A very young star in the formation phase (colloquially “baby star”) was unexpectedly discovered near the supermassive black hole Sagittarius A star (Sgr A*), at the center of our Milky Way.
The star is only several tens of thousands of years old, making it younger than humanity. The particularity of the baby star X3a is that, in theory, it should not be able to exist so close to the supermassive black hole. However, the team behind the study published in The Astrophysical Journal believes that it formed in a cloud of dust orbiting the giant black hole and sank to its current orbit only after it had formed.
“The paradox of youth” The vicinity of the black hole at the center of our galaxy is generally considered to be a region characterized by highly dynamic processes and strong ultraviolet and X-ray radiation. Precisely these conditions act against the formation of stars like our sun. Thus, scientists had long assumed that, over periods of billions of years, only old and evolved stars can settle by dynamic friction in the vicinity of the supermassive black hole.
Surprisingly, however, very young stars were found in the vicinity of Sgr A* already twenty years ago. It is not yet clear how these stars got there or where they formed. The appearance of very young stars very close to the supermassive black hole has been called “the paradox of youth”. The baby star X3a – which is ten times larger and fifteen times heavier than our Sun – could now close the gap between star formation and young stars in the vicinity of Sgr A*. X3a needs special conditions to form in the vicinity of the black hole.
Favorable region to form a “baby star” First author Dr. Florian Peisker explained in a statement: “It turns out that there is a region at a distance of a few light-years from the black hole that meets the conditions for star formation. This region, a ring of gas and dust, is sufficiently cool and shielded from destructive radiation.”
Low temperatures and high densities create an environment in which clouds of hundreds of solar masses can form. In principle, these clouds can move very quickly in the direction of the black hole due to cloud-to-cloud collisions and scattering that eliminates angular momentum.
In addition, very hot clusters formed near the baby star that could be absorbed by X3a. These clusters could also contribute to X3a reaching such a high mass. However, these clusters are only part of the X3a formation story. They still do not explain their “birth”.
The scientists assume the following hypothesis is possible: protected from the gravitational influence of Sgr A* and from the intense radiation, a sufficiently dense cloud could have formed in the outer ring of gas and dust surrounding the center of the Galaxy. This cloud had a mass of about a hundred suns and collapsed under its own gravity into one or more protostars.
“This putative decay time roughly corresponds to the age of X3a,” Peisker added. Observations have shown that there are many of these clouds that can interact with each other. Therefore, a cloud is likely to fall towards the black hole from time to time. This scenario would also fit with the stellar development phase of X3a, which is currently evolving into a mature star. It is therefore quite plausible that the ring of gas and dust acts as the birthplace of young stars at the center of our galaxy.
Journal Reference:
- Florian Peißker, Michal Zajaček, et al. X3: A High-mass Young Stellar Object Close to the Supermassive Black Hole Sgr A*. The Astrophysical Journal. DOI 10.3847/1538-4357/aca977