Can any nearby supernova cause a mass extinction?

The most dangerous parts of a supernova explosion are the outputs like X-rays and gamma rays. Even though they only share a small fraction of a supernova’s power, they are extremely dangerous. But they’re not going to disintegrate the Earth. We are simply too far away from any potential supernova for that to ever be a problem.

What can happen is that these forms of radiation pack enough energetic punch that they can tear apart molecules. Elements like nitrogen and oxygen prefer to float around as molecules in our atmosphere. But then once they get hit by X-rays, gamma rays, and cosmic rays, they get broken apart. And then they recombine in interesting and fascinating ways like various nitrogen oxides, including everybody’s favorite nitrous oxide, AKA laughing gas. And while everyone’s laughing and having a good time, our ozone layer gets stripped away.

That’s the danger of a too-close supernova: it breaks up our ozone layer. And without an ozone layer, it means the Earth is vulnerable to ultraviolet radiation from the sun. Our ozone layer protects us from the vast majority of ultraviolet radiation. There are a couple specific bands of wavelengths that do sneak through, which is why we need to wear sunscreen here on the surface, so we don’t get nasty tans and sunburns and skin cancer and all that.

But imagine no ozone layer, then you get all the UV radiation, the full output, and it’s bad. And it’s not just a matter of quicker tans and faster burns and higher rates of skin cancer. The problem is that photosynthetic microorganisms like algae become vulnerable. They get cooked and then they die. And since they form the very base layer of the food chain, you end up with a whole ecosystem collapse and a mass extinction.

What’s the risk? We’ve done the math. And it says that generally, for the typical strengths of supernova that tend to occur in our galaxy, a dying star has to be within roughly no more than 25 to 30 light years of the Earth to be able to strip away at least half of our ozone layer, which would be enough to trigger all the aforementioned bad things. And I do have some good news to help you sleep at night. You should know that there are no known supernova candidates within 30 light years of the Earth.

The nearest candidate that is about to go supernova is Spica. And it’s about 250 light years away.

That’s great. There are no supernova candidates in the danger zone. And there are no stars that will evolve to become supernova candidates that will approach within 30 light years of the Earth in their lifetime.

So that’s a relief.