At the heart of our galaxy, the Milky Way, is a “monster”: a supermassive black hole with a mass four million times that of our Sun, which absorbs all material, including gas, dust and stars wandering in its immense gravitational pull.
Scientists used the Event Horizon Telescope (EHT), a global network of observatories working collectively to observe radio sources associated with black holes, to study this mysterious resident of the Milky Way. This Thursday, through multiple press conferences around the world, astronomers announced that they finally had an image of this black hole, called Sagittarius A*, or SgrA*. A long-awaited event described as “revolutionary result”, by EHT officials.
This result provides proof that the object is indeed a black hole and provides valuable clues about the workings of these giants, which are thought to be at the center of most galaxies, the scientists say.
Indeed, this image is a long-awaited look at the massive object that sits at the very center of our galaxy. Scientists had previously observed stars orbiting an invisible, compact and very massive object at the center of the Milky Way. This strongly suggested that this object – known as Sagittarius A – was a black hole, and today’s image provides the first direct visual evidence of this.
Although we can’t see the black hole itself, because it’s completely dark, the glowing gas surrounding it reveals its signature: a dark central region (called a shadow) surrounded by a bright, star-shaped structure. ring. This new image shows light distorted by the black hole’s powerful gravity
“We were amazed at how well the size of the ring matched the predictions of Einstein’s theory of general relativity“said EHT project scientist Geoffrey Bower of the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei.”These unprecedented observations have dramatically improved our understanding of what is happening at the very center of our galaxy, and offer new insights into how these giant black holes interact with their surroundings..”
The Event Horizon Telescope collaboration was set up in 2012 with the aim of obtaining the first direct images of black holes. In 2019, the EHT team unveiled the very first photo of a black hole. The image – a bright ring of red, yellow and white surrounding a dark center – showed the supermassive black hole at the center of another galaxy called Messier 87, or M87.
A black hole is a region of spacetime with such intense gravity that nothing, not even light, can escape from it. “For this reason, we call it ‘black’. Since the material that falls on it (or inside), seems to disappear, it is reminiscent of a “hole”, explain the scientists of the EHT. The event horizon is the boundary surface between space and the “interior” of the black hole. This is the “region of no return”, i.e. anything that crosses the event horizon will forever be unable to escape.”
Since black holes emit no light, observations are difficult. The objective is therefore to image their “shadow” or “silhouette” created by the curvature of light under extreme gravity. “ This is only possible with a very sharp image of the black hole in the microwave spectrum, by virtually combining telescopes spread over the Earth into a single telescope as large as our globe. Individual telescopes are placed in remote, high altitude locations with a dry atmosphere to avoid the effect of water vapor. Their recorded signal is combined and processed to obtain the final image”, explain the EHT researchers.
Specifically, project scientists searched for a ring of light – superheated disturbed matter and radiation flowing at tremendous speed at the edge of the event horizon – around a region of darkness representing the actual black hole.
“The resolution of our global network of radio telescopes at this wavelength would allow us to determine the size of a ping-pong ball on the Moon.“, further specify the researchers.
ten million stars
Known as the spiral galaxy, the Milky Way seen from above or below resembles a spinning pinwheel, with our Sun located on one of the spiral arms and Sagittarius A* located in the center. The galaxy contains at least one hundred billion stars. And its central region, towards the constellations of Sagittarius, Ophiuchus, and Scorpius, has about ten million stars. But the central region is obscured for visible light seen from Earth due to the presence of interstellar dust, which has complicated observations and their analysis.
The most recent observations of stars orbiting Sgr A* give the black hole a mass of about four million solar masses. For this mass, the radius of the event horizon is about 6 million km. Its very first image should make it possible to better estimate its mass.
Sagittarius A* is located about 26,000 light-years (the distance light travels in one year, 9.5 billion km) from Earth. The M87 black hole is therefore much more distant and massive than Sagittarius A*, located about 54 million light-years from Earth with a mass 6.5 billion times that of our Sun. In releasing the photo of this black hole, the researchers said their work showed that Albert Einstein, the famous theoretical physicist, correctly predicted that the shape of the shadow would be almost a perfect circle.
“We have two completely different types of galaxies and two very different masses of black holes, but near the edge these black holes look surprisingly similar“says Sera Markoff, Co-Chair of the EHT Scientific Council and Professor of Theoretical Astrophysics at the University of Amsterdam, the Netherlands.”This tells us that general relativity governs these objects up close, and that any differences we see further away must be due to differences in the matter surrounding the black holes.“