And that’s two. On Thursday afternoon, an international team of astronomers presented the first image of the black hole at the center of the Milky Way. This image of Sagittarius A* (Sgr A*) comes three years after the black hole M87* at the center of constellation Messier 87 was revealed to the world. Sera Markoff, astrophysicist at the University of Amsterdam, is co-chair of the science council of Event Horizon Telescope (EHT), the telescope network used to take both photos.
Why did you want another photo of a black hole so badly?
“Among other things, to learn something about the evolution of the universe. Black holes do not sit still in a galaxy. They influence that galaxy – and thus how the universe looks. For example, black holes can stop galaxies from growing. New stars form from cold gas, but because black holes can give off a lot of heat, they can stop the formation of new stars.”
What struck you most about the photo?
“That the glowing ring of matter surrounding the black hole looked exactly like Albert Einstein’s general theory of relativity predicted. This was already the case with M87*, but whether that also applied to Sgr A* was still open to question. Sgr A*, our own black hole, is different from M87*: it is much smaller and in a different environment.”
What do you want to investigate further in the data?
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“For example, we want to compare the two black holes with each other, in order to be able to say something about the development of black holes. Sgr A* may be special to us, but every galaxy has such a black hole at its center and they almost all behave like ours: passively. M87*, on the other hand, is active, emitting a huge jet of matter, a jet. We think the hole in our galaxy used to do that and will do that again in the future. Presumably all black holes of this kind, ie in the center of a galaxy, go from a passive phase to an active phase over billions of years and then vice versa. By comparing the two black holes in different phases, we may be able to learn something about that change.”
The aim of the EHT was to photograph Sgr A*. That worked. Are you going to take more pictures?
“The picture presented now was taken with data from 2017. In the meantime, three telescopes have been added and we took new images of Sgr A* in 2018 and 2022. Perhaps that data is even better, or maybe we can make a movie of Sgr A*. Now that we have retrieved the first photos from the data, hopefully it will be faster next time.”
What do you hope to get out of those new images?
“Then, for example, we can say with even more certainty that Einstein’s general theory of relativity is correct. The photo of Sgr A* is interesting, but this black hole is relatively small. The radius of the event horizon of Sgr A* – the boundary around a black hole beyond which even light can no longer escape – is about six million kilometers. Due to its relatively small size, matter revolves quickly around the black hole, making it look different every time. If we have a video, we can see even better what exactly happens around the black hole in the Milky Way.”