This is Sagittarius A*, the supermassive black hole at the center of our galaxy

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This is Sagittarius A*, the supermassive black hole at the center of our galaxy

It is more than 4 million times more massive than the Sun.

Sgr A*, the supermassive black hole at the center of our galaxy.

The very first image of Sagittarius A*, the supermassive black hole at the center of the Milky Way, was released at several press events around the world by institutions associated with the EHT (Event Horizon Telescope) initiative. ), including the European Southern Observatory (ESO) and the Harvard-Smithsonian Center for Astrophysics.

This image provides direct visual evidence of the presence of the black hole at the heart of the Milky Way.

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Astrophysicist Julie Hlavacek-Larrondo, professor of physics at the University of Montreal and specialist in supermassive black holes, followed the announcement live with colleagues who attended at a meeting of the Center for Research in Astrophysics of Quebec.

“There were a hundred of us applauding with great emotion! As astrophysicists, we understand the technological challenge represented by this image, it is colossal. It is a source of pride to see what astrophysics is capable of accomplishing. »

— Julie Hlavacek-Larrondo

Enlarge image

The area of ​​sky in which Sagittarius A* is located is marked with a red colored circle in the constellation Sagittarius. Most of the stars visible to the naked eye under good viewing conditions are found there.

As early as 1974, astrophysicists had detected a compact and brilliant radio source in the center of the Milky Way. It was not until the 1990s, however, that this source was associated with the presence of a supermassive black hole, those regions of space where the gravitational field is so intense that it prevents any form of matter or radiation. to escape.

But how to confirm the presence and create an image of a celestial object that does not emit light? This is the task that the 350 astrophysicists associated with the EHT have been focusing on since a first observation campaign carried out in 2017 using the eight terrestrial telescopes located around the world and which form the virtual equivalent of a radio telescope several thousand kilometers in diameter.

This image shows the location of some of the telescopes that make up the 'EHT, along with a representation of the long baselines between the telescopes.

The goal of these scientists was to image the silhouette of a black hole caused by the curvature of light subjected to the extreme gravity of the celestial object. The first results were announced in 2019. At the time, the image of the black hole at the center of the galaxy Messier 87 (M87*) was presented, some 55.3 million light years from Earth.

By comparison, Sagittarius A* (Sgr A*) is only 27,000 light years from our planet.

The image that the astrophysicists managed to capture corresponds to the outline of SgrA*, what is called the event horizon. This region marks the immaterial limit of entry into the black hole.

“This ring of light represents matter and gas swirling around the black hole just before they get swallowed up,” says Prof. Hlavacek-Larrondo.

This outline of the black hole is considered one of the most violent places in the universe, and the point of no return beyond which everything – i.e. stars, planets, gas, dust and all forms of electromagnetic radiation, including light, would be irreversibly sucked out.

Sgr A* is 100 times closer to our planet than any other supermassive black hole, but its relative proximity did not make it easier to observe than M87*, much further away.

Size comparison of the two black holes imaged by EHT: M87* and Sgr A*. The image shows the scale of Sgr A* relative to M87* and other solar system elements such as the orbits of Pluto and Mercury.

The gas near the two black holes moves at the same speed, but the gas around M87* takes days or even weeks to orbit, while the gas around M87 takes only minutes to orbit. of Sgr A*, which is 1000 times smaller.

So M87* being a more stable target, all images end up looking the same. In the case of Sgr A*, the photo shown is an average of the various images the team extracted.

Astrophysicist Olivier Hernandez, director of the Rio Tinto Planetarium in Montreal, is thrilled by the amount of data collected that allows a comparison of the two monsters.

“When images of black holes are projected into the sky, they appear pretty much identical in size. But the Messier 87 black hole is located 55 million light-years away. […] In fact, the black hole at the center of our galaxy is very small by comparison.

— Olivier Hernandez

It is so small that the entire ring we see fits into Mercury's orbit. It's impressive, adds Mr. Hernandez.

The Rio Tinto Alcan Planetarium in Montreal.< /p>

The really technological challenge is that we are observing a varying distance… The gas around Sgr A* is literally spinning at the speed of light and we are seeing it. sees spinning live! That's why it took years to get the image, says the professor.

The team spent five years creating of the image using supercomputers to combine and analyze the data.

The image shown to the world surprised astrophysicist Olivier Hernandez, director of the Rio Tinto Planetarium in Montreal .

“We see the black hole from the front as was the case for M87*, when we thought we were seeing it on the edge because that technically we are in the disk of the galaxy. It’s very interesting, there are going to be good discussions about it over the next few years.

— Olivier Hernandez

These images show the first two images ever taken of black holes. On the left, M87*, and on the right, Sgr A*).

The astrophysicist underlines the extent to which the data collected correspond in every way to the theory of general relativity. Beyond the observations, there are a lot of very advanced simulations that are made to understand everything that is happening. These simulations lead to theoretical models whose [simulated] images correspond exactly to the images obtained, he believes.

“ It really is absolutely amazing technical and scientific work. And the result of all this tells us a little more about the very heart of our galaxy and validates the mass of our galactic black hole.

— Olivier Hernandez

The presence of three brighter spots on the outline of the hole also remains to be clarified, according to Mr. Hernandez. They are probably related to places where the magnetic field is more intense, but it may also be the places from which the flow of matter crosses the event horizon.

Some results also surprised Professor Hlavacek-Larrondo, who knew that Sgr A* was not the most active black hole.

“He's really quiet and swallows very little material. […] It is as if he was fasting. On a human scale, it is as if a human ate a grain of rice every million years.

—Julie Hlavacek-Larrondo

According to the scientist, this reality does not fit with current knowledge of inactive black holes.

“Our physics currently predicts much more variability in the black hole than is observed in the image. This means that there is knowledge in physics that is lacking. There is something we don't understand. »

— Julie Hlavacek-Larrondo

According to the professor, the EHT team will continue to analyze the data in the coming months and may create a video showing the black hole's environment and how it revolves around it.

“My dream really is to see those monsters gobble up their meal!” To see the invisible! This is a super fascinating side of physics. »

— Julie Hlavacek-Larrondo

It's a blurry picture, but what it reveals is so huge and far away. It is a black hole called Sagittarius A and it sits right in the center of our galaxy. A report by Normand Grondin.

  • The Milky Way is a spiral galaxy made up of more than 100 billion stars, dust and interstellar gas.
  • As a whole , it measures about 106,000 light-years in diameter.
  • At the center is a bright nucleus, called a bulge, from which spiral arms extend into a huge flattened disk.
  • Our Sun sits on one of its spiral arms and orbits the galactic center every 240 million years.
  • Sgr A* is more than 4 million times more massive than the Sun.

Sgr A* isn't the only black hole in the Milky Way. There are dozens of stellar black holes. The latter are formed during the gravitational collapse of massive stars at the end of their life which explode in supernovae. To date, 20 such black holes have been confirmed in our galaxy.

Despite this significant galactic breakthrough, black holes remain very mysterious celestial objects.

“How do they form? Where does the material they absorb go? How to define their gravitational singularity? What's on the other side? »

— Olivier Hernandez

As many unanswered questions that science can continue to dwell on in the coming years.

Since the first series of observations conducted in 2017, three observatories have been added to the EHT network and more will join in the coming years.

The image is the subject of six articles published in The Astrophysical Journal Letters .

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