What are the chances a close supernova could threaten life on Earth?
It seems likely that this may already have happened several times in the billion or so years of life on earth. There are several mass extinctions in the geologic record that do not bear the Iridium and soot signature of the asteroid impact in the Yucatan some 65 million years ago which seems to have caused the Cretaceous-Tertiary geological boundary associated with the demise of the dinosaurs. Nearby supernovae are a leading suspect for some of the other mass extinctions. The Cygnus Loop is the remnant of a supernova only some 25 pc (80 Ly) away, which would have been seen on Earth 20,000 years ago, in the time of Cro-Magnon man. Everyone would have seen for many days a star of magnitude -17; a thousand times brighter than the full moon, but still a thousand times dimmer than the sun. The spectrum of a Type II supernova peaks in the UV and the earth would have received 1000 times the normal UV flux at the top of the atmosphere, leading to greatly increased ionization of the upper layer
