Scientists have glimpsed the event horizon of a black hole for the very first time.

Key points: The world-first image shows the supermassive black hole at the centre of M87, a neighbouring galaxy to our own Milky Way

The world-first image shows the supermassive black hole at the centre of M87, a neighbouring galaxy to our own Milky Way The image is the result of a seven-year project linking telescopes all over the world to create 'a dish the size of the planet'

The image is the result of a seven-year project linking telescopes all over the world to create 'a dish the size of the planet' The fact the image matches so closely to predictions is a confirmation of Einstein's theory of general relativity

Until now, every image of a black hole you have ever seen has been an artist's impression.

"We've been studying black holes so long that sometimes it's easy to forget that none of us has actually seen one," said France Cordova, director of the US National Science Foundation, at one of seven simultaneous press conferences where the scientists announced their findings to the world.

"This is a huge day in astrophysics. We're seeing the unseeable."

The first ever image showing the event horizon of a black hole. It's the supermassive black hole at the centre of M87, a relatively nearby galaxy 55 million light-years from our own Milky Way. ( Supplied: Event Horizon Telescope )

The historic image shows a bright fringe of gas which is being squeezed, heated and accelerated as it falls towards the event horizon of a supermassive black hole at the centre of M87, a galaxy near our own Milky Way.

This cosmic monster sits 55 million light-years from Earth and is 6.5 billion times heavier than the Sun. Its event horizon is spherical in shape and about three times bigger than the path Pluto traces around the Sun.

But even though it's huge, it's incredibly difficult to see.

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Achieving the impossible

Observing black holes is a notoriously huge challenge because their gravitational pull is so strong that nothing — not even light — can escape once it crosses the event horizon, the point of no return.

Today's historic portrait is the result of decades of theoretical predictions and technical advances.

"This is an extraordinary scientific feat accomplished by a team of more than 200 researchers," said Dr Sheperd Doeleman from the Harvard-Smithsonian Centre for Astrophysics.

"We have achieved something presumed to be impossible just a generation ago."

He pioneered the instrument making it all possible: the Event Horizon Telescope (EHT), which is actually a network of radio telescopes spanning the globe.

Their combined observing power has been trained on two supermassive black holes, including the one in the centre of our own Milky Way galaxy, Sagittarius A*.

This is situated 26,000 light-years from Earth and is 4 million times the mass of our Sun, but by supermassive black hole standards, it is pretty small. The researchers say they are still analysing data from Sagittarius A*.

Their other target — the subject of Wednesday's image — is much bigger, but also much further away, at the centre of the nearby galaxy M87.

Over several nights in April 2017, the EHT turned its dishes towards M87 and collected vast quantities of data.

The files were so large they were too big for the internet; team members had to carry their findings around the world on hard drives.

This week, after two years of analysis, the EHT team called their global press conference.

Nobody outside the project knew exactly what they would be announcing, but they had declared it was "a groundbreaking result".

The finding is also described in a series of six research papers, all published today in a special issue of Astrophysical Journal Letters.

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What exactly are we looking at?

The bright ring in the image is caused by the incredible pull the black hole exerts on nearby matter.

It's surrounded by a swirling disc of gas, which gets superheated and emits bright radio waves as it accelerates towards the event horizon — getting very, very close to the speed of light.

"You can see that one side of that ring is brighter than the other, and that's the side that's coming towards us as the whole thing spins," explained University of Queensland astrophysicist Professor Tamara Davis.

"That was also predicted by relativity — that if it was spinning, and most things do tend to spin, then it would have one side that was brighter than the other."

Einstein's theory of general relativity first predicted the existence of black holes, as well as mapping out how heavy such objects would warp the fabric of space-time and bend the path of light.

It's those mind-bending ideas, Professor Davis said, that probably explain why we can see the orange ring in all its glory.

Although the blazing, spinning disc of material passes behind the black hole, from our perspective, the light actually curves right around the black hole — so that telescopes on Earth can still catch it.

"It gets emitted and bent, forming the visible ring that we can see, with the black hole in silhouette and the ring around it."

A telescope the size of a planet

The EHT initiative kicked off seven years ago with the aim of directly observing the immediate environment of a black hole.

One of the telescopes in the network is the James Clerk Maxwell Telescope on top of Mauna Kea peak in Hawaii, where Australian Jessica Dempsey is deputy director.

"We've made a dish the size of the planet," she told ABC's Catalyst earlier this year.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 4 minutes 36 seconds 4 m 36 s The stars must align to get the first glimpse of a black hole

By combining results from nine separate dishes, scattered from Antarctica to Europe, Dr Dempsey and her colleagues can create a virtual telescope 9,000 kilometres in diameter, making it the world's biggest camera.

"To give you an idea of how small a thing you can see, if you're sitting in a pub in Perth, you would be able to see a guy sitting in the pub in Sydney, not only would you be able to see him, you'd be able to see his eye colour, and you'd be able to see the brand of beer he was drinking," she said.

Getting this global telescope network in sync has been an exercise in precision. The operators had to know the timing of the signals at every one of these telescopes to a billionth of a second to make sure they were all looking at the same thing at the same time.

These locations included volcanoes in Hawaii and Mexico, mountains in Arizona and the Spanish Sierra Nevada, the Chilean Atacama Desert, and Antarctica.

"We are stacking impossible task on top of impossible task and this shouldn't work," Dr Dempsey said.

It shouldn't — but it did, as Wednesday's announcement made clear.

What does it all mean?

Today's discovery is a also test that goes to the heart of physics. The conditions near the event horizon of a supermassive black hole are so extreme that they put unprecedented pressure on Einstein's laws.

Professor Davis said she was "dumbstruck" when she saw the image.

"It's crazy. I didn't expect that it would be quite that good. It looks beautiful — and just exactly like the simulation says it should."

Even under these most extreme of conditions, the predictions and modelling have been spot-on.

"It would be a massive surprise to us if general relativity's predictions of what we expect to see were not correct," Professor Davis said ahead of the announcement.

"But that's why we're looking — because the really interesting physics comes from the surprises, the things that we don't know how to explain."