(Note, 3/28/2008: See post today with information suggesting that supernovas would have to be closer to have severe effects on Earth. This older post includes supernovas that were 800 to 7500 light years away, as well as closer ones.)
I have come across several references to supernovas that were both recent and within several thousand light years. The Vela supernova, for instance, exploded about 10,000 to 12,000 years ago (sources differ), and is only 800 light years away! Tycho’s supernova was seen in 1572, and is only about 7500 light years ago. Supernovas are the most extreme and powerful events in the universe since the Big Bang, briefly shining as brightly as an entire galaxy of hundreds of million stars. Massive stars explode so intensely that they transmute light elements into heavier elements, thus being the origin of all elements heavier than iron. There might be effects on weather, plants and animals on Earth, and even on climate and human civilizations. Does anyone know anything about this?
http://antwrp.gsfc.nasa.gov/apod/ap030703.html
Update 2/17/08 This sort of event cannot be too rare; I found references to past and possible future close supernovas, but all are fairly speculative. For example, one article (http://science.nasa.gov/headlines/y2003/06jan_bubble.htm) suggests that the Pliocene-Pleistocene transition, 2 million years ago, was supernova related: “Researchers estimate (with considerable uncertainty) that a supernova less than 25 light years away would extinguish much of the life on Earth. The blast needn’t incinerate our planet. All it would take is enough cosmic rays to damage the ozone layer and let through lethal doses of ultraviolet (UV) radiation. Our ancestors survived the Pliocene blasts only because the supernovas weren’t quite so close.
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“We know because we can still see the cloud today. It’s 450 light years from Earth and receding in the direction of the constellations Scorpius and Centaurus (hence the cloud’s name, “Sco-Cen”). Astronomer Jesús Maíz-Apellániz of Johns Hopkins University recently backtracked Sco-Cen’s motion and measured its closest approach: 130 light years away about 5 million years ago.
“Sco-Cen was still nearby only two million years ago when many plankton, mollusks, and other UV-sensitive marine creatures on Earth mysteriously died. Paleontologists mark it as the transition between the Pliocene and Pleistocene epochs. Around the same time, according to German scientists who have examined deep-sea sediments from the Pliocene era, Earth was peppered with Fe60, an isotope produced by supernova explosions.”
I also found a reference (http://www.newscientist.com/article/dn2311-supernova-poised-to-go-off-near-earth.html) to a potential supernova, only 150 light years away at the moment, that may occur “soon” (within a few hundred million years!). I doubt that destroying the ozone layer would destroy all life on Earth, but it would mess things up significantly. GML
