Milky Way’s supermassive black hole seen in new image

According to Einstein, all galaxies have a giant black hole at their centre, including ours.

However, the nature of black holes themselves - places where light and matter cannot escape - makes them extremely difficult to photograph. Therefore, it has not been until this week that scientists have been able to confirm the existence of a supermassive black hole at the centre of the Milky Way, and take a picture of it.

The team of astronomers, which was part of the global Event Horizon Telescope (EHT) Collaboration, used observations from a worldwide network of eight radio telescopes on our planet – collectively forming a single, Earth-sized virtual telescope – to take the stunning image.

Until now, scientists strongly suspected the existence of this black hole, as they observed stars orbiting around something invisible, compact, and very massive at the centre of the Milky Way. However, any previous efforts to capture the supposed black hole found it too jumpy to get a good picture. There were even alternative theories regarding the giant object, such as it being made of “dark matter”, but the image unveiled provides overwhelming evidence of the existence of the black hole, named Sagittarius A* (Sgr A*).

The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. Sgr A*, placed near the border of Sagittarius and Scorpius constellations, was also the exact size predicted by Einstein’s equations.

The University of Arizona’s Feryal Ozel called the black hole, “the gentle giant in the centre of our galaxy”.

The use of the word “gentle” refers to the fact that scientists had expected the Milky Way’s black hole to be more violent, but it turned out to be a "more cooperative black hole” than the team expected. Moreover, in contrast to other black holes, which gobble up galactic material, this one is “eating very little”; the equivalent of a person eating a single grain of rice over millions of years.

“It is the cowardly lion of black holes,” said project scientist Geoffrey C Bower of Taiwan’s Academia Sinica Institute of Astronomy and Astrophysics.   

It is precisely the fact that the black hole “is on a starvation diet” with very little material falling into its centre, that has allowed astronomers to photograph it. Although scientists cannot see the black hole itself, the glowing gas around it reveals a tell-tale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure.

To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The EHT observed Sgr A* on multiple nights in 2017, collecting data for many hours in a row, similar to using a long exposure time on a camera. 

The EHT team's results are being published today in a special issue of The Astrophysical Journal Letters. The same team that worked on the project also published the first image of a black hole in 2019, which reflected M87*, a black hole from a galaxy 53 million light-years away. The Milky Way black hole is much closer, about 27,000 light-years away.

Despite the huge distance between Sgr A* and M87*, the two black holes look remarkably similar.

"We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar,” said Sera Markoff, Co-Chair of the EHT Science Council. "This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.” 

To take the picture, the researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, this was not the case for the Milky Way's black hole. The image of Sgr A* is actually an average of the different images the team extracted. 

The effort was made possible by the work of more than 300 researchers from 80 institutes around the world. The team worked rigorously for five years, using supercomputers to combine and analyse their data while compiling an unprecedented library of simulated black holes to compare with the observations. Overall, the project cost nearly $60m (£49m) with $28m coming from the US National Science Foundation.

This feat of astronomy has greatly improved scientists' understanding of what happens at the very centre of our galaxy. The team has already begun using the new data to test theories and models of how gas behaves around supermassive black holes, a process thought to be fundamental in shaping the formation and evolution of galaxies.

The ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.