NASA / SCIENCE PHOTO LIBRARY NASA / SCIENCE PHOTO LIBRARY
Gamma ray extinction event. Computer illustration showing a burst of gamma rays from a massive star explosion (upper left) hitting the Earth (lower right). Scientists think that this ten-second burst of gamma radiation may have triggered the Ordovician extinction, 450 million years ago, 200 million years before the dinosaurs. The gamma rays would have initiated changes in the atmosphere, depleting the ozone layer and creating a brown smog of nitrogen dioxide. 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.
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