Black Holes Can Spawn Large, Odd Stars, Model Suggests

Anne Minard
for National Geographic News
August 22, 2008
Black holes, with their overwhelming gravitational pulls, generally aren't considered good neighbors for the types of dusty gas clouds where new stars are born.

But sometimes stellar nurseries can escape the maws of supermassive black holes and spawn abnormally large stars on eccentric orbits, a new model suggests.

The stars would be so massive that they'd die relatively young, burning out before the black hole had time to crush them with its gravity.

The finding could help explain why two unexpected groups of young stars, called disk stars, exist near the supermassive black hole at the center of our Milky Way galaxy.

One population contains stars that are just six million years old, and the other has stars that are even younger. (See an updated map of the Milky Way released in June.)

"The high tidal shear from the black hole should tear apart the molecular clouds that form stars," wrote the Scotland-based study authors Ian Bonnell from the University of St. Andrews in Fife and Ken Rice from the University of Edinburgh.

The computer simulations show that when a starmaking cloud approaches a large black hole like the one at our galaxy's heart, the tidal shear can cause the type of celestial commotion that's needed to make new stars. The oversize newborns then circle the black hole on the same bizarre, oblong orbit as their parent gas cloud.

The findings, reported in today's issue of the journal Science, could yield insight into the life cycles of black holes.

"Hopefully, by studying these things, we'll get a better insight into how black holes grow and form," Bonnell said.

Escape Artists

The black hole at the center of the Milky Way has the mass of about a million suns—small by supermassive black hole standards.

Still, its range of influence is wide, Bonnell said. Gas is drawn toward the black hole from a distance of up to 20 light-years.

Stars closer than about six light-minutes are expected to get pulled apart and turned into disks. Much closer than that, and even light won't escape the black hole, Bonnell said.

But sometimes, according to the models, a gas cloud behaves oddly as it approachs. It begins rotating, and gas at the leading edge experiences a kickback of energy that flings it outward from the black hole and forms new stars.

Normally, a population of new stars contains more small stars than large ones, with most reaching only a half or a third the size of the sun.

"Proportionally, stars bigger than the sun are rare," Bonnell said.

But in both populations of young stars near the Milky Way's black hole, researchers have noticed an unusual number of very large stars—and smaller ones are nowhere in sight.

Bonnell believes that's due to the heating of gas by the black hole's energy.

"You need much more mass for something to be gravitationally bound," he said, adding that the smaller stars probably never form in the first place.

The resulting stars are so close to the black hole—about a light-month away—that they would normally get crushed.

But they're also so massive that they'll likely explode as supernovae within ten million years, well before they would have been crushed, Bonnell said.

Big Questions

Philip Armitage at the University of Colorado at Boulder wrote an accompanying commentary for Science in which he praises the authors of the new study.

But he adds that when it comes to the populations of odd stars near the Milky Way's black hole, he takes the findings with a grain of salt.

"As satisfying as the new results are, the case for disk fragmentation as the origin for the disk stars remains unproven," he writes.

He also points out that previous work has shown that most gas within a hundred light-years of the Milky Way's black hole never approaches it.

Study co-author Bonnell agrees. "That's one of the big questions, is why would there be this supply of gas coming in," he said.

One theory suggests that orbiting gas clouds occasionally collide, lose their momentum, and fall into the black hole.

(Related: "Massive Cloud to Strike Milky Way, Scientists Say" [January 14, 2008].)

Richard Alexander, an astronomer at Leiden Observatory in the Netherlands who was not involved with the new study, said the theory of gas clouds forming stars near black holes isn't new.

But the new paper contains "the first simulations which attempt to model the whole story, from the infall of a gas cloud to the formation of a disk of stars."

Now, he said, there is a viable explanation for star formation in the neighborhood of the black hole.

Alexander suspects that in the coming years the most convincing evidence will be drawn from the behavior of the stars themselves.

"In a few years we will have a much clearer [observational] picture of how these young stars are moving, and that in turn will tell us a great deal about how they formed in the first place."

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