Black holes are among the most profound predictions of Einstein’s general theory of relativity. Originally studied as a purely mathematical consequence of the theory rather than as physically relevant objects, they were soon regarded as generic and sometimes unavoidable results of the gravitational collapse that initially forms a galaxy.
In fact, most physicists have suspected that our own galaxy orbits a supermassive black hole at its center. There are also other ideas, such as “dark matter” (an invisible substance believed to make up most of the matter in the universe). But now an international team of astronomers, including one I led from the University of Central Lancashire, has unveiled the first image of the object lurking in the center of the Milky Way — and it’s a supermassive black hole.
This means that there is now overwhelming evidence for the black hole called Sagittarius A*. While it may seem a little scary to be so close to such a beast, it’s actually about 26,000 light-years away, which is reassuringly distant. Because the black hole is so far from Earth, it appears to us in the sky about the size of a donut on the moon. Sagittarius A* also seems rather inactive – it doesn’t devour 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 – which together form a single virtual telescope on Earth – to create the stunning image. The breakthrough follows the collaboration’s 2019 release of the first-ever image of a black hole, dubbed M87*, at the center of the more distant Messier 87 galaxy.
Looking into the darkness
The team observed Sagittarius A* over several nights and collected data for hours on end, 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 around it reveals a telltale feature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new image captures light deflected 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 how black holes work, which are believed to be at the center of most galaxies.
The surprising thing about this image is that it is so similar to the M87* image we published three years ago – this certainly came as a surprise. The reason for the similarity is that while the M87* black hole is about 1000 times larger, the Sagittarius black hole is about 100 times closer. Both obey Einstein’s general theory of relativity, which shows that Einstein was right on a scale of a factor of 1000. For a physicist this is important. Relativity has been around for a century and it still turns out to be correct. I think even Einstein himself would have been surprised by that!
The publication of the image of the Sagittarius A* black hole is a hugely exciting achievement of the collaboration. When I first saw the image, I thought: this says a lot. I couldn’t wait to write about it and interpret the image. We had many meetings to come to a consensus on what it tells us. For starters, we met face to face in different parts of the world. Then COVID hit and suddenly no one had anywhere to go. So online meetings became the norm, just like in every other aspect of life. This certainly slowed us down.
My role was to help write two of the six papers published in the Astrophysical Journal Letters: the first, Introduction to Observation; and the third, in which we discuss how we have made a picture of the observations, and how reliable that picture is.
In addition, I was a “contributing author” for all six papers. This is an administrative role, handling all correspondence between our team of more than 300 astronomers and the scientific journal that published our findings. This had its challenges as I had to deal with every typo and every typesetting error.
I also had to channel comments from my colleagues. As the majority of employees are based in the US or East Asia, this meant they were working at night in UK time. That’s why I came to work every morning to find about 100 late-night emails from coworkers — a daunting start to any day.
Anyway, we got there in the end – and the dazzling result was worth all the work.
(Author: Derek Ward-Thompson, Professor of Astrophysics, University of Central Lancashire)
Disclosure Statement: Derek Ward-Thompson does not work for, consult, hold stock in, or receive funding from, any company or organization that would benefit from this article, and has not disclosed any relevant affiliations outside of their academic appointment.
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