Image credit: LIGO/CALTECH/MIT/SONOMA STATE (AURORE SIMONNET)
It was not easy to deduce the existence of black holes, and even less to discover that the center of many galaxies hides one of enormous size
Hera was not overly fond of her stepson Heracles. Knowing this, the messenger of the gods, Hermes, decided to take advantage of a day when Hera was sleeping to put the baby on her and to suck on her stepmother’s breasts, feeding on her. Hera was soon awake and, as soon as she found Heracles on her lap, she jerked his breast away from her, spilling milk into the sky. The whitish substance stained the celestial vault and on clear nights, you can see a kind of white road that crosses the sky, the Milky Way. This is how the Greeks told it and, luckily, we now know that history has nothing to do with gods. When we look up at the cosmos and come face to face with that huge elongated blob, what we are seeing is our own galaxy.
If we imagine the galaxy as a disk within which we live, we can look in three directions. Either we point up or down the disk or towards its edges. In the latter case we will find many more stars than if we look in the other direction, where the limit of our galaxy is closer. If we look at the edge of the disk, to our eyes, it appears as a great cluster of tiny stars crowded around a large strip in the sky. That is what we see in the sky and not Hera’s milk. However, this information accompanies us. Aristotle already suggested that it must be a large number of really distant stars, Galileo verified it in 1610 with his rudimentary telescope and Edwin Hubble confirmed 310 years later that the Milky Way is only a finite star cluster and separated from others that populate the universe. Now that we knew what it was, the question was clear: What is at its center if we are not there?
Broadly speaking, a black hole is a region of spacetime with such a density that not even light can escape its gravitational pull. At first it was just an assumption, a disturbing consequence of the field equations and relativity that Einstein had given birth to in the early 20th century. However, over time the mathematical deduction took shape and those first dark stars began to be taken seriously as science, receiving the name of black holes.
A monster that swallows light It was 1971 and two scientists were wondering the same thing: what is at the heart of our galaxy? Their names were Donald Lynden-Bell and Martin Rees, and to answer this question they began to compare the location of some extremely dense and distant astronomical objects capable of emitting very strong pulses of light (quasars) and compare this with the rest of the light. that comes to us from our galaxy. Taking this data, the doctors came up with a hypothesis that Lynden-Bell himself had already flirted with in the past. Those data were compatible with the idea that, at the center of each galaxy, there was a large black hole.
4 million soles
Visible light is the part of the electromagnetic spectrum that our eyes recognize, but beyond it there are other very interesting radiation. Radio waves, which are also a kind of light, can pass through objects, making them extremely useful to astronomers. It was precisely thanks to this radio astronomy that two scientists named Bruce Balick and Robert Brown confirmed the reveries of Lynden-Bell and Rees. Observing the center of our galaxy they found that it emitted a large amount of energy. Sagittarius A * (pronounced Sagittarius A star) was Brown’s name for this central region of our galaxy, and it soon became a recurring topic of study in the community. What could be emitting so much energy? Was it one of those elusive black holes? Or maybe just a cluster of millions of stars really put together?
From how the stars around this region moved and other data sets, the experts eventually concluded that their observations were compatible with the presence of a single, hugely massive object. We now know that it was a black hole with the mass of 4 million suns concentrated in a small region of space.
Over time, we not only deduced that many other galaxies must host supermassive black holes at their centers, but that, unlike ours, they could be active. What’s more, in 2019 we obtained the first direct image of a black hole thanks to a network of radio telescopes known as the Event Horizon Telescope.
Just a year ago, taking data from the same telescope, researchers managed to superimpose on that famous image another showing the direction of the black hole’s magnetic fields. Hopefully, at this rate, it won’t take long to “photograph” the center of our galaxy and complete the journey that Lynden-Bell began.
Some argue that the 2019 image was actually the first taken of a black hole. Leaving aside that the image is not exactly that of the black hole itself, the problem seems to lie elsewhere. In 1979, a CNRS researcher named J. P. Luminet published the result of a computer simulation of a black hole. The image in the article is highly reminiscent of the one taken in 2019. What’s more, the researcher suggested that his simulation may not be so different from what we could see in M87 *, which coincidentally is the black hole in the famous 2019 image.