Click the image to see a video animation of a gamma ray burst. Credit: NASA/SkyWorks Digital
Without many of us knowing, Earth's ozone layer was saved from being ripped away and the resultant widespread mass-extinctions were avoided when a scientist at The Ohio State University announced that our planet was not at risk of being struck by gamma ray bursts (GRBs). Now we just have to worry about the damage we're causing ourselves.
"It can always happen, just the chance of that happening in the universe these days is very rare to basically non-existent," said Krzysztof Stanek, an associate professor of astrophysics at Ohio State. "It's never zero, but the chances were small before, and now we don't have to worry about it."
Gamma ray bursts—beams of radiation that are flung from the north and south poles of certain stars as they spin during supernova explosions—were first observed in the late 1960s and have been the subject of much speculation and wild theorizing since.
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Primarily, Stanek said, it was not known whether or not a GRB could take place near Earth, and if it could, what the effects on terrestrial life would be. In fact, according to Stanek, some scientists theorized that the Ordovician-Silurian extinction event, which took place 450 million years ago, killing over 100 families of marine life, was caused by GRB.
Stanek's work has resolved some of that speculation, building on the previously held belief that GRBs tended to occur in small, less metallic galaxies, because metal-rich galaxies produce powerful metal-based stellar winds with the capacity to slow down the spinning of the stars that allows GRBs to occur.
Stanek and his team statistically analyzed four recent GRBs taking place in galaxies close enough to ours to be effectively studied. According to Stanek, the very occurrence of GRBs in these small galaxies, where new stars do not tend to form, makes even a set of four significant.
"When you form stars in the current universe, it mostly happens in the Milky Way and similar galaxies," Stanek said. "But the fact that every time a GRB happens, it happens in a Small Magellanic Cloud-like galaxy, with four events it becomes very highly significant. It's maybe a 0.1% chance that that's random."
Stanek's team was able to conclude for the first time that the Milky Way would never be a GRB site, thus putting to rest the conjecture that they caused past extinction events.
"By studying the properties of these four [GRBs], we are now able to say in a statistically significant way that they are only seen in small, dinky galaxies which have very few metals," said Stanek, who has submitted his findings to the The Astrophysical Journal. "They do not happen in galaxies like ours or any normal galaxies as we think about them in the local universe."
Bohdan Paczynski, a Princeton astrophysicist who studies GRBs, called Stanek's research "simple" and "original."
"It was known for some time that GRBs explode in dwarf, low-metallicity galaxies," he said via e-mail. "But it was Stanek who in his recent paper made a very obvious—after the fact—connection: Our galaxy is safe!"