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Supernova/mass extinction business

Larry Smith wrote, apropos of theories about a supernova at the Permian-
Triassic boundary:

>An exploding star about 185 trillion miles away would create enough gamma
>radiation to thin the ozone for many years, he said.

As I recall, this is less than 35 light-years.  A nova within that
range would have left us at the middle of a huge and clearly visible
shell of gas that has never, to my knowledge, been reported by any
astronomers, nor have they identified any black holes or pulsars in
that range.  I don't have a star atlas here at work, but I can't even
recall a white dwarf star at that range, either.  Do we have any
evidence for this besides a really spiffy analysis of what the
radiation _might_ have done if it were close enough?  Do have any
real candidates for the dead star(s) and what extinctions they are
alleged to have participated in?

225 million years is about one galactic rotation at the Sun's location, and
is vastly longer than the lifetime of an identifiable explosion signature
in the interstellar medium. A relative motion of only a few km/s would
have carried sun and hypothetical remnant too far apart by now for a
reasonable ID, and the lifetimes of hot-gas bubbles from the explosions
would rarely exceed a million years. Evidence for this one would have to
come from terrestrial chemistry (though it would certainly add plausibility
if one were to find matching signatures of irradiation on other planets...)

Bill Keel                         Astronomy, University of Alabama