The international collaboration of astronomers EHT and the European Southern Observatory (ESO) proved Thursday in image the existence of Sagittarius A *, a supermassive black hole in the heart of our Milky Way. And this, three years after the first photo of a black hole located in a distant galaxy.
“I can present to you the image of the trou noir Sgr A* at the center of the galaxy,” EHT project manager Huib Jan Van Langevelde announced to applause at a press conference in Garching, Germany.
It was a strange ballet of stars in the center of the Milky Way which first made it possible to understand that there was probably a black hole there: Andrea Ghez and Reinhard Genzel received the Nobel Prize in Physics in 2020 for their “discovery of a compact supermassive object at the center of our galaxy”.
The team had even managed to determine its exact mass. But, thanks to the observations unveiled today, its size could be assessed. Two essential parameters to prove with certainty its existence.
Sagittarius A*, which owes its name to its detection in the direction of the constellation Sagittarius, has a mass of approximately four million suns and is 27,000 light years from Earth. Its existence has been assumed since 1974, with the detection of an unusual radio source at the center of the galaxy.
Matter swallowed
Technically, you can’t see a black hole, because the object is so dense and its gravitational force so powerful that not even light can escape from it. But we can observe the material that circulates around it, before being swallowed up: its silhouette stands out on this luminous disc.
“We have direct evidence that this object is a black hole,” explained Sara Issaoun, from the Harvard Center for Astrophysics, describing “the cloud of gas [autour du trou noir] which emits radio waves and which we have observed”.
“The object is surrounded by a surface called the event horizon”, explains astrophysicist Roland Walter, from the Observatory of the University of Geneva, specialist in black holes. At the microphone of Forum, he explains that the shadow in the middle is “the signature of the presence of the black hole”. It was more difficult to grasp, because it is “much smaller, less luminous and there is a lot of material between it and us”, he remarks.
Telescopes have to observe it much longer to get something usable: “By recording this data and correlating it, we can reconstruct this image”.
>> Size comparison of black holes Messier 87* and Sagittarius A*:
Comparison of the size of black holes M87*, at the heart of the galaxy Messier 87, and Sagittarius A* (Sgr A*), at the center of the Milky Way. For scale, the orbits of Pluto and Mercury, as well as the diameter of the Sun and the current position of the Voyager 1 spacecraft, the furthest spacecraft from Earth. Due to their relative distance from our planet, the two objects appear to be the same size in the sky. [Lia Medeiros, xkcd – EHT collaboration]
Second historical image
The technique used for this image is millimetric radio interferometry. The EHT, an international network of eight radio-astronomical observatories, brought in 2019 the historic image of M87 *, a supermassive black hole of six billion solar masses in its galaxy, Messier 87, located 55 million years ago. -light.
>> Lire: Humanity sees a black hole for the first time in history
Now equipped with eleven observatories, the EHT has portrayed Sgr A* with its four million solar mass… a featherweight in the bestiary of supermassive black holes.
Scientists see it as proof that the same mechanisms of physics are at work at the heart of two systems of very different sizes: “Black holes are absolutely fascinating objects. They are predicted by Einstein’s general relativity, but Modern physics relies heavily on quantum mechanics. The two are not compatible and do not predict the same thing about what happens on the surface of a black hole,” notes Roland Walter.
“And the black hole is the place in the Universe where we manage to put these two theories together” and maybe even find a theory that unifies them! This is not currently the case: “It’s a place where you can really do fundamental physics”, rejoices the astrophysicist.
>> A short film explaining how the EHT works (in English, with French subtitles):
Stéphanie Jaquet and the agencies