There's a monster lurking in the heart of our galaxy, and we may get our first glimpses of it tonight.
An international team of scientists has announced it will reveal a "groundbreaking" discovery at our galactic centre at 11pm AEST — and you'll be able to watch it live.
There are rumbles the Event Horizon Telescope (EHT) will unveil images of Sagittarius A*, the supermassive black hole in the middle of the Milky Way.
In 2019, the EHT captured the first-ever images of the swirling mass of superheated dust and gas surrounding a supermassive black hole at the centre of a galaxy 55 million light-years away called M87.
The image of M87's black hole silhouetted by a bright "ring" of emissions bent by gravity was a "huge day in astrophysics", according to France Córdova, director of the US National Science Foundation at the time.
"We're seeing the unseeable."
But top of the bucket list for the EHT — a planet-wide network of 12 observatories — is the black hole lurking at the centre of our galaxy.
Sagittarius A*, or Sgr A* for short, lies 27,000 light-years away, and is a lightweight in the supermassive stakes, at only about 4 million times the mass of our Sun.
Will we see its event horizon? If so, will it look like the image snapped of M87?
Getting a glimpse of our cosmic monster will help us understand the evolution of our galaxy, says Joss Bland-Hawthorn, an astrophysicist at the University of Sydney who studies Sgr A* but is not involved with the EHT.
"No-one knows for sure [what the image] will be, but everyone imagines it's going to be different from just seeing a shadow," Professor Bland-Hawthorn said.
Here's what we know so far about the behemoth parked in the middle of our galaxy.
How do we know a black hole is there at all?
Most galaxies have a supermassive black hole more than a million times the mass of our Sun at their heart.
While astronomers suspected for decades our galaxy might harbour one, it was not pinned down until two separate teams in the US and Germany started studying the precise movements of stars around our galactic centre in the 1990s.
"They tracked these stars over the course of 20 or so years, and realised that the stars were moving under the influence of a very, very massive, dense, dark blob," Professor Bland-Hawthorn said.
Their measurements indicated the blob was about 4 million times the mass of our Sun.
Andrea Ghez and Reinhard Genzel, who led the teams, shared the 2020 Nobel Prize in Physics for their work.
What do images we already have tell us?
We cannot see black holes directly. At their centre, gravity is so strong that not even light can escape.
To snap a portrait of a black hole, the EHT detects light beamed out by hot gas swirling around the edge of the black hole's disc.
But until now, our view of the black hole at the centre of our galaxy has been limited to the effects of its immense gravitational tug on material whizzing around it.
In 2010, we got the best look at the centre of our galaxy when the Chandra X-ray Observatory took a snapshot that showed the remains of a massive explosion near Sgr A* and large bubbles of hot gas extending for a dozen light-years on either side of the black hole, as well as mysterious X-ray filaments.
While our galaxy's central black hole flings nearby stars back out into space, and there may be a weak jet of X-rays coming from its centre, it is relatively quiet compared to other black holes.
But the large bubbles first identified in X-rays by Professor Bland-Hawthorn's research, then in gamma rays by the Fermi Gamma-ray Space Telescope, show it wasn't always like this.
Professor Bland-Hawthorn said these billowing clouds, known as Fermi bubbles, showed something happened at the centre of the galaxy 2 to 3 million years ago.
"The evidence is overwhelming that [the bubbles were] powered by a past explosion from the black hole," he said.
How does Sgr A* compare to the black hole in M87?
At 6.5 billion times the mass of the Sun, the black hole in the centre of M87 is one of the largest supermassive black holes in our neck of the universe.
So how does it compare to Sgr A*?
For one, M87's black hole is more massive — way more.
Generally, the bigger the galaxy, the heftier its central black hole. Our galaxy, the Milky Way, is about half the size of M87 and contains about a 10th of the stars, so it follows that our supermassive black hole would be smaller too.
Sgr A* is also nowhere near as active as M87, which is feeding on gas and stars and blasting out radiation.
"So we don't have a lot of radiation that helps us generate some kind of a shadow," Professor Bland Hawthorn said.
And because SgrA* is changing much faster than the lumbering M87*, it has been much more challenging to get images that aren't blurred.
What might we see tonight, then?
The EHT team previously modelled what it suspected the black hole might look like in short radio wavelengths of 1.3 millimetres.
Like the image of M87's black hole, the simulation of Sgr A* is fuzzy due to the distortion of gas and dust around the black hole and its crushing gravity.
Given the hype, images of Sgr A* will likely be more than the gold-and-orange donut of M87's emission ring, Professor Bland-Hawthorn said.
"I'm going to predict some kind of a jet phenomenon or some kind of focused collimated something, like a jet or a bubble blasting off right from the black hole region."
Professor Bland-Hawthorn and colleagues have published evidence of a weak jet coming from the centre of the Milky Way.
Jets are a phenomenon seen in other galaxies, including M87 and Centaurus A's central black hole, as captured by the EHT and reported last year.
James Miller-Jones, an astrophysicist at Curtin University and the International Centre for Radio Astronomy Research, expects to see images of a supermassive black hole with a more dynamic environment than what the EHT captured in 2019.
That's because the timescales during which a black hole's surrounds change — such as variations in the hot, turbulent gas swirling around it — get longer the more massive the black hole is.
For hefty M87*, changes in its environment are predicted to happen on the scale of days to weeks.
The more diminutive Sgr A*, on the other hand, should experience changes in the order of minutes, Professor Miller-Jones said — certainly fast enough to be detected during a handful of hours of observation.
"It is unclear whether the observed emission will come from the hot gas swirling around it, or from a low-level jet, either of which could vary with time."
Yet another possibility is that the supermassive black hole is not one, but two black holes, circling each other in a colossal waltz.
Pairs of black holes dwelling in galactic centres are not unusual, says Alister Graham, a Swinburne University astrophysicist who studies supermassive black holes.
For instance, last year he and colleagues reported that the galaxy NGC 4424 houses a pair of supermassive black holes, brought together during a galactic merger.
So if more than one supermassive black hole was found in the centre of our galaxy, "it would be a remarkable measurement and one that has long been expected", he added.
"Our Milky Way has many debris trails from smaller galaxies … which have been captured and torn apart.
"Some of these captives may well have delivered massive black holes that are now on their way to, or already at, the centre of our galaxy."
