[RubyO (OP)]

Jeff sent us this question:

How close to Earth would a supernova need to be in order to affect life here? What effects would a supernova have?

Can anyone shed some light on it for him?

[evan_au]

A supernova observed by Chinese astronomers in 1054 was seen as a sign of the death of the emperor.
I guess that is indirectly affecting life on Earth...

This is now identified with the Crab Nebula, at a distance of 6,500 light-years.
See: https://en.wikipedia.org/wiki/SN_1054#Sources

It is thought that a supernova within 30 light years would pose a serious danger to life on Earth.
See: https://en.wikipedia.org/wiki/Near-Earth_supernova

[Janus]

Jeff sent us this question:

How close to Earth would a supernova need to be in order to affect life here? What effects would a supernova have?

Can anyone shed some light on it for him?

Pretty close.   A supernova 26 light years away would remove 1/2 of the ozone layer.  The gamma radiation that does this decreases by the square of the distance, so at 52 ly, it will be 4 times weaker.  Besides the weakening of the ozone layer which would allow for more UV to reach the surface, The absorption of this radiation by our atmosphere can produce a smog-like effect which would block some wavelengths of light, leading to a cooling of the planet, possibly triggering "mini-ice age".
None of the potential supernova candidates closest to the Earth represent any threat. While close enough to be spectacular to the naked eye, they are far enough away to be of no threat.   The closest of these is IK Pegasi at 150 ly away.   While this presently puts it close enough to have a small effect on us. But it is not expected to be a threat until quite far in the future, and in that time its relative motion will carry it to a safe distance from us.

Gamma ray bursts are more dangerous over much longer distances because they are concentrated in beams.  A GRB in our galaxy aimed at the Earth, could have effects similar to that of a nearby supernova.

All of the GRBs we have detected so far have occurred in distant galaxies, and estimating from these observations, we would not expect a GRB to occur in our galaxy any more often than every 10,000 to 1,000,000 yrs, and to be a threat to us, it would have to be aimed at us.  So this is a pretty low-level threat.