[MUSIC PLAYING] The cosmos has so many catastrophes in store for our fragile, little planet.

Among the scariest is that one day, we will almost certainly find ourselves in the path of a gamma-ray burst's death ray.

[MUSIC PLAYING] 10 00:00:24,740 --> 00:00:28,590 The end of the world may not be nigh, but it will come.

In fact, there are quite a few ends of the world on their way, each one more interesting and less avoidable than the last.

There's a good chance we can prevent the most imminent, like asteroid strikes, or at least deal with their effects, like the damage caused by gamma-ray bursts and supernovae.

But the later ones will be beyond any conceivable technology to prevent.

For example, the gradual heating and eventual death of the sun.

Hopefully, our super advanced and probably not-quite-human descendants will be able to escape those by traveling to other star systems.

But what about the Milky Way's inevitable collision with Andromeda, or the final burning out of the last stars, or the evaporation of the last black hole and decay of the last proton?

The ends of the world, end of the universe, will come.

Better to be forewarned.

We'll get to each of these inevitable cosmic catastrophes, but let's start with the one that could happen any time-- a supernova or gamma-ray burst frying our atmosphere.

Every 100 million years or so, a good deal of Earth's life gets wiped out.

At five or six different points in time over the past half billion years, a large fraction of species simply vanished from the fossil record.

Some of these mass extinctions were due to giant asteroid impact, including the most recent, which wiped out the dinosaurs 65 million years ago.

But this is the most preventable end-of-world scenario.

In fact, we've already talked about asteroid impacts and how to deflect them.

However, at least one mass extinction may have been caused by something we'll never have the technology to stop.

The Ordovician-Silurian extinction 440 million years ago may have resulted from the Earth being blasted by the intense radiation jets from a distant exploding star.

It may have been caused by a gamma-ray burst.

In this episode, we're going to look at the evidence that the OS extinction was caused by a GRB, and we'll also figure out how long before the next gamma-ray burst hits.

But first, let's go over the proposed scenario.

As many of you know, a supernova is the explosion that follows the catastrophic collapse of a massive star at the end of its life.

Now, some won't die that way, but any star more than around eight times the Sun's mass will.

The resulting explosion sprays high-energy light, so ultraviolet, x-rays, gamma rays, and near-light-speed particles-- so cosmic rays-- into the surrounding interstellar space.

Any planet within a few tens of light years of a supernova is in trouble.

It's even more dangerous if the star was rapidly rotating before it exploded.

In that case, the powerful magnetic fields can channel the explosion into narrow jets that massively focus and amplify the blast.

Roughly once per day, the jet from such an explosion in a distant galaxy reaches the earth and is detected by the Swift or Fermi satellites.

The observed faint flash of gamma rays from exploding stars can last anywhere from a couple of seconds to a few minutes.

These are long-duration gamma-ray bursts.

Short-duration bursts that last less than two seconds are caused by merging neutron stars.

So, how close is too close for a GRB?

Well, the main danger of a burst within the Milky Way is not the direct radiation itself.

Essentially, all of the gamma rays and x-rays are going to be blocked by our atmosphere.

Some extra ultraviolet radiation will reach the ground, but not at seriously dangerous levels.

Instead, the danger is in the long-term effects on the atmosphere.

Gamma rays break apart nitrogen and oxygen molecules in the atmosphere, which then recombine into various oxides of nitrogen.

Those molecules are the real killers.

Nitric oxide catalyzes the destruction of ozone molecules, depleting the ozone layer that protects us from solar UV.

And nitrogen dioxide absorbs visible light, reducing the energy received from the sun.

These dangerous molecules can remain in the atmosphere for a few years.

And that's potentially long enough to cause a UV increase deadly to many species and to initiate runaway global cooling.

Also, they result in nitric acid rain.

It's estimated that a typical gamma-ray burst within 10,000 light years could deplete ozone enough to cause up to a 30% increase in ultraviolet at sea level.

And this is enough to devastate the most sensitive organisms, including phytoplankton, the basis of the marine food chain and Earth's main oxygen producer.

That alone is enough to cause a mass extinction event, and this could be exacerbated by the global cooling triggered by a few years of NO2 absorption of sunlight.

So why do some scientists think that the Ordovician-Silurian extinction event resulted from a GRB?

Well, a couple of pieces of evidence fit nicely.

Looking at the fossil record, there seems to be a strong correlation between the likelihood of a given species going extinct and the exposure that species would have had to ultraviolet light.

Species in the late Ordovician that lived near the ocean's surface or in shallow water were more likely to go extinct or went extinct earlier than those living in deeper water.

The same pattern isn't clear for the other mass extinctions.

One explanation for this unusual extinction pattern is that deeper-dwelling organisms had more protection against the increased UV following a gamma-ray burst.

Now, I should mention that the OS extinction is definitely associated with the beginning of an ice age.

Scientists agree that many of the extinctions of that era resulted from the change in climate.

The Ordovician was a very warm period.

And the relatively sudden onset of glaciation is hard to explain without some triggering event.

That event may have been the increase in sunlight absorbing NO2 after a GRB.

Also, extinctions appear to have started before that ice age really got under way.

That fits the hypothesis of a GRB.

Extinction started due to the sudden UV exposure and continued due to climate change.

Whether or not this particular extinction event was due to a gamma-ray burst, we're pretty confident that the earth does get blasted periodically.

Based on the rates of GRBs we see in other galaxies and on the population of stars in the Milky Way, it's estimated that every billion years, Earth finds itself in the path of between one and three GRBs within 10,000 light years.

Unfortunately there is no way to tell whether a GRB will be pointed our way until it happens.

The nearest potential GRB in the brewing is 8,000 light years away, so within the danger zone.

This is a Wolf-Rayet star, WR 104.

It's a massive star in the last phase of its life, currently blasting off its outer shells into a pinwheel-like nebula.

The exposed inner star shines several times hotter and hundreds of thousands of times brighter than the Sun.

This star is part of a binary system, and it's this binary orbit that produces the spiraling nebula.

The fact that the spiral appears to be face on suggests that the axis of the entire system is pointed directly at the Earth.

The rotational axis of the star will define the direction of the jet in the event that this Wolf-Rayet star does produce a gamma-ray burst.

If it does, the orientation of the system suggests we could be right in its firing line.

Well, no need to pack up and leave the solar system just yet.

Firstly, WR 104 could have up to half a million years of life in it.

Although, it's hard to tell exactly how close a star like this is to exploding.

Also, further observations with the Keck telescopes indicate that the system's orbital axis isn't pointed directly at the Earth.

That doesn't necessarily mean we're safe.

It's the star's rotational axis that defines the direction of the jet.

But the orbital axis of a binary system and the rotational axis of its stars are often correlated, so we may have dodged a bullet in this case.

Gamma-ray bursts are much less common than regular supernovae.

And in fact, regular supernovae can do just as much damage as a GRB.

However, for a supernova to produce the same effects, it needs to be much closer, within 20 to 30 light years.

There are definitely no stars in that range that could explode anytime soon.

However, the Sun isn't stationary.

It orbits the Milky Way, and its galactic neighbors come and go.

Maybe in a few 250-million-year orbits, a stellar time bomb will wander into our vicinity.

However, it's really the GRBs that are most likely to hit us first and hit us more often.

We should certainly expect one in the next half to one billion years, even if it's not WR 104.

And when that happens, well, we won't see it coming.

And anyway, there's nowhere in the solar system to hide.

But with any luck, we'll have advanced to the stage where geoengineering of the entire atmosphere is possible.

Perhaps we'll be able to rebuild the ozone layer and clean the bad molecules from the sky.

Maybe we can hold out a little longer against the series of calamities flung at us, one after the other, from outer space-time.