Discover the first images of a black hole

What does a black hole look like? We now have the answer. Astronomers from around the world, united under the Event Horizon Telescope project, unveiled on Wednesday the result of a cross-observation aimed at capturing the image of a black hole, a first in the history of astronomy. a huge step forward for humanity », announced Carlos Moedas, European Commissioner for Research, who chaired in Brussels one of the six press conferences held around the world, simultaneously. « Here is the first image of a black hole. You have probably seen many images of black hole simulations so far. But here it is finally true », proudly presented Heino Falcke, one of the scientists responsible for the discovery, unveiling the image of what he describes as a « ring of fire. » A fuzzy ring, but that’s no wonder. « At this huge distance where M87 is, it’s like looking at a mustard seed in Washington from Brussels. »

The unveiled image actually shows the « silhouette » of the black hole: a black disc, with an asymmetrical halo of light around it. Massive, gluttonous, overpowered: « black holes are stranger than what science fiction authors can imagine, but they are scientific realities », underlined Carlos Moedas. Black holes are objects that don’t emit light, but they act as « gravitational lenses », that is, they will deflect light rays as they pass close to their body – which is what we call the horizon of the black hole. Basically, a black hole “absorbs” matter that comes to its horizon; but if it misses, it is deflected. Concretely, under the effect of the enormous gravitational attraction of a black hole, stars that are too close are flattened, stretched and then dislocated, the gas brought to extreme heat. Gas and chunks of stars spiral around the black hole and finally sink into it, generating a brilliant burst of ultraviolet light.

halo of light

“The light from the halo on the image comes from gas and matter close to the black hole which heat up by being accelerated by the gravitation of the black hole, details the Belgian physicist Geoffrey Compère. The most intense light has a high frequency radio spectrum, it is this light that is observed by telescopes. Because of the black hole, this light has a characteristic shape: the silhouette of the black hole, black in the middle, and with a halo of light around it with an asymmetry due to the rotation of the black hole and that of its accretion disk (matter in orbit). This rotation induces a driving effect of the light rays in the direction of the rotation, which creates this asymmetry”.

Black holes have been theorized, modeled, detected but never directly observed in the spectrum of light. According to the law of general relativity published in 1915 by Albert Einstein, who theorizes their functioning, the gravitational attraction exerted by these monsters is such that nothing can escape, neither matter nor light, whatever the wave length. Result: they are invisible. To circumvent this handicap of size, the astronomers seek to observe the monster by contrast, on the matter which surrounds it. Another peculiarity: a black hole is a celestial object that has an extremely large mass in a very small volume. It’s a bit like the Earth being squeezed into a thimble. To see such small objects, you need a large telescope.

Hibernating bear or excited toddler?

In April 2017, eight telescopes spread across the world simultaneously targeted two black holes with one goal: to try to get an image of them. For two years, the scientific community has been waiting for the result. “A photo is definitive proof of the existence of black holes”, enthuses Jean-Pierre Luminet, astrophysicist at the French CNRS, author of the first digital simulation of a black hole in 1979. “Even in the scientific community, there is still a lot of resistance”, adds the scientist who already wanted at the time “to show the black hole”.

By combining eight telescopes spread across the globe, the EHT has succeeded in creating a virtual telescope approximately 10,000 km in diameter, close to the size of the earth. With in particular the 30-meter telescope of IRAM in Europe, the powerful ALMA radio telescope built in Chile (co-managed by Europe, the United States and Japan) but also structures in the United States, Hawaii and Antarctica , the Event Horizon Telescope covers a large part of the globe. The larger a telescope, the more detail it can see.

And astronomers have chosen two targets: the two black holes, which seen from Earth, are the largest. One, Sagittarius A*, is nestled in the center of the Milky Way, 26,000 light-years from Earth. Its mass is equivalent to 4.1 million times that of the Sun. Its radius is one tenth of the distance between the Earth and the Sun. The other is one of the most massive black holes, 1,500 times more than Sagittarius A*. It has no name and is located 50 million light-years from Earth, in the heart of the M87 galaxy. It is much bigger than Sagittarius A* but much further from us.

But seeing the very first results, they decided to focus on the black hole of M87, explains Heino Falcke. « We knew that the results would be extraordinary. The problem was that Sagittarius is a thousand times smaller but a thousand times closer so we had the same silhouette as that of M 87, but it turns a thousand times faster. Take an image from this source , it was like taking a picture of a toddler, who keeps moving all over the place for 8 hours. Try to get a steady picture! Whereas M87’s black hole was like taking a picture of a bear, a hibernating bear, a thousand times slower. He moved little during those eight hours. It was much easier. We were very lucky he was so big. He could have been more small, we didn’t know that before. Once we saw that, we decided to focus on it. Now we take a little more time to study Sagittarius. »

« You may be wondering how this image was constructed, continues his colleague Monica Moscibrodzka. In fact, the data provided by the EHT is like an incomplete puzzle, in fact we only see parts of the image. image and we have to fill in the holes to reconstruct the physically possible image that matches our data. It’s a complex process. We do it first alone, then as a team, to avoid human bias. »

Through their observations, astronomers sought to identify the immediate environment of a black hole. According to the theory, when the matter is absorbed by the monster, it emits light. The EHT project, capable of capturing the millimeter waves emitted by the environment of the black hole, aims to define the perimeter of the celestial object. Project scientists were also pleased that Einstein’s theory of gravity, which predicted the existence of black holes, « completely passed » this important test of these observations. Scientists now expect a telescope larger than Earth – something that can only be done with telescopes launched into space – to improve the resolution of images taken of black holes.

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