We can catalog black holes according to the amount of mass they contain. The best known are stellar black holes . On the other hand we have supermassive black holes , with a mass of millions and even billions of times that of the Sun. They are not small at all, like the previous ones, but they reach a radius similar to the distance to the star closest, Alpha Centauri. Nor are they scattered haphazardly around the galaxy, but rather are located in its center.
What astrophysicists still don’t know is where they came from, what their origin is. There are different hypotheses, from that they are the result of hundreds or thousands of small black holes that merged in ancient times, to large clouds of gas that quickly collapse and accrete the surrounding mass, through to a collapse of a star cluster. However, none of them resolves the main and most serious handicap of their formation: a huge amount of matter is needed in a sufficiently small volume. In the current universe, this situation is quite unusual, but there was a time when it was more likely to be found: when the universe was young. And where? Astrophysicists believe thatsupermassive black holes get fat because accelerated accretion of gas and dust occurs, just what is observed in objects that abounded in the early universe, quasars. Acronym for quasi-stellar objects, they are one of the most powerful sources of energy we know and whose engine is thought to be a supermassive black hole. If so, they must have formed very soon, because the observation of the most distant quasars shows that supermassive black holes had already appeared in the Universe when it was less than a billion years old. Since then, these objects have done nothing but get fat. As if it were a galactic garbage dump, they collect the masses of gas and dust that surround them -a very abundant material in the center of galaxies-, which allows them to grow to sizes like the one found in the center of the Milky Way. , whose mass is 4.5 million times that of our Sun. The Sun also feeds on its smaller cousins, since orbiting around it is a whole host of stellar black holes that contribute little by little to increasing its mass. According to estimates by astrophysicists, it must be in the order of 20,000.
Within the group of super black holes, astrophysicists distinguish a peculiar subgroup: the so-called ultramassive black holes (UMBH), like the one found in the center of our neighbor M87, a giant elliptical galaxy located just over 53 million years away. light that has a mass that reaches 6,000 million solar masses. Can a UMBH grow without limits? Apparently not. Theoretically, its ‘fatness’ has a ceiling: around 50,000 million solar masses, according to Andrew King, of the University of Leicester, England.
But something is wrong with astrophysicists studying the population of black holes in the universe. We have those that appear as a result of the death of a star, which have masses between 5 and 80 times that of the Sun. Then we have the superholes, between 100,000 and billions of solar masses. And between both groups… nothing? Aren’t there intermediate mass black holes, 1,000 or 10,000 times the mass of the Sun? This absence does not convince astronomers, who are struggling to find evidence that a type of intermediate black hole exists. Some point to mysterious ultraluminous X-ray sources as its astronomical fingerprint.
In the 1980s, the Einstein satellite, an X-ray space telescope, discovered a new class of extremely bright X-ray source within some galaxies. It was quite a surprise, because they were not close to supermassive black holes. They were very intense sources that emitted equally in all directions. In addition, the objects were so luminous that by force they had to expel part of the matter they contained. In short, none of this could be explained by mere stellar processes; there must be something else. From quasars to supernova remnants to intermediate-mass black holes: these were the usual suspects for the mysterious objects. With the launch of new more powerful and precise satellites such as NuSTAR, Chandra or XMM-Newton, a new suspect has joined the list: neutron stars, stellar corpses whose size is that of a radio sphere that of an average city. and with a mass of 2-3 soles. Resolving what they are is complicated because they are not very abundant in the universe: in our galaxy, for example, none have been detected, and at most there is only one ultraluminous X-ray source per galaxy.
Now, if these objects are not intermediate black holes, where are they? Or better yet, are they really up there? In 2014, a team of astronomers claimed that what appeared to be a group of massive stars orbiting just 3 light-years from the center of our galaxy, GCIRS 13E, was one of those elusive black holes, in this case 1,300 masses. solar. But the evidence they provided was circumstantial, and most astronomers are reluctant to accept their conclusions. Since then, other astronomers have proposed intermediate black hole candidates, without much success. The last one is from 2017, when it was announced that there could be a black hole of a few thousand solar masses in the globular cluster 47 Tucanae. They had deduced its existence by studying the movement of the stars in the cluster, but after a more detailed analysis of its dynamics, it was ruled out that it was a black hole.
On the other hand, it is also necessary to explain where they came from: here astronomers have let their imagination run wild and among others is the colorful hypothesis that they could be formed when the stars of a group collide in a chain. Be that as it may, the enigma continues.
If a dense mist of mystery covers these mysterious black holes, another more impenetrable mist is that of micro black holes. In 1974 Stephen Hawking postulated that black holes the size of a pinhead and with the mass of a mountain could exist. Evidently their existence is not due to stellar collapse, and in the years that followed, theorists struggled to find some mechanism that would generate them. The Big Bang gave them that mechanism: they could have formed less than a second after the cosmic jolt, hence they are also called primordial black holes. The essential ingredient is a fluctuation in the density of the Universe that induces a gravitational collapse. But the challenge is, obviously, to discover how to detect them… if they exist, of course.