Even if the existence of black holes is indisputable today for the scientific community, they continue to exert a particular fascination for cosmologists as the enigmas about them still remain numerous.
The question of how supermassive black holes form, in particular, is a current area of active research. This shadowy area could be illuminated thanks to the model of black hole stars.
Supermassive black holes (TNSM) are a category of black holes whose mass can vary from a few hundred thousand to a few billion solar masses. According to the current model of galactic formation, these black holes are found at the center of the majority of galaxies. Thus, the center of the Milky Way is home to Sagittarius A*, a supermassive black hole of about 4 million solar masses (1).
The origin of TNSM is currently officially unknown and is the subject of much research. For such black holes to form, a huge amount of matter must be in a very small volume of space. Several hypotheses have been developed to integrate this prerequisite.
The primordial black hole model postulates that at the end of the Big Bang, areas of space-time subjected to colossal energy densities collapsed and formed intermediate black holes of a few tens of thousands of masses. solar. These would then have merged or accreted matter, leading to the formation of TNSM (2).
The population III star model, on the other hand, predicts the existence of extremely massive stars that would have formed soon after the Big Bang (at around z=20) from the collapse of pre-galactic halos (3). Another model predicts the collapse of stellar clusters under the effect of relativistic dispersion velocities leading to the formation of TNSM (4).
Finally, a last model proposes to provide an explanation for the origin of TNSM:the theory of stars with black holes or quasi-stars.
The black hole star theory postulates that a particular type of extremely massive star would have formed long before the appearance of the first "classical" stars. These quasi-stars result from the gravitational collapse (via a mechanism called "Jean's gravitational instability") of giant gas clouds in the first moments of the universe. Once the protostar is formed, its core is extremely dense and compacted under the effect of gravitational contraction.
The latter is also very unstable, the cause of this instability being the internal production of electron-positron pairs resulting in a strong energy disturbance. This instability is all the more increased by the increasing flux of neutrinos emitted by the core, subjected to the various internal and external pressure forces. Eventually, the instability becomes critical and the core collapses, forming a black hole in the center of the star.
The energy released by the gravitational collapse of the core is absorbed by the peripheral layers of the star, these being dense enough to resist the influx of energy and thus allow the star to maintain its integrity in place. and place to explode in supernova. At the end of this process, the black hole is therefore surrounded by the outer layers of the protostar. In other words, a quasi-star is nothing but a star whose heart is a black hole.
Once the central black hole is formed, it begins to absorb the material that makes up the star. This absorption generates a release of energy pushing back the peripheral layers and causing the expansion of the star until it reaches between 50 and 100 times the size of a red giant (3), i.e. a diameter of about 10 billion kilometers for a surface temperature of about 3700°C (5). This energy release also compensates for the effect of gravitational contraction, creating a balance similar to that generated by the thermonuclear reactions of a classical star.
As the central black hole absorbs more and more matter, the lifespan of a quasi-star is estimated at 7 million years (6). After this time, the black hole becomes an intermediate black hole, reaching around 10,000 solar masses after absorbing the entire star. If the protostar is initially massive enough, the black hole can accrete matter until it directly becomes a TNSM (7).
