For the first time, a photo of Sagittarius A*, supermassive black hole at the center of our galaxy



Three years after the first photo of a supermassive black hole, named M87* (1), the Event Horizon Telescope (EHT) does it again, but this time with a much closer giant. The international project, supported by Europe and which brings together hundreds of astrophysicists, unveiled on Thursday May 12 a snapshot of Sagittarius A* (2), the black hole at the center of the Milky Way, our galaxy.

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This is the first image of this cosmic titan, and the first irrefutable proof that the heart of our galaxy is indeed home to a black hole. The data collected will provide valuable insight into the functioning of these celestial objects, which are thought to also occupy the center of most other galaxies.

A Franco-German telescope put to use

Among the eight observatories used to photograph Sagittarius A* is that of the Institut de radioastronomie millimètre (Iram), founded by the National Center for Scientific Research (CNRS) and the Max-Planck-Gesellschaft in 1979. Its 30-meter telescope , located near Granada, is the one that provided the most accurate images.

To get a photograph of the black hole, the EHT observed it for several nights, collecting data for long hours. “By definition, a black hole emits nothing and especially no light”, recalls Alain Riazuelo, astrophysicist at the Institute of Astrophysics in Paris. On the shots, we can therefore only distinguish a black shadow, which stands out in the middle of a brilliant cluster. This cluster corresponds to the accretion disk, that is to say the heated matter being swallowed up by the black hole in the middle.

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The challenge was getting a sharp image, as the gases circulating around Sagittarius A* travel at the speed of light and circle the giant in minutes. Also, this black hole is located 27,000 light-years from Earth. In the sky, it is no more visible than a donut placed on the surface of the Moon.

Origin of galaxies

The images obtained will make it possible to study the behavior of gas around a supermassive black hole, a process which could play a key role in the formation of galaxies. Scientists now have data on a small supermassive black hole, ours, and on a heavyweight in its class, M87*, which will also allow them to analyze how gravity behaves in extreme environments.

The detection of a radio signal from the center of the Milky Way dates back to the 1930s, but it took until 1974 to determine the exact source of these waves, thanks to the work of Americans Bruce Balick and Robert Brown. Since then, this radio source has been considered a black hole. The Nobel Prize in Physics was also awarded in 2020 to the American Andrea Ghez and the German Reinhard Genzel, who both separately highlighted it.

6.5 billion times more massive than the Sun

Objects still very poorly understood, black holes represent a very large mass in a very small volume. A bit like compacting the Sun in a house. There are two types of black holes: those called “stellar”, the most numerous in the universe, which form when a star dies and collapses on itself; and those called “supermassive”, which are found in the center of galaxies and which are much larger.

“Many questions still surround the formation of these supermassive black holes, some of which are very old,” recognizes Alain Riazuelo. Stellar black holes are too small to be imaged, unlike supermassive ones, even if the operation remains complicated.

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On the image published in April 2019, which represents the black hole M87 * in the center of the galaxy of the same name, we see a kind of golden ring, the accretion disk. “Inside, the black hole is very large, about 6.5 billion times more massive than our Sun,” recalls Alain Riazuelo. In comparison, Sagittarius A* has a mass which is “only” 4 million times that of the Sun, itself 333,000 times more massive than the Earth.

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