“The last supernova seen with the human eye was documented by Johannes Kepler in 1604. That was at a distance of about 20,000 light years. And while it shone brighter than any star and was visible in daylight, it certainly caused no issues on Earth.
“However, were a supernova to go off within about 30 light years of us, that would lead to major effects on the Earth, possibly mass extinctions. X-rays and more energetic gamma-rays from the supernova could destroy the ozone layer that protects us from solar ultraviolet rays. It also could ionize nitrogen and oxygen in the atmosphere, leading to the formation of large amounts of smog-like nitrous oxide in the atmosphere.
“Supernovae happen about once every 100 years in the Milky Way. But the Milky Way is a big place, roughly 100,000 light years across. Given that, and the fact that the Sun is near the outskirts of the Milky Way where few stars massive enough to become supernovae are born, having a supernova within 30 light years of the Sun should, on average, happen only once in every 100 million years.”
Answered by Dr. Mark Reid, a Senior Astronomer at the Harvard-Smithsonian Center for Astrophysics