This image is taken by the Event Horizon Telescope (EHT) and shows the shadow of the black hole at the centre of our own galaxy, the Milky Way -Event Horizon Telescope Collaboration
This image is taken by the Event Horizon Telescope (EHT) and shows the shadow of the black hole at the centre of our own galaxy, the Milky Way -Event Horizon Telescope Collaboration

Our galaxy is huge. Our Sun is just one star amongst more than 200 billion in the Milky Way. We like to think that we are special. But our Sun, along with the Earth, is located in the outskirts of our galaxy. We are so far away from the centre of the galaxy, that even travelling at the speed of light, it would still take 26,000 years to get to it.

But the centre of the galaxy is a busy place. There are a lot more stars in that region compared to our place in the galaxy. But there, right at the centre, is also a giant black hole.

Now the existence of black holes was first predicted as a consequence of Einstein’s General Theory of Relativity. The gravity of such objects is supposed to be so intense that not even light can escape from it — hence the name, ‘black hole’. This inference was so strange that Einstein himself doubted their actual existence.

But nature does not rely on scientific reputations. Later work showed that stars much larger than our Sun would indeed end up in the strange state of black holes. Our Sun will never become one. Instead, it will end its life as a white dwarf, a weird object but considerably less strange than a black hole. I don’t know if we should feel happy or sad about our Sun.

Astronomers have unveiled the first image of the supermassive black hole that resides at the centre of our own Milky Way galaxy. This result offers new insights on how giant black holes interact with their surroundings

As you can imagine, it is challenging to find black holes. But they cannot hide themselves completely. When material falls on to a black hole, it usually forms a disk. The material in the disk is sped to incredibly high speeds before it falls into the ‘event horizon’ — the place of no return. Nothing can escape the event horizon, but the disk around it can get very hot due to friction, and can betray its presence by emitting X-rays or other forms of light.

In our own galaxy, astronomers have occasionally detected stars orbiting invisible partners. Cygnus X-1, for example, is one such system, where the disk is so hot that it emits light in the form of high-energy X-rays. We do not see the black hole directly, but all indications suggest that it is one.

These black holes are the end-state of stars that are 20 to 50 times more massive than our Sun. However, astronomers have also discovered black holes that are millions or even billions of times more massive than our Sun. These are called supermassive black holes and they reside almost exclusively at the centre of all large galaxies. Their formation is still a mystery. One such supermassive black hole was depicted in the 2014 sci-fi movie Interstellar.

In 2019, however, astronomers took the image of the shadow of the event horizon of a supermassive black hole at the centre of M87, a galaxy located 55 million light years away. This was the first time humans had directly seen the area right around the event horizon.

Now the existence of black holes was first predicted as a consequence of Einstein’s General Theory of Relativity. The gravity of such objects is supposed to be so intense that not even light can escape from it — hence the name, ‘black hole’. This inference was so strange that Einstein himself doubted their actual existence. 

It took an incredible effort. Astronomers combined eight telescopes on four continents to create that image. In effect, they turned the entire Earth into a telescope. This combination of telescopes is called the Event Horizon Telescope (EHT).

The Milky Way also has a supermassive black hole at its centre. We have known about its existence as we have seen its gravity speed up stars and gas clouds around it. In fact, the 2020 Nobel Prize was shared by Andrea Ghez and Reinhard Genzel for observations that showed the existence of this supermassive back hole. We call this black hole Sagittarius A*, shortened to Sgr A*, but pronounced Sagittarius A Star.

But what does it look like? Can we see the light right next to its event horizon?

You would think that imaging our very own supermassive black hole would be easier than the one located at the heart of M87, 55 million light years away. That is not the case. First, our own Sgr A* is much smaller in mass. It is only four million times the mass of our Sun. In comparison, the one in M87 is 1,500 times bigger, or more than six billion times the mass of our Sun.

In 2019, astronomers took the image of the shadow of the event horizon of a supermassive black hole at the centre of M87, a galaxy located 55 million light years away. This was the first time humans had directly seen the area right around the event horizon. It took an incredible effort.

This is important, as it results in material moving faster closer to the event horizon, making it difficult to combine images from the set of telescopes that make up EHT. Second, since we are located in the disk of the galaxy, we have to see through a lot of material between us and Sgr A*.

This past week, the EHT team finally released the first-ever image of the shadow of the supermassive black hole at the centre of the Milky Way. It looks like a doughnut. I should clarify that this is not visible light. Instead, astronomers are seeing this in radio waves.

The central part is dark because any light across that region falls into the black hole, creating a shadow or a silhouette of the event horizon. The size of this dark region is about 65 million km across, comparable to the orbit of planet Mercury in our solar system. Amazingly, the size of the doughnut fits perfectly well with the predictions of Einstein’s General Theory of Relativity.

This is a stunning confirmation of our physics and prior observations. Nature is under no obligation to follow our musings. And yet, here we are.

The next time you are feeling a bit down, perhaps after reading the world news, think about this: tiny beings on a small planet, orbiting an ordinary star in the far outskirts of the Milky Way, have confirmed the existence of a mysterious supermassive black hole at the centre of our galaxy.

The team of Earthlings behind the EHT should get news headlines across the galaxy.

The writer is Professor of Integrated Science & Humanities at Hampshire College in the US. He is also an astronomer affiliated with the Five College Astronomy Department (FCAD) in Massachusetts and hosts shows on the YouTube channel Kainaat Astronomy in Urdu

Published in Dawn, EOS, May 22nd, 2022

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