A Star Grinder in the Centre of the Galaxy

Farther than one tenth of a light year from SgrA*, we can see very massive, so-called O-type, stars. Such stars weigh twenty and more Sun-masses and are ten or more times larger than our Sun. These stars must have formed in the past 5 million years as they cannot live longer than that. Beside the O-type stars, we can also find stars there that are only a few times heavier than our Sun, the so-called B-type stars. Such stars can live much longer.

Surprisingly, closer than one tenth of a light year to SgrA*, astronomers see only B-type stars that are younger than 50 million years and no O-type stars at all. This change in the population of stars with distance from the central super-massive black hole remained a mystery for decades. Another puzzle is that far away from the centre of our Galaxy, astronomers have found 200 million years old B-type stars that are racing away from it with many hundreds of kilometres per second. But such old B-type stars are not seen near SgrA*, so how could they have been ejected from there?

The solution lies in a hidden population of about ten thousands of smaller black holes lingering around the central super-massive black hole. These black holes cannot be observed directly as they are the dark corpses of previous very heavy stars and they orbit around SgrA* with a speed of a thousand kilometres per second.

The team of Jaroslav Haas has calculated that these black holes collide with the heavy and large O-stars, thereby destroying them within just a few million years. „The smaller B-stars can survive much longer, in fact for some 50 million years. This explains why the heavy O-type stars are missing at distances smaller than about one tenth of a light year from SgrA*, with only B-stars surviving there,“ according to Jaroslav Haas.

SgrA* (EHT Collaboration)

Pavel Kroupa, who is a co-author of this study, explains: „These results give us an entirely new understanding of the immediate surroundings of the central super-massive black hole.“ And Jaroslav Haas adds: „We can even estimate how the number of black holes changes with the distance from the central super-massive black hole and we found that this number must increase going further away, reaching a maximum to then decline again.“

Ladislav Šubr, also a co-author of this publication, explains: „This density profile is a result of the complicated dynamical processes near the central super-massive black hole and our results will allow us to perform new computer simulations to better understand these.“

Ph.D. student Myank Singhal, who also co-authored this work, finds: „This is so extremely exciting and it will allow me to do new calculations that will help us understand this violent pack of thousands of black holes at the centre of our Galaxy that is continuously destroying the stars down there.“ And Ladislav Šubr adds: „The existence of distant 200 million years old B-stars racing away from SgrA* is now indeed nicely explained by strong ejection events where the companion B-stars that remained close to SgrA* have since been destroyed by the wild black holes.“

The authors call the cluster of thousands of black holes in the Galactic centre the „Star Grinder“ because the black holes destroy stars through the collisions. Some such collisions may have been already observed through the dusty clouds which appear very close to SgrA*. „The population of thousands of black holes inevitably leads to many fascinating new effects to be observed and studied in the centre of our Galaxy such as bright flaring up of otherwise unseen stars as black holes pass in front of them,“ Jaroslav Haas concludes.

Press Release