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Spec Tech: How Close is too Close for Supernovae?

January 20, 2011

Every day throughout the universe old stars are exploding in fantastic, trembling concussions of mass and energy. Suffice it to say that you wouldn’t want to be next door when this happened. But just how close is too close? Is there a minimum safe distance from supernovae for an Earth-like planet?

The exact answer is—of course—not exact. This question is currently a topic of debate and ongoing study by scientists. But we have some guesses and some rules of thumb. To come to a conclusion about safe distance we need to understand a couple of things: the different kinds of supernovae and what they do to the Earth that is actually dangerous.

Type I and II and Jets

Supernovae are generally divided into two categories imaginatively named Type I and Type II. Type II are giant stars that have run out of hydrogen to burn and collapse in on themselves releasing tons of energy. Type I are caused by white dwarfs that have built up so much mass from nearby gas and dust that they collapse into neutron stars or black holes. Of the two Type I are the more powerful explosions.

There is one more complication. Supernovae are not symmetrical. When collapsing stars tend to flatten out into disks. These disks tend to form jets of super fast matter. The reasons for this are not entirely clear but we see them everywhere. It turns out that a large amount of the energy of a supernova is directed out in these jets. The difference between a great show in the sky and certain doom is whether or not the jet is pointed at you.

Effects of Supernovae on Earth

So what happens when a supernova goes off ‘near’ the Earth? What comes out of supernovae is mostly made up of photons (gamma rays, x-rays, etc.), neutrinos, relativistic protons (‘cosmic rays’), and a shockwave of non-relativistic matter. It turns out that we only need to worry about one of these.

Neutrinos don’t interact with normal matter very often. In fact there are billions of them streaming through you as you read this. And after passing through you they will continue to pass right through the Earth and out the other side. From the perspective of the neutrino the universe is empty. So we don’t worry about those. And luckily most of the energy of a typical supernova is contained in these harmless particles.

Protons and other particles moving near the speed of light are pretty dangerous. They can rip through you creating a shower of radiation that tears apart DNA. But being made of big things like protons they tend to bump into stuff before getting too far. The gas and dust that surrounds our sun and permeates the galaxy tends to dissipate cosmic rays and other heavy particles before they reach Earth. So no problem there.

The non-relativistic matter is just the rest of the explosion. It’s moving pretty slow, and will never make it outside the immediate space around the supernova. Basically unless you are in orbit around the exploding star itself it’s of no concern.

The photons on the other hand are pretty troublesome. Supernovae expel a nasty concoction of high energy light. Everything from UV to gamma rays come pouring out in huge quantities. This radiation, if strong enough, can tear apart our upper atmosphere. Ironically it’s the atmosphere that protects us from this kind of radiation on a daily basis. The Ozone layer blocks most of the UV from the sun and has been keeping us safe for millions of years. But if the atmosphere gets hit with enough radiation, say from a supernova, it will start to wear away that protective coating leaving life quite vulnerable. So it’s not the supernova itself that hurts us, but the lasting devastation to our blanket of protection from everyday radiation.

How Close

The size of the danger zone from supernovae

The size of the danger zone from supernovae with jets (called Gamma Ray Bursts, or GRB's)

How close is too close then? At what point does the radiation become so intense that we lose our ozone layer and fry all the plant life on Earth and starve to death? If we are not in the cross hairs of a jet from a Type I supernova— essentially the worst case scenario—then it would have to be pretty close. Since the rest of the energy and radiation from a supernova not in the jets is distributed more of less evenly in every direction the actually amount that would get to Earth drops off very quickly with distance. The generally accepted number of danger for a bright Type II or normal Type I is about 30 light years. Anything closer than that and we would have to worry about mass extinctions caused by the suddenly missing ozone layer and climate change associated with the upper atmospheric radiation. There is some evidence that a supernova went off not too long ago (geologically speaking) about 80 light years away without any ill effects beyond some mild climate change (a few decades of slight cooling then warming again). 10 light years is just next door neighbors. In fact there are only a handful of stars this close to Earth and none of them are going to explode anytime soon.

Things are considerably worse if a jet is pointed right at us. Right now Scientists think that we could see nearly complete ozone loss and perhaps even a mass extinction if the offending supernova is as close as 10,000 light years! For scale it is about a tenth of the way across the whole galaxy. There are millions of stars of all kinds in this radius, but thankfully the chances of one pointed at us and going off in the lifetime of our species is next to zero. It’s expected that this sort of thing only happens once or twice every billion years. So don’t panic in real life! Panic in your writing, of course, is up to you.

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