Image of the super massive black hole at the centre of the Milky Way released by a global research team EHT Collaboration
12th May 2022. Our home galaxy, The Milky Way, has long thought to contain a super massive black hole. This is something observed not just in the Milky Way, but in many other if not most galaxies.
Using a range of telescopes across a wide range of frequencies including radio and infrared, astronomers have been zeroing in on the heart of our galaxy. Stars close by were observed orbiting what became known as Sagittarius A* or shortened to Sag A *. The EHT uses a variety of telescopes working together to effectively form a telescope with an aperture about the size of the Earth itself. Years were spent combing data from all these sources as well as using supercomputers to combine and analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.
First image of the super massive black hole in the centre of the Milky Way
In 2019 the Event Horizon Telescope (EHT) produced the now iconic image of the area surrounding the black hole in the elliptical galaxy M87. While we can’t see a black hole itself as it either emits or reflects light, we can see gas swirling around, which is evidence of the existence of the black hole. However, capturing the giant at the heart of the Milky Way proved far more difficult.
Why was the black hole in M87 imaged before much closer Sagittarius A*?
As the Earth is located some 27,000 light years from the centre of the galaxy, there’s vast amounts of interstellar gas and dust blocking our view. The other aspect still intriguing astronomers is that while the super massive black hole is approx. 4 million times the mass of the Sun, the flow of matter into it is extremely low.
One of the EHT team members, Harvard astrophysicist Michael Johnson, commented during the press conference during the release of the image that “We only see a trickle of material making it all the way to the black hole. In human terms, it would be like eating just one grain of rice every million years.”
The speed of the gas orbing the Milky Way’s black hole also posed problems capturing this image. EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, US, explains: “The gas in the vicinity of the black holes moves at the same speed — nearly as fast as light — around both Sag A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sag A* it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sag A* was changing rapidly as the EHT Collaboration was observing it — a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The new image also show that the size of Sagittarius A*’s event horizon is 51.8 microarcseconds on the sky.
“Our image is in very close agreement with theoretical predictions,” said Nobel Prize laureate Andrea Ghez,Özel, who has described it as the biggest test of Einstein’s theory of general relativity ever made, noting that the theory passed with flying colours.
“It’s a great laboratory for trying to understand how gravity works in the vicinity of a supermassive black hole,” Ghez said.
The black hole at the centre of M87 is also some 1,000 time more massive than the one in the middle of the Milky Way. Despite it being further away from us, it appears the same size in the sky.
Black hole in the M87 and The Milky Way
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, said in a statement. “We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
Sag A* in comparison with both M87* and other elements of the Solar System such as the orbits of Pluto and Mercury. Event Horizon Telescope Collaboration