How Puzzling Stars Formed near Galactic Black Hole

Gas clouds pulled into orbit around black holes could survive long enough to make stars
star forming around black hole

Courtesy of Science/AAAS

Researchers say they have figured out how a mysterious clutch of massive stars could have come into existence a few trillion miles from the supermassive black hole at the center of the Milky Way.

This group of stars—about 100 of them in an elongated disk—has posed a challenge to theories of star formation, which predict that stars emerge when clouds of hydrogen molecules coalesce under their collective gravitational attraction.

The gravity around a supermassive black hole weighing millions of times more than the sun should have shredded such a cloud like paint dropped on an eggbeater before it got a chance to make stars.

To address the mystery, researchers from the University of St. Andrews and the University of Edinburgh, both in Scotland, simulated the fate of a hydrogen cloud as massive as 10,000 suns that suddenly wafted near a black hole. They found that although much of the cloud would splatter [see image], shock waves and other turbulence would drain the inner 10 percent of angular momentum, causing it to take up orbit around the black hole and giving time for stars to form.

The results, reported in Science, also explain why the observed stars are all several times more massive than the sun, when stars everywhere else in the universe typically come in a mix of sizes: Squeezing by the black hole would heat the gas, and only a massive enough patch of gas would have the gravity necessary to overcome its own outward pressure and collapse into a star.

Researchers have been unsure what to make of these stars, which they estimated at around six million years old. That's too young for them to have formed farther out from the black hole and spiraled in, study author Ian Bonnell, an astrophysicist at St. Andrews, says. "They shouldn't really be there unless you could form them there directly," he says.

The finding fits with other simulations indicating that large stars would tend to form around a black hole, although the detailed explanations differ, says astrophysicist Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

But what the simulations still don't explain is the origin of about 25 massive but randomly oriented stars even even deeper in the galactic core that provided key evidence of the Milky Way's black hole.

Researchers suspect these may be the remnants of pairs of stars that strayed near the black hole and were violently separated, with one member of the pair kicked out of the Milky Way and the other taking residence around the black hole. "That seems like the most promising option," Genzel says, "as crazy as it sounds."

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