How is a picture taken of a black hole – and what do we see?

Five years ago, the Event horizon telescope (EHT) aimed its many radio telescopes at two black holes, one in the galaxy Messier 87 and the other in our own galaxy, the Milky Way.

But what did the telescope really see?

Why did it take until this year before a picture of the center of the Milky Way was published?

And what comes next?

Before answering these questions, a little about black holes.

Black holes are super compact objects

A black hole is a very compact area in space from which nothing can escape. Around a black hole is an event horizon, the boundary between the hole and the rest of space. Due to the strong gravitational force, nothing, not even light, can escape the black hole if it ends up within the event horizon.

The existence of a supermassive object in the center of the Milky Way was first proven by Reinhard Genzel and Andrea Ghez, a discovery that in 2020 each gave them a quarter of the Nobel Prize.

The other half went to Roger Penrose for his proof that black holes are a direct consequence of the general theory of relativity.

But it was not until this year before we received photo evidence that the center of the galaxy, this supermassive object, actually consists of a black hole.

– Today, the Event horizon telescope is pleased to share the first direct image of “the friendly giant” in the center of our galaxy – Sagittarius A *, said Feryal Özel, professor of astronomy at the University of Arizona, at one of the press conferences which was held in connection with EHT presenting its new results.

How to photograph a black hole?

EHT is a global collaboration and uses radio telescopes from several research stations around the world to make observations of objects far out in space.

The observations are made using a technique called long-range interferometry. Interferometry involves linking different telescopes to create a new, larger one.

By combining measurements from eight radio telescopes, the EHT functions as a telescope as large as the earth. The resolution of a telescope is proportional to its size. EHT has a resolution better than 60 microwave seconds, which is equivalent to being able to distinguish an orange on the moon’s surface from the earth.

What do we see in the picture?

Since nothing can escape the gravity of a black hole, it is not really the black hole we see. What is visible is the hot gas circulating around Sagittarius A * just outside its event horizon – neither matter nor light on the other side of this boundary can leave the black hole and therefore we can not photograph the black hole itself.

– The picture shows a light ring surrounding the darkness. This ring consists of hot gas that swirls around the black hole. Light that is close enough to the black hole to be swallowed by it eventually crosses its horizon, leaving the dark void in the middle, said Feryal Özel.

When were the observations made?

The observations were made as early as 2017 when the telescope for five nights collected data from both Sagittarius A * and the black hole in the center of the galaxy Messier 87 – M87 *.

Sagittarius A * is around 26,000 light-years away and weighs like four million suns. But even though the M87 * is significantly larger, the pictures look the same.

– The black hole in Messier 87 is 1,500 times more massive, which makes its event horizon 1,500 times larger. But it is also 2,000 times further away from us, which makes these two pictures look very similar, said Feryal Özel.

Why did it take so long before the picture was published?

The image of M87 * in the galaxy Messier 87 was presented as early as 2019. The reason why the image on our own black hole has been delayed is because Sagittarius A * is significantly more dynamic and varied than M87 *.

Around the two black holes, gas moves at speeds close to the speed of light. But while it takes days or weeks for the gas to circulate M87 *, the gas at the smaller black hole Sagittarius A * gets around in a matter of minutes.

As the rapidly orbiting gas creates fluctuations in brightness and pattern, EHT produced thousands of images of the center of the Milky Way – instead of just one – during its observations. Several years of processing have now led to the image on Sagittarius A * which is an average of the amounts of images taken by the telescope.

The image now confirms once and for all that it is actually a black hole that our galaxy circulates around.

Do you remember researcher Katie Bouman? She went viral when the first image on a black hole was presented, when she shared an image of herself as she sat at the screen and saw how the computer created the image, using the algorithm she had been involved in developing.

Katie Bouman is currently an assistant professor at Caltech and works with image processing. In this video, she tells how the new image on a black hole has been developed.

What can we learn from the picture?

By comparing EHT’s collected data with simulations made using supercomputers, the researchers have concluded that the black hole probably rotates counterclockwise about an axis that points almost straight towards the earth.

Additional data has been collected since 2017, and EHT continues its global collaboration so that we can learn more about Sagittarius A *. Among other things, the researchers want to find out if the black hole has, or has had, jets. Many black holes, such as M87 *, have two jets of matter that are ejected in opposite directions from its surface.

Jet jets could be caused by an intense heating of the gas falling into the black holes. Sagittarius A * may have had jets before, something that clouds of matter both above and below the center of the Milky Way point to. Today, the jets – if they remain at all – are probably weaker.

In addition to learning more about the Sagittarius A *, the new image confirms that the previous image of the M87 * was not just a coincidence.

– Now we know that it was not a coincidence; it was not just any aspect of the environment that happened to look like the ring we expected to see. In both cases, it is what we see the heart of the black hole, said Feryal Özel.