The Galactic Center is essentially the Wild West of the Milky Way. It is dominated by a supermassive black hole with a mass of 4.3 million times the mass of the Sun, and it is considered the most hostile environment in the galaxy.
Black holes, as we know, are where stars risk certain death. Stars that venture too close to one can be reduced to streams of gas and dust by intense tidal forces, ending what could have been a long and fruitful life of merging atoms.
Imagine the surprise astronomers felt when they found the opposite – a star not dying but in the process of being born near the monstrous Milky Way, Sagittarius A* (Sgr A*).
X3a, as this new cosmic body is known, is only a few tens of thousands of years old, and is barely dispersed in cosmic time. However, it is so close to Sgr A* that its very existence challenges our understanding of not only star formation but the processes of black holes.
Despite its dynamic density and strong ultraviolet and X-ray radiation preventing gas from fusion into the star’s seed, X3a not only does not exist but is located where a young star would not be expected to form.
At 10 times its radius, 15 times its mass, and 24,000 times the luminosity of the Sun, it’s not quite a small star either.
According to a team led by astrophysicist Florian Becker of the University of Cologne in Germany, the reason is relatively simple. X3a did not form out of place: it formed far from the black hole and migrated inward.
“It turns out that there is a region a few light years from the black hole that satisfies the conditions for star formation,” Peißker explains. “This region, a ring of gas and dust, is cold enough and protected from damaging radiation.”
The details of star formation are still obscure, but we know that certain conditions must be met. A star forms in a dense, cold molecular cloud in space, when a denser mass collapses and rotates under its own gravity and begins to gravitationally attract more material from the cloud around it.
The immediate vicinity of a supermassive black hole is not a particularly good environment for these conditions. Sgr A* is surrounded by a disk of dust and gas known as an accretion disk, which rotates around it at high speeds and emits intense light. UV photons exert radiation pressure and photoevaporation processes that can Reducing star formation And accumulator discs emit a lot.
After a certain distance from the black hole, the material is thick enough to protect against these devastating effects and keep temperatures cool enough for stars to form.
According to the team’s analysis, X3a could have formed in this region, a ring of material around the center of the galaxy. In this ring, an even denser cloud can clump together, generating enough mass in a region small enough for a gravitational collapse that starts the star formation process.
This cloud began with a mass of about 100 suns, and its gravitational collapse would have resulted in the formation of many young stars.
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But the X3a did not stay put. It began migrating toward Sgr A*, still surrounded by material as it grew. On the way, you may have encountered other dense clumps and clouds that formed in the same environment, allowing the young star to accumulate more mass. It’s still in this growing stage now, surrounded by material.
It’s that lump of matter, called X3, that first caught astronomers’ attention, before they recognized the tiny star inside. Multiple infrared and near-infrared instruments can discern the star’s long-wavelength light that can penetrate the thick cloud envelope around it. Analysis of this light revealed a chemistry consistent with a baby star.
“With its high mass of about ten times that of the Sun, X3a is an interstellar giant, and these giants are evolving very quickly towards maturity,” says astronomer Michel Zajec at Masaryk University in the Czech Republic.
“We were fortunate to have detected the massive star in the center of the comet-shaped envelope. Next, we identified key features associated with a young age, such as the young star’s envelope orbiting it.”
The discovery of X3a may help astronomers solve another decades-old mystery. About 20 years ago, very young stars were observed near Sgr A*, where it was previously thought that only very old stars could exist. X3a suggests that distant young stars, followed by their migration towards Sgr A*, may not be a particularly uncommon event.
And it may not only happen in our galaxy. Structures around Sgr A* have been identified in many other galaxies, and they could host clusters of their own young stars. It’s an idea that could change our understanding of the dynamics of galactic cores.
Future work will test the team’s star formation model, not just for the Milky Way but for the wider universe.
Research published in Astrophysical Journal Letters.
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