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Gamma ray bursts striked the Earth, causing mass extinction. Some scientists believe this happened 450 million years ago. These gamma-ray bursts, astrophysicists recently learned, originate in distant galaxies and are unfathomably powerful--as much as 10 quadrillion (a one followed by 16 zeros) times as energetic as the sun. The bursts probably result from the merging of two collapsed stars. Before the cataclysmal event, such a double star might be almost completely undetectable, so we'd likely have no advance notice if one is lurking nearby. Once the burst begins, however, there would be no missing its fury. At a distance of 1,000 light-years--farther than most of the stars you can see on a clear night--it would appear about as bright as the sun.

The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged for up to five years, harmful ultraviolet radiation from the Sun would kill smaller life-forms and disrupt the food chain. Scientists say that a gamma-ray burst might have caused the Ordovician extinction 450 million years ago, some 200 million years before dinosaurs.

Gamma-ray bursts are the most powerful explosions known. Most originate in distant galaxies, and a large percentage likely arise from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely-directed beams of gamma rays that race off into space. Gamma-ray bursts in our Milky Way galaxy are indeed rare, but the scientists estimate that at least one nearby likely hit the Earth in the past billion years. Life on Earth is thought to have appeared at least 3.5 billion years ago. This research, supported by a NASA Astrobiology grant, represents a thorough analysis of the "mass extinction" hypothesis first announced by members of this science team in September 2003.

Nir Shaviv is a senior lecturer in physics at the Hebrew University in Jerusalem, Israel:

"Once every few decades a massive star from our galaxy, the Milky Way, runs out of fuel and explodes, in what is known as a supernova. Cosmic rays (high-energy particles like gamma rays) spew out in all directions and if the Earth happens to be in the way, they can trigger an ice age. If the Earth already has a cold climate then an extra burst of cosmic rays could make things really icy and perhaps cause a number of species to become extinct. The Earth is at greatest risk when it passes through a spiral arm of the Milky Way, where most of the supernova occur. This happens approximately every 150m years. Paleoclimate indicators show that there has been a corresponding cold period on Earth, with more ice at the poles and many ice ages during these times.

"We are nearly out of the Sagittarius-Carina arm of the Milky Way now and Earth should have a warmer climate in a few million years. But, in around 60m years we will enter the Perseus arm and ice-house conditions are likely to dominate again."