A computer simulation shows the evolution of a molecular cloud with a solar mass of 10^4 falling toward a supermassive black hole with a solar mass of 10^6. Here, the cloud is within 0.5 parsecs of the black hole. Color scale is from 0.1 g cm^-2 to 1000 g cm^-2. A small population of stars is being ejected from the system in the this image.
Deep in the center of our galaxy, circling suspiciously close to the giant black hole lurking there, is a group of massive stars.
Now scientists have designed a model that shows for the first time how these stars might have formed in such an extreme environment.
Astronomers have long puzzled how these massive stars came to be in the vicinity of a huge black hole. They couldn't have formed as most stars do, from a tenuous cloud of gas, because this cloud would have been ripped apart by the savage gravitational forces from the black hole nearby.
One guess was that the stars originally formed elsewhere as a cluster and later spiraled inward. But no trace has been found of the trail of stars this process would have left behind.
A new explanation
Now Ian Bonnell, an astronomer at the University of St Andrews in Scotland, and William Ken Rice of the University of Edinburgh have created a computer simulation that offers a possible explanation for how the stars could have formed. They detailed their model in the Aug. 22 issue of the journal Science.
In their model, a giant gas cloud plunges in toward the black hole. As it gets close, much of the cloud is ripped apart by the black hole's gravity, though some portion of it survives because of the turbulence of the gas in the cloud.
This remnant forms an oval disk of gas orbiting around the black hole, gravitationally bound but beyond the range within which it would be sucked in. Variations in the density of the material in the disk then cause it to condense into stars and break up, leaving the stars in an oval orbit around the black hole where the disk used to be.
?"We've been trying to see how we could bridge the gap between normal star formation and star formation in this kind of environment," Bonnell told SPACE.com. "The mechanism we present should be able to provide a variety of possible properties which will definitely encompass the ones we observe in our galaxy."
Though the researchers found a way that the stars near our galaxy's central black hole could have formed, the simulation is not proof that this is what happened.
"As satisfying as the new results are, the case for disk fragmentation as the origin for the disk stars remains unproven," wrote astronomer Philip Armitage of University of Colorado, Boulder, in an accompanying commentary article in the same issue of Science. "We do not know whether the initial conditions assumed by Bonnell and Rice are realized in the galactic center."
Bonnell agreed that the next step would be to try to determine how probable their proposed scenario is compared to other suggestions for how the stars formed.
"What we need to do next is to worry about the likelihood of these other events happening," Bonnell said. "For the process we've outlined, the probability is high enough that it has a real chance of happening. I don't think it's anything that will prove to be that controversial in the long run. I have a feeling it seems quite likely."
A widespread occurrence
The scientists are also interested in whether this process may have occurred in other galaxies. It is difficult to study the central regions of other galaxies because they are so far away, but it could be that this type of star formation is widespread in the universe, Bonnell said.
"Supermassive black holes like the one in our galactic center are fairly common in galaxies," he said, "though our black hole is not as big as the black holes in other galaxies, which could be thousands of times more massive."
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