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I did a little looking and it appears stars outside of 10 parsec/33 ly will have very little effect on the Earth.

What it did say was that inside that range the gamma rays would affect our ozone layer which would reduce our radiation protection. How close would it have to be for the actual gamma rays to be a serious primary threat to the Earth, as in the gamma ray burst directly kills a large swath of Earth's life? I've been thinking about a story where this happens.

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  • $\begingroup$ Look at Greg Egan's novel Diaspora. There, homo sapiens are destroyed by a near supernova-like event, with very plausible description provided. $\endgroup$
    – Irigi
    Nov 4, 2014 at 16:53

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Awesome question. I love this kind of thing.

Wikipedia of course has a a page on near-Earth supernovae, and the first body paragraph briefly examines the effects of life on Earth. The only really important thing there is just what you said - that the gamma rays from a supernova could deplete the ozone layer. Not too specific. But look further down the page. In this section, it says

Recent estimates predict that a Type II supernova would have to be closer than eight parsecs (26 light-years) to destroy half of the Earth's ozone layer.

I checked the referenced estimates on the arXiv pre-print used. It starts off with

Estimates made in the 1970’s indicated that a supernova occurring within tens of parsecs of Earth could have significant effects on the ozone layer.

They then say that they now have improved tools; later on they say

We find that for the combined ozone depletion from these effects roughly to double the “biologically active” UV flux received at the surface of the Earth, the supernova must occur at <∼ 8 pc.

which they restate later as

Summing our gamma-ray and cosmic ray depletions for DSN = 10 pc, and taking into account that our adopted energy is larger than that found in the latest SN study mentioned earlier, we obtain a fiducial “critical distance” to significantly disrupt ozone of Dcrit ≃ 8 pc for a SN with a total gamma-ray energy ∼ 1.8 × 1047 erg.

And in the conclusion, we again find

Our primary finding is that a core-collapse SN would need to be situated approximately 8 pc away to produce a combined ozone depletion from both gamma-rays and cosmic rays of ∼ 47%, which would roughly double the globally-averaged, biologically active UV reaching the ground.

Not a huge difference at first, but we'd see some pretty bad effects after a while - say, a day or two.

Looking here, we find

A supernova explosion of the order of 10 pc away could be expected every few hundred million years, and could destroy the ozone layer for hundreds of years, letting in potentially lethal solar ultraviolet radiation.

Ouch!

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  • $\begingroup$ yes, didn't read all of that at the Wiki, and I do understand that this will be a huge problem but a longer term one, though I had't thought/known of it before this. +1 $\endgroup$
    – bowlturner
    Nov 4, 2014 at 16:24
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    $\begingroup$ (Wikipedia): "In Earth's case, gamma rays induce a chemical reaction in the upper atmosphere, converting molecular nitrogen into nitrogen oxides." There is also this problem, ozone layer aside. I would not underestimate effects of tons and tons of acidic rains falling to the surface. They, however, do not write in what amounts it appears. Atmosphere has mass of 5x10^18 kg. If 5% of that reacted into oxides of nitrogen, it would be pretty lethal, I guess. $\endgroup$
    – Irigi
    Nov 4, 2014 at 16:49
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    $\begingroup$ @Irigi Very good point. I'm more concerned about the ozone layer because the nitrogen oxides will only be formed once, whereas, the UV radiation let in by the depleted ozone layer from the sun will come in for a long time. $\endgroup$
    – HDE 226868 ♦
    Nov 4, 2014 at 16:51
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    $\begingroup$ But none of this answers the really interesting question: ARE there any stars within this distance of Earth that might become supernovae in the near future? $\endgroup$
    – jamesqf
    May 10, 2015 at 18:11
  • $\begingroup$ @jamesqf Just looking quickly at Wikipedia, I see . . . none. The closest given is IK Pegasi B, at 150 light-years. It's a white dwarf, though, so it would undergo a Type Ia supernova, rather than the more commonly thought-of core-collapse supernova. $\endgroup$
    – HDE 226868 ♦
    May 10, 2015 at 18:16
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It depends what you mean by "severely harm". A supernova at a distance of 10 parsecs might not cause any significant physical damage, but if it happened to be in the opposite direction to the sun then the absence of any real night or darkness for several months could be very damaging to wildlife. A Betelgeuse supernova would be expected to be about as bright as the full moon, but 10 parsecs is 1/20th the distance to Betelgeuse, so a supernova at that distance would be four hundred times as bright as the full moon (though still only 1/1000th as bright as the sun).

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