Expert: James Gort - 4/1/2007

Question
I've been reading up on the Permian-Period extinction theory and was wondering how close a supernova would have to be to (mildly)affect life on Earth? Also, how close would a supernova have to be to the Earth to cause mass extinctions of species or total extinction of all life?

Geoff

Answer
Hi Geoff,

There's been considerable research into this lately. We'll look at it from two ways:  The first is the total radiation (luminosity) released from a typical supernova.  We know the average absolute bolometric magnitude of a type 1 supernova is -18.4.  The bolometric magnitude of the sun is about +4.7.  That difference gives a luminosity ratio of 1.7 billion times.  If we use the inverse square law to determine the distance ratio, a supernova at 42,000 times the distance of the sun would be as bright as the sun appears to us.  Since one light year equals 63,240 astronomical units, that means the supernova would have to be a little less than one light year from us to appear as bright as the sun.  Since the nearest star is 4.3 light years away, even the nearest star exploding as a supernova would not appear as bright as the sun, and therefore would do little damage - at least from visual radiation!

Now, for part two.  The real danger is in gamma-ray (or even x-ray) bursts (GRB).  These are short duration high-energy bursts, which are very directed in space.  I refer you to an excellent paper that can be found on the internet:  http://people.roma2.infn.it/~aldo/dar01.pdf  In it, they look at the damage of a star such as Eta Carina (6000 light years away) directing its GRB towards earth.  They compute that earth's hemisphere (facing the star) would be bombarded with the equivalent of one kiloton of TNT per square kilometer, enough to destroy the ozone layer, start huge fires, etc.  They also look at muon flux at ground level, and conclude that it's ten times the lethal dose for human beings!  Although Eta Carina is (or was) a massive star, it appears that a good estimate is any star directing its GRB towards earth and located closer than, say, 1000 light years would probably be lethal to life on earth.  At 10,000 to 50,000 light years, it would at least mildly affect life.  

But the real caveat in all this is "directing its GRB".  These beams are extremely narrow, and it would be EXTREMELY rare for a supernova to occur within 1000 light years of earth (an estimate is that type 1 supernovae occur once per 50 years per galaxy) AND have its beam of gamma rays directed towards earth.

So, to answer your question, the star doesn't need to be all that far away (an astronomical terms) to affect life, but it would be an extremely rare occurrence.

Hope that helps!

Prof. James Gort