Stardate 20040108.1055

(On Screen): The American Astronomical Society has been meeting in Atlanta the last few days, and there have been a lot of astronomy stories in the news. Today's cool story:

The second-largest extinction in the Earth's history, the killing of two-thirds of all species, may have been caused by ultraviolet radiation from the sun after gamma rays destroyed the Earth's ozone layer.

Astronomers are proposing that a supernova exploded within 10,000 light years of the Earth, destroying the chemistry of the atmosphere and allowing the sun's ultraviolet rays to cook fragile, unprotected life forms. ...

Fossil records for the Ordovician extinction show an abrupt disappearance of two-thirds of all species on the planet. Those records also show that an ice age that lasted more than a half million years started during the same period.

Melott said a gamma ray burst would explain both phenomena.

He said a gamma ray beam striking the Earth would break up molecules in the stratosphere, causing the formation of nitrous oxide and other chemicals that would destroy the ozone layer and shroud the planet in a brown smog.

"The sky would get brown, but there would be intense ultraviolet radiation from the sun striking the surface." he said. The radiation would be at least 50 times above normal, powerful enough to killed exposed life.

In a second effect, the brown smog would cause the Earth to cool, triggering an ice age, Melott said.

The extinction "could have been a one-two punch," said Bruce S. Lieberman, a paleontologist at the University of Kansas and a co-author of the theory. "Our theory builds on earlier theories" that included an ice age.

This is Eta Carinae:

Or perhaps I should say that this is part of the planetary nebula that Eta Carinae has created in the process of dying. There's a star right in the middle of the nebula, in the narrow section between the two lobes. It's actually been throwing off huge amounts of gas for a long time now, and the nebula that has resulted is immense.

It's quite a star. Estimates range from 100 to 150 solar masses, and stars that big live fast, die young, and go out with a bang. Eta Carinae will almost certainly go supernova soon, but no one knows when. It could be a few million years. It could be a few thousand years. It may already have happened, and the light – including gamma rays – from that event could be speeding towards us right now.

Eta Carinae is about 9,000 light years away.

Sleep tight, children... Eta Carinae certainly isn't.

Update: Before everyone gets the dickens scared out of them, I'd like to point out that the Crab Nebula is the result of a supernova whose light arrived here in 1054 AD. It's about 6300 light years away.

Update 20040109: According to this paper (sent by John) it isn't actually a danger, but this article (sent by Pat) is not so sanguine.

Update: Jay Manifold comments.