Most of the time, the stars are peaceful, located as they are so very far from our little planet.

Once once in a while, though, a star will lash out in a fit of unimaginable violence, threatening the existence of any unfortunate creature or being that happens to live within its very, very large danger zone.

Scientists are still sifting through the clues, but there is mounting evidence just such an event has occurred at least once on Earth.

About 250 million years ago, at the end of the Permian period and just before the dinosaurs appeared, some great catastrophe befell the planet, which at the time was already 4.25 billion years old.

For some reason, about 90 percent of living things in the ocean and 70 percent of land dwellers died out - and rather suddenly.

The most famous extinction, which ended the Cretaceous period and took out the dinosaurs some 65 million years ago, deposited a thin layer of iridium nearly everywhere around the world and blasted a crater into the Yucatan Peninsula in Mexico, thereby leaving ample evidence of a major asteroid impact.

The cause of the great Permian die-off is much less certain - as is the Ordovician extinction, which happened about 200 million years earlier - but there is a new theory about what the cause might have been.

Scientists at NASA and the University of Kansas in Lawrence said the trigger for some of the great ancient extinctions could have been a star explosion called a gamma-ray burst.

In a paper published in the latest issue of Astrophysical Journal Letters, Brian Thomas, a Ph.D. candidate at K.U. and colleagues said they calculated gamma radiation from a relatively nearby star explosion, hitting the Earth for only 10 seconds, could deplete up to half of the atmosphere's protective ozone layer.

If so, the Kansas team wrote, the ozone-recovery process could have taken at least five years. With the ozone layer damaged, ultraviolet radiation from the sun could kill much of the life on land and near the surface of oceans and lakes and disrupt the food chain.

The life-recovery process could take much, much longer - perhaps millions of years.

Gamma-ray bursts are the most violent events in the universe, bigger even than supernovae and far more dangerous, because they travel at the speed of light.

In an instant, a single star - perhaps 10 to 15 times more massive than the sun - emitting a burst can briefly shine more brightly than an entire galaxy. The radiation is so intense it can kill living organisms thousands of light-years away.

Fortunately, within the Milky Way, the bursts are quite rare, occurring maybe once or twice every billion years. Across the entire universe, however, the bursts occur about once a day.

In November 2004, NASA launched its SWIFT spacecraft to seek and observe the short-lived bursts. In just the past three months, SWIFT has detected more than 25.

Despite their rarity, the Kansas scientists estimate at least one gamma-ray burst probably hit Earth within the past billion years.

"A gamma-ray burst originating within 6,000 light-years from Earth would have a devastating effect on life," said K.U. research team member Adrian Melott of the Department of Physics and Astronomy.

"We don't know exactly when one came, but we're rather sure it did come - and left its mark. What's most surprising is that just a 10-second burst can cause years of devastating ozone damage."

Thomas said a gamma-ray burst may have caused the Ordovician extinction, killing 60 percent of all marine life. At that time, living things were confined largely to the oceans, although some primitive land plants had emerged.

Thomas, along with Charles Jackman of NASA's Goddard Space Flight Center used sophisticated computer models to calculate that gamma rays can break down molecular nitrogen in the atmosphere into nitrogen atoms, which react with molecular oxygen to form nitric oxide - which destroys ozone.

This process remains self-perpetuating for some time, they said. Five years later, at least 10 percent of the ozone layer remains still destroyed.

The resulting UV radiation would kill anything on the land surface and anything living within a few feet of the ocean's surface - including plankton, which constitutes the foundation of the marine food chain.

Once a mass extinction has occurred, the biosphere requires a long, long time to recover.

"The bursts we detect today originated far away billions of years ago, before the Earth formed," Thomas said, "(but) among the billions of stars in our galaxy, there's a good chance that a massive one relatively nearby exploded and sent gamma rays our way."

A sensible question might be when is the next one expected?

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