The team observed Sagittarius A* over several nights, collecting data for many hours at a time, similar to using a long exposure time on a still camera.
Black holes are one of the most profound predictions of Einstein’s theory of general relativity. Initially studied as a mere mathematical consequence of the theory and not as physically relevant objects, they soon came to be seen as generic and sometimes inevitable results of the gravitational collapse that initially forms a galaxy.
In fact, most physicists suspect that our own galaxy revolves around a supermassive black hole at its center. There are other ideas as well, such as “dark matter” (an invisible substance thought to make up most of the matter in the universe).
But now an international team of astronomers, including a team I led from the University of Central Lancashire, has revealed the first image of the object lurking at the center of the Milky Way, and it is a supermassive black hole.
This means that there is now overwhelming evidence for the black hole, dubbed Sagittarius A*. Although it may seem a bit scary to be so close to such a beast, it is actually about 26,000 light-years away, which is reassuring.
In fact, because the black hole is so far from Earth, it appears to us to be about the same size in the sky as a donut would be on the Moon. Sagittarius A* also seems quite inactive: it is not devouring much matter from its environment.
Our team was part of the global Event Horizon Telescope (EHT) collaboration, which has used observations from a global network of eight radio telescopes on our planet – collectively forming a single virtual Earth-sized telescope – to take the stunning image. This breakthrough comes after the collaboration published in 2019 the first image of a black hole, called M87*, at the center of the more distant galaxy Messier 87.
looking in the dark
The team observed Sagittarius A* over multiple nights, collecting data for many hours at a time, similar to using a long exposure time on a camera. Although we can’t see the black hole itself, because it’s completely dark, the glowing gas surrounding it reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright, ring-shaped structure.
The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun. The discovery also provides valuable clues about the workings of black holes, which are thought to reside at the center of Earth. most galaxies.
The amazing thing about this image is that it looks very similar to the image of M87* that we published three years ago, which certainly surprised us. The reason for the resemblance is that while the black hole in M87* is about 1,000 times larger, the one in Sagittarius is about 100 times closer. Both obey Einstein’s theory of general relativity, showing that Einstein was right by a factor of 1,000 on the size scale.
For a physicist this is important. Relativity has been around for a century and continues to prove its accuracy. I think even Einstein himself would have been surprised by this.
The release of the Sagittarius A* black hole image is a tremendously exciting achievement of the collaboration. When I first saw the image, I thought: this tells us a lot. I couldn’t wait to start writing about it and interpreting the image. We had many meetings to reach a consensus on what it tells us.
At first we met face to face in different parts of the world. Then Covid came and suddenly no one could go anywhere. So online meetings became the norm, just like any other aspect of life. This definitely set us back.
My role was to help write two of the six papers that have been published in the Astrophysical Journal Letters: the first, presenting the observation; and the third, in which we discuss how we made an image from the observations, and how reliable that image is.
In addition, I was a “contributing author” for all six articles. This is an administrative role, in which I handled all the correspondence between our team of more than 300 astronomers and the academic journal that published our findings. This was challenging as I had to deal with all the typos and composition errors.
I also had to channel feedback from my colleagues. Since most of the contributors are based in the US or East Asia, this meant they were working overnight on UK time. From there