We can finally see the black hole in the center of our galaxy, and it’s an achievement

<span class=The first image of Sgr A *, the supermassive black hole in the center of our galaxy. eso2208-eht-mwa, CC BY-SA“src =” https://s.yimg.com/ny/api/res/1.2/cEnzAeLyA4ENvXUFB1ZShA–/YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTk2MA–/https://s.yimg.com/uP2/2E2/6/ – ~ B / aD00MDc2O3c9NDA3NjthcHBpZD15dGFjaHlvbg – / https: //media.zenfs.com/fr/the_conversation_fr_articles_180/dfe21ffb34f33b13a3bc825c429f0bcf “data-sn2/n/2 = /YXBwaWQ9aGlnaGxhbmRlcjt3PTk2MDtoPTk2MA–/https://s.yimg.com/uu/api/res/1.2/dC3PNjSuXh5E6LHbb7PZQw–~B/aD00MDc2O3c9NDA3NjthcHBpZD15dGFjaHlvbg–/https://media.zenfs.com/fr/the_conversation_fr_articles_180/dfe21ffb34f33b13a3bc825c429f0bcf”/ >
The first image of Sgr A *, the supermassive black hole in the center of our galaxy. eso2208-eht-mwa, CC BY-SA

In the center of our galaxy, the Milky Way, sits a supermassive black hole that we orbit. Until now, no one had been able to observe it, and with good reason, a black hole absorbs all the ambient light. Still, a research team has managed to produce the first image of “our” black hole, Sagittarius A * or SgrA *. More precisely, we can see its immediate surroundings with this orange halo.

Already in 2019 wrote a similar picture of a black hole story. It was the black hole M87 *, in the center of the galaxy Messier 87. Located a thousand times longer than the center of our own galaxy, it is also a thousand times larger, so the two objects have the same size “apparently” in the sky: they appear “the size of a donut placed on the Moon as seen from Earth”, to use the comparison of astronomers from Event Horizon Telescope who made the picture.

The collapsing case

The Imager SgrA * turned out to be much heavier than the M87 *, but it’s not a matter of apparent size, as we’ve just seen.

This is because we never see the black holes ourselves (the black hole is in the dark area in the middle of the image), but instead the material which collapses towards their center and which, while accelerating, emits electromagnetic waves, that reaches us. This substance rotates at a very high speed (10% of the speed of light) around the center of SgrA *. Since SgrA * is 1000 times smaller than M87 *, matter makes a turn much faster (in just a few minutes around SgrA * instead of a few days or even a few weeks around M87 *), and the radiation that reaches us is more unstable over time.

It is therefore necessary to observe SgrA * for a shorter time to obtain an average image of the same quality as M87 *. This is the difficulty: since the variability of SgrA * is greater, each image of SgrA * is different from the others, and we can not add them easily, we need to add similar images, which are necessarily shorter (to overcome this variability) and therefore more rare (because most images are different from each other).

Substance that collapses on the black hole is heated in the accretion disk and emits light. This is the growth disk, which has inhomogeneities: the disk is more or less thick depending on the distribution of the substance, if the substance falls preferably somewhere on the disk, the disk will be thicker in this place with more light intensity, and which is found in the pictures in the form of more or less intense areas: the orange halo.

Images in the radio domain

Supermassive black holes emit mainly in visible radiation, ultraviolet, even X-rays, but also within radio waves, it is this part of the signal that could be detected from Earth thanks toEvent Horizon Telescope : a network of 11 radio telescopes distributed on the earth’s surface, each composed of several antennas. One of them is in France: it is the NOEMA telescope on the Bure plateau in the Hautes Alpes, which includes 12 antennas.

The signals from the different antennas are combined using a supercomputer according to the principles of interferometry. The larger the antenna array, the higher the resolution.

Event Horizon Telescope is thus a telescope that “largely” resembles the size of the Earth … to achieve sensitivity (the ability to distinguish very weak signals) and resolution (the ability to distinguish between two very weak signals). close) unsurpassed.

The farm: interfering signals from very distant telescopes

Interferometry is a very sensitive technique: in order to cause two electromagnetic waves to interfere, the peaks of their respective waves must not be too far apart (we then say that the waves are contiguous). It’s a bit like combining the oscillations between them, created by two rocks falling into the water, if they fall at different times, the oscillations do not interact or only slightly, if on the other hand they are lost at the same time, so oscillations interact, we say, that they are then coherent.

The phenomenal amounts of data generated are then stored on hard drives and transported by air to the computer center where the interferometry calculations take place.

But these calculations are not enough to generate the images presented by the collaboration Event Horizon Telescope. The researchers actually modeled thousands of black hole configurations with different speeds and axes of rotation to compare the signals that would escape such black holes with the signals that were actually detected.

What color is a black hole?

The colors of the images are therefore false colors, as radio waves are detected, invisible to the human eye. They indicate the temperature in the different zones around the black hole, called brightness temperature. For SgrA *, the temperature goes from 0 Kelvin (this is the absolute zero point for temperatures, ie about – 273 ℃) to 14 billion Kelvin; while for M87 * it varies from 0 to 6 billion K. This is always because SgrA * is smaller and the collapsing substance is closer to its black hole.

In the future, the team hopes to be able to make films of black holes to study their dynamics, changing structures and variations in the growth disk, observe the rays emitted perpendicular to the growth disk or even their magnetic fields. This may make it possible to test the general theory of relativity in more detail.

The original version of this article was published on The Conversation, a nonprofit news site dedicated to sharing ideas between academic experts and the general public.

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