As on great occasions, several scientific institutions had called simultaneous press conferences eight days in advance for this May 12 in different cities around the world: Washington, Munich, Tokyo, Mexico and Madrid, among others. The objective was to announce a "revolutionary" finding, which changes our way of understanding the universe" in the words of Rosa Menéndez, president of the CSIC, the Spanish convening body. This has been the case: it is the first image of Sagittarius A*, a supermassive black hole whose existence in the center of our galaxy, the Milky Way, was assumed; but no visual evidence. Until now.
Observed a supermassive black hole hidden in a ring of cosmic dust
The finding is the work of the Event Horizon Telescope (EHT). In reality, the EHT is a synchronized network of radio telescopes – from various scientific institutions – that extends through Mexico, Chile, the South Pole, the United States and Spain, which participates with the Observatory of the Institute of Millimetric Radio Astronomy located on Pico Veleta, in Granada. That network works like a virtual telescope the size of the Earth.
The photo shown today has been generated by combining the information provided by the EHT network. This is the same method that was used in 2019 when the same team unveiled the first image of a black holealso supermassive: on that occasion it was the one located in the center of the elliptical galaxy Messier 87, located about 54 million light years away.
A 'monster' that catches everything
Much closer, about 26,000 light years from Earth, and with a mass about four million times greater than that of the Sun, Sagittarius A* is a monster that attracts celestial bodies, electromagnetic waves, gases, dust and everything that does not moves faster than the speed of light… (in our universe, nothing moves faster than light).
The force of attraction unleashed by this supermassive black hole is what gives the Milky Way – the galaxy in which our Solar System is found – its characteristic spiral and elliptical shape. In fact, when a black hole catches a cosmic object (for example, a star) a process called 'spaghettification' takes place – I'm not kidding – whereby the object is stretched into thin and elongated shapes, like spaghetti.
Until now, astronomers worked with the hypothesis that the 'central body' of our galaxy was a supermassive black hole. That hypothesis was supported by indirect data, such as the 10-year movement of the star S2 and its elliptical orbit around the enigmatic central region. Today, finally, researchers –and the rest of Humanity– can see Sagittarius A*.
Achieving the first visual proof of the center of our galaxy has not been easy. The challenge was even greater than in the case of the Messier 87 black hole, because there is a lot of dust and cosmic gas clouds between the Earth and the center of our galaxy.
This finding should serve to contribute to one of the big questions surrounding black holes: Did they form with such high masses from the beginning, or does their mass accumulate over time?
Distribution of the EHT, a virtual Earth-sized telescope
Black holes are not holes, nor are they empty. Quite the contrary. They are extremely dense cosmic objects of matter, with enormous masses, but compact in their dimensions. Their volume is minuscule compared to their enormous mass (from that of a large star to millions of times that of the Sun), so they have a very high gravitational force, so much so that they even trap the photons that make up light. That's why we see them black: they don't reflect anything, not even nearby stars. Its ability to attract reaches the curve of the space-time continuum, forming what is known as a 'singularity': a compression of matter so strong that its density tends to infinity.
The theoretical limit where matter and electromagnetic waves, including light, are captured is called the black hole's 'event horizon'. The last possible orbit of a cosmic object would reach there. Once the object crosses that theoretical threshold it is no longer visible to us on this side of the hole.
How are they formed? It is not entirely clear. There are two types of black holes: stellar and supermassive. The stellar ones would be formed from great extinct stars. When a large star runs out of 'fuel' and stops producing energy, its matter compresses, increasing its density and therefore its gravity (beginning to attract other cosmic objects). In order for a star to form a black hole, it must first have a mass about 10 times greater than our Sun, from which it can be deduced that our 'king star' will never become a black hole. The second type of black holes, supermassive black holes, contain as much matter as millions of suns (between one and one hundred million solar masses). Its origin is still unknown.