Mankind is safe--at least from the devastating, destructive power of long-lasting gamma-ray bursts (GBR). Although these powerful cosmic flashes pack more energy than the sun will release in its entire 10-billion-year lifetime, astronomers have concluded that our Milky Way galaxy has enough metal to preclude such a detonation.
A long gamma-ray burst, which lasts a few seconds to tens of minutes, is the birth cry of a black hole, produced when a massive, whirling star explodes as a supernova (ScienceNOW, 5 October 2005). Just about every day, a long burst flares up, usually in a remote galaxy. Astronomers had assumed that thousands of them must have lit up in the Milky Way over the past few billion years, and perhaps close enough to Earth to pelt it with lethal radiation. So could they have caused mass extinctions?
No, says Krzysztof Stanek, an astronomer at Ohio State University in Columbus. He and colleagues analyzed four long-duration gamma-ray bursts that lit up nearby galaxies, including one detected by NASA's Swift satellite on 18 February this year. All four galaxies contained almost no "metals"--astronomical parlance for atoms heavier than hydrogen and helium that are produced over the eons by stellar evolution. Apparently, long bursts only occur in pristine galaxies. This observation nicely fits in with theories saying metal-rich stars lose too much mass and rotational punch during their lives to blow up as gamma-ray bursts when they die.
According to Stanek and his team, by the time the sun and Earth formed, our Milky Way galaxy was already metal-rich enough not to host any long gamma-ray bursts. In a paper submitted for publication in The Astrophysical Journal, they write: "We can probably cross GRBs off the rather long list of things that could cause humankind to 'join the dinosaurs' on the extinct species list."
"It's a reasonable argument," says physicist Adrian Melott of the University of Kansas, Lawrence, who has suggested that the mass extinction at the end of the Ordovician period, 443 million years ago, was caused by a gamma-ray burst. Still, he thinks Stanek's conclusions may be a bit too strong. After all, every few hundred million years or so the Milky Way gobbles up a dwarf galaxy that's poor in metals, he says, and those might still produce long gamma-ray bursts.
So is mankind safe from gamma-ray bursts? Maybe not. But, says Melott, "I don't lie awake worrying about this."
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