An international team of researchers led by Dr. Florian Peißker of the Institute of Astrophysics at the University of Cologne has discovered the heaviest and youngest young star near the black hole at the center of our Milky Way galaxy. The star is in the formation phase near the supermassive black hole Sagittarius A* (Sgr A*) at the center of our Milky Way galaxy.

The star is much younger than us – only tens of thousands of years old. The unique thing about the young star X3a is that according to the theory it should not be so close to the supermassive black hole. The team claims that it developed in a cloud of dust that circled the massive black hole and sank to its current orbit only after it had already formed.

Most people agree that the region around the black hole at the center of our Milky Way galaxy is characterized by strong X-ray and UV radiation and very active processes. It is precisely these conditions that hinder the formation of stars such as our sun. That is why scientists had long expected that only old, developed stars could crash into the supermassive black hole region through dynamic friction for billions of years.

However, quite surprisingly, very young stars were found in the vicinity of Sgr A* as early as twenty years ago. It is still unclear how these stars got there or where they formed. The appearance of very young stars close to the supermassive black hole has been called “the paradox of youth.”

A gap between star formation and the newborn stars near Sgr A* can now be closed by the baby star X3a, which is 10 times larger and 15 times more massive than our Sun. To form X3a in the immediate vicinity of the black hole, special conditions are required.

The baby star X3a in its envelope, X3
The baby star X3a in its envelope, X3 (shown in blue). The envelope is blown by stellar winds, giving it a cigar-like shape. On timescales of less than 10 years, clumps can form and are in turn engulfed by X3a. Copyright: Florian Peissker

First author Dr. Florian Peissker explained: “It turns out that at a distance of a few light years from the black hole there is a region that meets the conditions for star formation. This region, a ring of gas and dust, is sufficiently cold and shielded from destructive radiation.”

Clouds with hundreds of solar masses can form in a low-temperature, high-density environment. Due to cloud-to-cloud collisions and angular momentum scattering, these clouds could theoretically move very quickly toward the black hole.

In addition, very hot clumps developed near the newborn star, which X3a could later accrete. Therefore, these clusters may also have helped X3a acquire such a high mass in the first place. However, these clumps are only a small part of X3a’s formation story. His “birth” is still not explained.

The scientists believe that the following scenario is possible: shielded from the gravitational influence of Sgr A* and intense radiation, a sufficiently dense cloud could have formed in the outer ring of gas and dust around the center of the Milky Way. This cloud had a mass of about a hundred suns and collapsed under its gravity into one or more protostars.

Peissker said, “This so-called fall time roughly corresponds to the age of X3a. . Observations have shown that many of these clouds can interact with each other. Therefore, a cloud probably falls in the direction of the black hole from time to time.”

Dr. Michal Zajaček of Masaryk University in Brno (Czech Republic), a study co-author, clarified: “With its high mass of about ten times the mass of the Sun, X3a is a giant among the stars, and these giants evolve very quickly to maturity. We were lucky enough to see the massive star in the middle of the comet-shaped circumstellar envelope. Then we identified key features associated with early age, such as the compact circumstellar envelope that orbits it.”

The method described could also be applicable to galaxies with similar dust and gas rings. So many galaxies have very young stars at their centers. This star formation model for our Milky Way and others will be put to the test through planned observations with NASA’s James Webb Space Telescope or the Very Large Telescope at the European Southern Observatory in Chile.

Magazine reference:

  1. Florian Peissker, Michal Zajaček, et al. X3: A young, high-mass stellar object close to the supermassive black hole Sgr A*. The Astrophysical Journal. DOI 10.3847/1538-4357/aca977