Sagittarius A*, the restless friend of the Milky Way who avoided the photo

Image developed by the Event Horizon Telescope (EHT) of Sagittarius A*.EVENT HORIZON TELESCOPE COLLABOR (EFE)

On April 10, 2019, the collaboration of the Event Horizon Telescope (EHT) revealed to the world the first image of a black hole, that of the galaxy M87.

This Thursday, the same team has shown the image of the black hole that resides in the center of our own galaxy, the Milky Way.

This is "our" black hole, a giant that contains four million suns inside and is located about 26,000 light years from us, in the constellation of Sagittarius, hence we know it by the nickname Sagittarius A, or SgrA * for friends.

And it is that SgrA* has been a friend to EHT researchers for many years.

Taking a picture of him has been much more difficult than with M87, and the reason is that he won't sit still!

In the photo of a black hole we see the incandescent gas that revolves around the event horizon, the door of no return out of the Universe.

This gas moves around the black hole at close to the speed of light, the maximum possible, but while in M87 it takes weeks to go around, in SgrA*, about 1,000 times smaller, it takes only minutes.

This causes the photo to change as the EHT watches for an entire night.

It's like trying to take a photo at night of a child who won't stop moving.

EHT researchers have spent years developing new algorithms that have allowed us to counteract the rapid movement of gas around SgrA*.

We have made literally tens of millions of images with simulated data to refine our programs and models, akin to searching for the lens and filter to give us that perfect snapshot.

Once everything was ready, we have made thousands of images of SgrA*.

Each of these images is slightly different, but the vast majority show the characteristic ring of light from a black hole with slight variations in the distribution of brightness along it.

The image shown to the world is an average of these images, revealing the giant at the center of our galaxy.

Black holes are one of the most extravagant and enigmatic objects that we can find in the Universe, but at the same time they are the simplest.

According to Einstein's theory of relativity, the size we see of the black hole is given only by its mass and, to a much lesser extent, how fast it rotates (its

).

Previous studies, worthy of the Nobel Prize in Physics in 2020, have allowed us to know in great detail the mass of SgrA*, so we knew very well (with much greater precision than for M87*) how big SgrA* should have been.

The ring of light in the EHT image is exactly 52 microarcseconds in size (about the size of a donut on the Moon), in perfect agreement with relativity theory.

SgrA* is about 1,000 times smaller than M87, and yet the two black holes are exactly the same, only their size changes.

For the first time we confirm this fundamental property of relativity.

Einstein is still right!

There are no memories sweeter than those of childhood and, for that child who spent summer nights in the farm looking at the stars and dreamed of deciphering the enigmas of the Universe, what greater reward is there than participating in the team that has obtained the first images of black holes?

Sometimes reality surpasses your wildest dreams!

But this is only the beginning.

In the coming years we will continue to expand and improve the network of antennas that make up the EHT, presenting increasingly sharper images with which to perform increasingly precise tests of general relativity.

And of course we want to make black hole movies!

(Hollywood trembles…) In the next few years we will see how the black hole in M87* engulfs part of the gas around it, and we will learn how it is possible to extract energy from black holes to form those

that emanate from its vicinity and extend beyond the size of the galaxy.

And what better way to do this journey of scientific discovery in company, and none better than that of my colleagues and friends at the Institute of Astrophysics of Andalusia, Antonio Fuentes, Guang-Yao Zhao, Rocco Lico, Thalia Traianou, Ilje Cho, and Antxon Alberdi.

An example of professionalism and dedication, without which it would not have been possible to obtain the image of the black hole in the center of our galaxy.

José L. Gómez

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