Most of us learn that black holes are such potent concentrations of mass and gravity that everything around them gets sucked in, willingly or not. This should especially true near the supermassive black hole, weighing in at some 3½ million Suns, that lurks at the center of our galaxy.
But within the last decade astronomers have come to realize that swarms of stars are buzzing around that cosmic maw in precariously tight orbits. Many of these appear to be quite massive and therefore young, no older than several million years.Cosmologists have scratched their heads in collective disbelief over this observation. Either these fat stars form elsewhere and somehow migrate inward (improbable, given their youthfulness), or else they condense from great clouds of molecular gas that sweep through the core region. Yet shouldn't the black hole's gravitational force rip the clouds to smithereens as they're dragged to their doom?
Apparently not, based on the results of a new computer simulation that appear in the August 22nd issue of Science. Ian Bonnell (University of St. Andrews) and Ken Rice (University of Edinburgh) followed the evolution of massive gas clouds as they fell into the Milky Way's core. It turns out that some of the gas isn't gobbled up by the supermassive black hole but instead forms an elliptical disk around it — and stars then form in the disk.Remarkably, many suns end up in orbits no more than 0.1 light-year in radius. And apparently a new burst of star formation occurs whenever a gas cloud dives through the core.
It just goes to show what you can do with more than a year of number-crunching using one of the world's most powerful supercomputers — in this case the Scottish Universities Physics Alliance (SUPA) SGI Altix supercomputer. As Rice notes in an online press release, the key was careful modeling of the gas's heating and cooling during its ordeal. "This tells us how much mass is needed for part of the gas to have enough gravity to overcome its own gas pressure," he says, "and thus form a star."