by The galactic center is basically the wild west of the Milky Way. The environment is dominated by a supermassive black hole 4.3 million times the mass of the Sun, and is the most hostile in the galaxy.
Black holes are, as we know, where stars risk certain death. Stars that venture too close can be teased into streams of gas and dust by the extreme tidal forces, ending what could have been a long and productive life fusing atoms together.
Imagine the surprise the astronomers felt when they found the opposite – a star that is not dying, but in the process of being born right near the Milky Way’s own monstrous maw, Sagittarius A* (Sgr A*).
X3a, as this new cosmic object is known, is only a few tens of thousands of years old, barely a blink of an eye in cosmic time. Yet it is so close to Sgr A* that its very presence challenges our understanding of not only star formation, but the operations of black holes.
Despite the dynamical intensity and powerful UV and X-ray radiation that would prevent gas from coalescing into the seed of a star, X3a not only exists, but does so in a place where no baby star is expected to form.
At 10 times the radius, 15 times the mass and 24,000 times the luminosity of the Sun, it’s not exactly a tiny infant star either.
According to a team led by astrophysicist Florian Peißker at the University of Cologne in Germany, the reason is relatively simple. X3a did not form where it is: It formed further from the black hole and migrated inward.
“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,” explains Peißker. “This region, a ring of gas and dust, is sufficiently cold and shielded from destructive radiation.”
The specifics of star formation are still something of a mystery, but we know that certain conditions must be met. A star forms in a dense, cold molecular cloud in space, when a denser clump collapses, spins, 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 considered a particularly good environment for these conditions. Sgr A* is surrounded by a disk of dust and gas known as an accretion disk, which swirls around at high speeds and emits intense light. Ultraviolet photons exert radiation pressure and photoevaporation processes that can limit star formation and accretion disks emit much of it.
Beyond a certain distance from the black hole, the material is thick enough to protect against these destructive influences and keep temperatures cool enough for star formation.
According to the team’s analysis, X3a could have formed in this region, a ring of material around the galactic center. In this ring, a denser cloud could have coalesced, generating enough mass in a small enough area for the gravitational collapse that starts the star formation process.
This cloud started out with a mass of about 100 suns, and its gravitational collapse could have triggered the formation of several baby stars.
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But the X3a was not left behind. It began to migrate in towards Sgr A*, still surrounded by material as it grew. On the way, it could have encountered other dense clumps and clouds that formed in the same environment, allowing the baby star to gather even more mass. It’s still in that growth phase now, surrounded by material.
It’s that blob of material, called X3, that first caught astronomers’ attention, before they identified the baby star inside. Several infrared and near-infrared instruments could discern the long-wavelength light of the star that could penetrate the thick cloud around it. Analysis of this light revealed chemistry consistent with a baby star.
“With its high mass of around ten times the mass of the Sun, X3a is a giant among stars, and these giants are evolving very quickly towards maturity,” says astronomer Michal Zajaček at Masaryk University in the Czech Republic.
“We were lucky to see the massive star in the middle of the comet-shaped circumstellar envelope. We then identified key features associated with a young age, such as the compact circumstellar envelope rotating around it.”
The discovery of X3a could help astronomers solve another decades-long mystery. About 20 years ago, very young stars were discovered in the vicinity of Sgr A*, where it was previously thought that only very old stars could exist. X3a suggests that the formation of young stars further out, followed by their migration in towards Sgr A*, may not be a particularly unusual occurrence.
And it might not just happen in our galaxy either. The structures around Sgr A* have been identified in many other galaxies, and they may host populations of their own baby stars. It’s a notion that could change our understanding of the dynamics of galactic nuclei.
Future work will test the team’s star formation model, not just for the Milky Way, but for the wider universe.
The research is published in The Astrophysical Journal Letters.
