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Stardust from a Nearby Supernova?

Week of July 5, 1999

Compared to say, biology or botany, astronomy is not a hands-on type of science. After all, astronomers study objects that are usually pretty far away. Unlike a scientist who can go out in the field and collect specimens of molds, or slice a leaf and look into the cell structure, astronomers usually must be satisfied to sit back and wait for data about objects to come to them.

This can be pretty frustrating. After all, if you study something like supernovae, you might have to wait hundreds of years for some star to explode. Then it takes its time, letting its light crawl here from some ridiculous distance like hundreds of thousands of light years. (That's not strictly true for every aspect of astronomy; roughly a few hundred tons of meteor dust falls to Earth every day, but that's the only thing from outer space that does come to us.)

At least we used to think so. A new finding shows that maybe something else has found its way here. No not some alien virus that will turn us into zombies and eventually having its story made into a movie starring Sharon Stone. Actually, it's a bit more frightening (and amazing) even that that.

A team of German researchers have found that Pacific floor sediments contain measurable amounts of iron-60. Iron is a common enough element, but iron-60 is a special isotope (or different form) of iron. It takes a lot of heat and pressure to make it, a lot more than you can find on Earth, or even anywhere in our solar system. The team, led by Gunther Korschinek, says that the only place you can get that sort of energy is from a supernova. In the huge fireball that results from a star exploding, the temperatures are high enough to cook iron into the weird isotope of iron-60.

There are two amazing things about this finding, if it turns out to be correct. One is that iron-60 is radioactive, with a half-life of roughly a million years. That means that the scientists could estimate just when the iron-60 was first forged: about 5 million years ago. That's very recent in astronomical terms. But the other amazing thing is that there was enough iron to measure at all. The Milky Way Galaxy is big, and supernovae relatively rare. On average, they are pretty far away, like thousands of light years. But this one must have been close! The dust and junk from the explosion, traveling outward at a few thousand kilometers per second, must have gotten here to Earth in a short length of time, or else it would have all radioactively decayed away by the time the scientists found it. That in turn means the supernova was, according to the German team, less than 100 light years away!

That's incredibly close. If true, our simian ancestors would have seen the supernova as a star about as bright as the full Moon! It would have hurt your eyes to see it. What a sight! The problem is, at that distance, there might be adverse effects on us here on Earth. No one is really sure, since luckily one hasn't gone off that close in a while, but the deleterious affects could include loads of x-rays bombarding us, gamma rays partially ionizing our atmosphere (playing havoc with communications), satellites getting knocked out (not a problem five million years ago, I'll admit) and the less known problems of a star so bright it can easily give off enough light to read by-- how will this affect the life cycles of animals? Some animals have breeding cycles tied to the phases of the Moon. What happens when a new, constant source of light appears?

Actually, some people have tried to reason out the consequences. Michael Richmond has a website (referenced in another section of my own website) with a nifty if somewhat frightening list of risks associated with nearby supernovae. Take a look! Then you might have a feel for what it must have been like, 5000 millennia ago, when a new star shone down on Earth.

I'll try to stay on top of this story as it interests me professionally too. I have seen no confirming reports, but the paper is due out soon in the journal Physical Review Letters. The initial announcement appeared in Sunday July 4th's Washington Post, but there is no web link that I could find. If you hear more about it and have a web address, please let me know!

©2008 Phil Plait. All Rights Reserved.

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