This is the second major discovery made by the Event Horizon Telescope (EHT), a global network of telescopes based at different locations, from Hawaiian mountains to the Chilean desert to the South Pole.

During an unprecedented, week-long observation campaign in April 2017, scientists used eight synchronised telescopes – which have a combined diameter as big as the Earth and resolution high enough to see a baseball on the moon – to zoom in on two supermassive black holes: one at the centre of our galaxy, and the other in the nearby galaxy Messier 87 (M87*).

The latter’s picture was released in 2019. In comparison, Sgr A* is more difficult to observe. Despite being much closer to us, it is relatively small and therefore the brightness of the surrounding gas has been changing rapidly, said Jiang Wu from the Shanghai Astronomical Observatory, during an online press briefing organised by the observatory.

“It’s like trying to take a clear picture of a mountaintop when the wind rises and the clouds begin to gather around it,” he said.

One of the telescopes used in 2017 was the James Clerk Maxwell Telescope, located 4,000 metres (13,000 feet) above sea level on Maunakea, Hawaii, and operated by the East Asian Observatory (EAO), of which China is a partner. It has a 15-metre (49-foot) wide main dish and is the world’s largest telescope specifically designed to work in the submillimetre wavelength.

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“EAO is really delighted to have been a part of the M87* and Sgr A* black hole shadow imaging experiments,” said Paul T.P. Ho, EHT board member and director of the EAO.

“The Asian regions have played a strong role in pushing for the highest angular resolution in astronomy, to make one of most important discoveries in science, and to open a new field of studying directly the black holes,” Ho said.

Seventeen astronomers from seven research institutions in mainland China contributed to data processing, theoretical analysis and modelling of the groundbreaking work.

Sgr A* is the closest and probably the most important supermassive black hole to humans, and scientists’ understanding of it has improved a lot over the past years, said astrophysicist Yuan Feng from the Shanghai observatory, who is also a member of the EHT collaboration.

Before direct imaging became possible, astronomers were detecting black holes via their influence on nearby stars, which orbit a black hole much faster than they would around a normal object of similar size.

Since the early 1990s, researchers in Germany and the US have separately monitored the orbits of stars near the galactic centre to estimate the location, mass and other key parameters of Sgr A*. Their work was awarded the Nobel Prize for physics in 2020.

The newly released image matches with theoretical predictions for Sgr A*, which is yet another testament to Albert Einstein’s theory of general relativity, Yuan said.

“Supermassive black holes are perfect natural laboratories for us to understand gravity,” which is the first fundamental force that humanity recognised but remains the least understood, he said.

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But M87* and Sgr A* might be the easiest black holes to observe from Earth: one is really big and one is really close. To hunt down more black holes in the future, EHT needs to go down to shorter wavelength for sharper pictures, Ho said.

Meanwhile, several new telescopes have joined the network to boost its resolution and sensitivity since 2017.

Scientists may even be able to take videos of Sgr A* in the future to better understand its dynamic process, said Shen Zhiqiang, director of the Shanghai observatory, who has been leading China’s efforts in the project.

“We are also planning to build China’s own submillimetre telescope so that more Chinese scientists can join the global, continuous observation of Sgr A*,” he said during the online press briefing in Shanghai.