The first image of the black hole in the center of our galaxy

The Event Horizon Telescope (EHT), an international consortium of multiple radio telescopes, has managed to observe the “shadow” of the Sagittarius A* black hole, located at the center of our Milky Way, for the first time. The image is reminiscent of the one the EHT always photographed in 2019, when it published the first-ever image of a black hole. At the time, the observation concerned a black hole in the center of the Messier Galaxy (M87), about 55 million light-years from Earth. Instead, the new observation concerns a colleague, but is closer to us, at least in astronomical terms: 26,000 light-years away and an estimated mass of about 4 million times the mass of the Sun.

The presence of a black hole in the center of the Milky Way has been hypothesized for some time and found various confirmations, thanks to analyzes of how celestial bodies move in the galaxy, but its presence has never been observed.

To detect what is happening near the black hole from a distance, radio telescopes are used, and large antennas, unlike conventional optical telescopes, use a parabola to detect radio waves emitted by objects (radio sources) located in space. Their use allows us to observe what happened long ago over enormous distances, such that it would take tens, hundreds, and sometimes even thousands of years to reach the light.

An optical installation of the likes of the radio telescopes used by EHT Worldwide (ESO)

In general, the larger the proverbs, the more accurate the notes. That’s why researchers wondered a few years ago if it was possible to turn the entire planet into some kind of large antenna, for a more powerful instrument. From this idea was born the Event Horizon Telescope (EHT), which brings telescopes from Chile to Antarctica to Hawaii, synchronized with atomic clocks in order to collect data on galaxies they are pointed at.

The radio telescope network used by the EHT (ESO)

black holes
However, the black hole cannot be seen or imaged directly, because these objects have such a gravitational field that nothing inside them can escape from it: not even light. We can imagine it as a sphere with the actual massive object inside. The spherical surface delineates the boundary under which conditions occur in which nothing can escape, or turn back if it ends up in the black hole: this sphere is called the event horizon.

The massive object in the center of the sphere, which creates a distortion in spacetime (the four-dimensional structure of the universe is explained, if you’re completely confused here), is instead defined by a “singularity”: it’s so named because we know the properties, but we know they differ from Those that regulate the behavior of matter as we know it. It is also assumed that the singularity density tends toward infinity.

Schematic representation of a black hole, click on the image to enlarge (ESO)

We do not know what happens inside the boundary of the event horizon, on the other hand from the outside we can observe what happens to matter (gas and dust) when it ends up at the boundary, and based on its reactions, we understand something more about the black hole. Thanks to EHT, it was possible to do this with the M87 * and now with the Sagittarius A *.

Sagittarius A*
The black hole at the center of the Milky Way is much closer to Earth, but it’s smaller and less active than the one at the center of M87, so much so that it’s more difficult to observe. Matter near the event horizon changes very quickly, and so the shadow of the black hole itself changes rapidly, which makes observations with synchronized radio telescopes even more complicated. The research team also had to deal with interstellar dust in our galaxy, which interfered with the observations.

And it was precisely the work of “cleaning” the data that required the research groups a lot of time. Sagittarius A* was observed in 2017 around the same time as the black hole at the center of M87 was observed. For the latter, it took nearly two years of analysis, while for the other it took nearly five years. However, research groups began work earlier on the M87, because it was considered less complex.

Arch A* as seen by the EHT (ESO)

As Katie Bowman, one of the astrophysicists involved in the initiative explained: “Taking a picture with the EHT is like listening to a song being played on a piano with many keys missing.” The collected data contains numerous gaps, which must be filled with complex calculations and to exclude interference from other celestial bodies. So research groups have worked for a long time without being sure of a sufficiently accurate and acceptable result.

EHT collected 3.5 million gigabytes, which is a huge amount of data that cannot be transferred over the Internet. The files were saved on hundreds of hard drives and then transmitted by radio telescopes to research and analysis centers in the United States and Germany. The data were then analyzed and selected incrementally to the point of being able to extract relevant information: arc image A*.

From the image and other data, EHT researchers have found that the Milky Way’s black hole is not particularly voracious when compared to other black holes around the universe. The amount of matter in its vicinity that ends inside it is rather limited, as another astrophysicist, Michael Johnson, explained at the press conference presenting the results: “If Sagittarius A* was a person, he would consume a grain of rice every million years.” Of course, everything must be related to the massive mass of the black hole, which is much higher than the mass of a person. Sagittarius A* is generally found to be popular and calm.

Comparison of M87* and Sagittarius A* (ESO)

In addition to the direct impact on our galaxy, the new result is important because it confirms the reliability of the systems developed by the EHT to study black holes. In the coming years, project managers are confident that they will be able to add new radio telescopes, creating a more accurate system that could make it possible to observe the black hole over time, and with better accuracy, to understand its evolution.

The first image of a black hole

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