Is Earth Safe from a Nearby Supernova?

How close can you be to a supernova before things get sticky, and perhaps more pertinent, are there ticking stellar time bombs close enough to Earth to hurt us?

There are quite a few ways a supernova can dish out cosmic catastrophe.

The most dangerous is high-energy radiation such as x-rays or gamma rays. Although our planet’s atmosphere would act as a buffer to absorb some of the deadly blast, doing so would literally change the chemistry of Earth’s air—and not in a good way.