Milky Way's Sagittarius A* Black Hole Imaged

How the supermassive black hole at the centre of the galaxy has been imaged by the Event Horizon Telescope team...
24 May 2022

Interview with 

Ben McAllister, Swinburne University of Technology


An artist's impression of a supermassive black hole


Over 100 years ago Albert Einstein predicted the existence of black holes: regions of the Universe so dense, with gravity so strong, that nothing - not even light - can escape. Whilst we’ve been pretty sure that black holes exist for a long time, it is actually only fairly recently that we’ve been able to see them. You might remember back in 2019 when an international collaboration of scientists released the first ever image of a black hole. It was captured with a device called the Event Horizon Telescope. This first black hole was 55 million light years away and huge, two factors that made the job a little easier. But now the same team are back, and they’ve pushed the envelope to show us what our own galaxy’s black hole looks like. Physicist Ben McAllister has the story…

Ben - Picture a vast, spinning galaxy, our home - the Milky Way. Billions of stars, spread across thousands of light years of space, rotating around a central point. At that central point, we now know lies an extremely large, extremely dense object: a supermassive black hole. And we know this because we’ve actually seen it.

Or rather, we can’t actually see the black hole itself - it’s completely dark because no light can escape -  but we can see its ‘shadow’ - a dark region surrounded by a bright ring of glowing gas just far enough away, and travelling just fast enough, to avoid being pulled in.

But, I’m getting ahead of myself. The Event Horizon Telescope collaboration - made up of hundreds of astronomers from all over the world - has recently revealed the first ever direct image of the supermassive black hole at the centre of our galaxy - called Sagittarius A-Star, about 4 million times the mass of the Sun. This is an astonishing feat, following up their similar work a few years ago, imaging a much larger black hole in a neighbouring galaxy.

So, how do you take a photo of something completely dark, 27,000 light years away? You need an exceptional telescope - or, in the case of Event Horizon, eight telescopes, distributed across the planet and all working together, “meshing” their data. Essentially, Event Horizon creates a ‘virtual telescope’ the size of the Earth.

The data used to make this new image were actually collected back in 2017, around the same time as the data for the last black hole image, but it’s taken longer to process. Since our own black hole is about a thousand times smaller than the last one, the fast moving gas which orbits it does so in a much shorter time frame, and so the image is changing a lot more rapidly, making the data processing more difficult.

Nevertheless, the researchers were stunned with how well the image agreed with Einstein’s predictions - made about 100 years ago - and said that these observations added to a deeper understanding of how these truly amazing, extreme cosmic objects behave.

It’s also incredible to think that the0. light the telescopes were imaging from the gas around Sagittarius A-Star has been travelling for 27,000 years across space to reach our network of radio dishes, distributed across the planet, and that astronomers can combine all that data to form a handful of beautiful, albeit slightly blurry images.

The Event Horizon team isn’t stopping there though! They are already working on new data processing techniques to improve the pictures, as well as telescope upgrades to take sharper images, and maybe even real-time movies.

So whilst we haven’t replicated Christopher Nolan’s ‘Interstellar’ just yet, we might not be far off seeing a real-life black hole on the big screen!


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