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Thursday, January 08, 2004

Super Star

Today, Steven Den Beste notes a story about the possible role of supernovae in the Ordovician extinction, 440 million years ago. It's suggested that the supernova exploded within 10,000 light years from Earth, zapping the Earth with gamma rays and destroying the ozone layer. He then points out that the massive, dying star Eta Carinae is 9,000 light years from Earth.

Now, 10,000 light years is a hell of a long way for gamma rays to travel and still screw up the Earth's atmosphere. Steven has overlooked a subtle but important point. From the story:

[Adrian Melott, of U. Kansas] 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 operative word here is beam. The strength of radiation emitted in a uniform sphere decreases as the inverse of the square of the distance travelled. In other words, the strength of the radiation at any point 10,000 light years away from the source is one hundred millionth its strength at a point 1 light year away.

So an explosion would have to be pretty impressive to affect us 10,000 light years away. (This depends on your standard for "effect", so I don't know exactly how likely it is.) However, there's a way to get around this, and that's by "beaming". That is, instead of blowing its energy promiscuously in all directions, the supernova releases it in (relatively tight) beams. That way, the same amount of energy is released in a much smaller area, so the energy density arriving at Earth would be much higher, and deadlier.

The thing is, since the beams cover a smaller area, the chances of being right in the beam's path are very small. In the case of Eta Carinae, an explosion would almost certainly be channeled along the axis of the current outflow (that is, the axis of the two clouds in the picture Steven reproduces), rather than directed toward us (not that we might not see some of it, if the beam opening angle is big enough).

Also, not all supernovae beam their energy (that we know of: if the beam is at right angles to us, we probably won't see it).

Worry about Osama under your bed if you must, but don't worry too much about Eta Carinae.