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So we know that many people are putting hard constraints on the galactic habitability zone based on the presence of nearby supernova/gamma ray bursts. But if they only affect the ozone layer, then I doubt that it's as hard of a constraint as many people think it is.

For one thing - there is practically no ozone layer around the planets of red dwarfs (and possibly even low-mass K-stars like Epsilon Eridani and Alpha Centauri B - IMHO, K-stars offer the best prospects for life on other planets.)

With this information, I am wondering wondering about the outcome, particularly in regard to life: would a nearby supernova really do so much damage to planets around those stars?

For instance, would a supernova really cause more damage than, say, the K/T extinction event 65 million years ago? Also, given that much marine life is shielded from UV rays by layers or ocean water, is it really going to cause significant amounts of damage to such life in that environment?

As a side note, maybe this about surviving gamma ray bursts is relevant for complex life too (although this response might be imperfect for now.)

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It really depends on the range. A star going supernova is going to absolutely obliterate any planet that's closely orbiting it. For solely ozone/atmospheric damage, you're going to have to be several light years away. Probably anything within 10 light years is going to suffer a severe extinction event at the very least. 15-20 maybe a touch of atmospheric damage. I vaguely recall 25 ly being the point where you're pretty much safe.

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Dr Phil Plait covers the effects of a supernova near a habited planet extensively in his book Death from the Skies. Basically, it would have to happen at a distance closer than 25 light years. Given that constraint, and looking at our system, no stars are candidates for a supernova explosion that would wipe us out. A GRB is a different beast entirely, and the distances involved are closer to thousands of light years. However, then the magnetic fields surrounding that candidate star must align perfectly to cause such an extinction event. The effects on the earth are very unlikely to occur due to the rarity of a GRB in our galaxy and the alignment required. Although it has been posited that one such event already occurred.

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