughout the universe.
Despite the brightly spiralling gas and dust in the image, Sgr A* was not "eating" as much matter as the team had expected.
While some black holes can be remarkably efficient in converting gravitational energy into light, Sgr A* traps and hangs onto nearly all of this energy.
"It converts only one part in 1,000 into light," Dr Johnson said.
And unlike the gargantuan black hole in the galaxy M87, an image of which was released in 2019, Sgr A* is not blasting an enormous jet of X-ray energy into space.
But it might have a weak jet, Dr Dempsey said, based on as-yet unexplained peculiarities in how it rotates and accretes matter.
If a jet is indeed there, the EHT can't yet see it, but research published late last year suggests a weak jet might be present.
While the EHT was gazing at the black hole, three X-ray telescopes kept an eye on it too. They spotted X-ray flares — or outbursts — from Sgr A*. Signs of a jet? Perhaps.
Black hole blanks to fill
James Miller-Jones, an astrophysicist at Curtin University and the International Centre for Radio Astronomy Research, said measuring the polarised light thrown off by the black hole's surroundings would tell us about its magnetic field.
It's something the EHT team reported, last year, about M87.
"Sgr A* seems to have a strong, dynamically significant magnetic field, which means it's a magnetic field strong enough to affect the motion of the plasma around the black hole," Professor Miller-Jones said.
Alister Graham, an astrophysicist at Swinburne University of Technology, hoped to find out just how fast Sgr A*'s spin was.
"Black holes can spin at significant fractions of the speed of light, but I sensed [the EHT team] was unable to get an accurate read on this."
Another mystery that's yet to be solved is pinpointing the launch site of plasma jets that blew up the colossal twin bubbles in the Milky Way, he added.
So how will we answer these questions? First, let's look at how astrophysicists managed to peek through a cosmic curtain of stars and gas to the black hole within our galaxy.
(Radio) lights, (telescope) camera, action!
Over a handful of nights in April 2017, when skies were clear, eight observatories from Antarctica to Europe simultaneously focused their gaze onto the centre of our galaxy, each tuned to record light with a wavelength of 1.3 millimetres.
These are radio waves — invisible to our eyes, but spat out in abundance by the incredibly hot, turbulent gas swirling around and falling into the black hole, which produces the donut-like image.
Because the EHT observatories were separated by vast distances, each telescope received the same radio signals from the Milky Way's centre at slightly different times.
Each radio signal data point was "stamped" at its telescope by an atomic clock so precise that over the course of 100 million years, it would lose only a second.
When it was time to combine the data, these time stamps let physicists synchronise the slew of signals and generate a sharper image.
This linked-telescope technique, called Very Long Baseline Interferometry, essentially produces a telescope the size of the planet — and one with a resolution so high, it could, in theory, spot a ping pong ball on the surface of the Moon.
So how can it be improved? Funny you should ask …
Did someone say more telescopes?
The EHT has been training its radio-ready eyes on Sgr A* again — and on yet more objects — in the years since its first observations in 2017.
More observatories have joined the EHT network since, which is already making a "really huge" difference, Dr Dempsey said.
More "eyes" means the EHT can collect more light, increasing its sensitivity and its ability to spot fainter features.
"The more elements we bring in, the more sensitive we become, and the more certain we can be of fitting what we see … to the model," Dr Dempsey said.
"And the most critical part for Sgr A* is we can do those snapshots faster."
This means the team will eventually be able to take images on timescales they need to produce a movie that captures dynamic features such as the rotation of the black hole, and the gases tumbling around it.
Already, the EHT has a spatial resolution some 5,000 times better than the Hubble Space Telescope, giving the EHT a "whopping improvement" in the ability to spy objects at vast distances, Professor Graham said.
But to make out finer details, we'll need more telescopes. Not on Earth, though.
"Having a radio telescope in space will offer further gains in resolution, as will having one on the Moon," Professor Graham said.
That's because the further apart the network's telescopes are, the better their spatial resolution is.
Plans are afoot to send a 10-metre-wide radio telescope dish some 1.5 million kilometres into space, where the gravitational tug of Earth and the Sun will hold it in place.
When incorporated into the Earth-based network, the telescope — called the Millimetron Space Observatory — should give the EHT a 150-fold improvement in resolution.
The mission is led by the Russian Academy of Sciences, and is, so far, slated for launch in 2030.
Seeing in a different light
Tuning the EHT's radio dishes to pick up light of different wavelengths will give astrophysicists different representations of the black hole too.
Detecting shorter wavelengths — less than a millimetre — should provide a sharper view through our galaxy's disc, Professor Miller-Jones said.
Comparing the brightness of the black hole's gassy ring at different wavelengths — say, if it appears brighter in one wavelength than the other — could reveal some of its physical processes.
"With the next generation [EHT] facility, it will be very exciting to test our models of the environment around the black hole, and what we understand about the processes of how gas flows around it," Professor Miller-Jones said.
"All of that will be very, very interesting in the years to come."
So there will be, no doubt, plenty more never-before-seen insights into some of the most mysterious phenomena in the universe — including our galaxy's black hole.
"I personally love results that open up more questions than answers — and this [new image] is definitely one of those," Dr Dempsey said.
