[MUSIC PLAYING] MATT O'DOWD: This episode
is supported by 23andMe. 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] 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. Thanks to 23andMe for
supporting PBS Digital Studios. 23andMe comes from the fact
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genomic analysis company created to help
people understand what their DNA says about them. 23andMe can help connect
you with your family and remind you of what
you have in common, which can be particularly
important this time of year. You may not share a lot
of the same opinions, but you definitely share
a lot of the same DNA. 23andMe has a special holiday
offer now through December 26. You can go to 23andme.com to
check out their holiday offer and get kits for your family. Show your support for
this show by checking out 23andme.com/spacetime. Last week, we talked about our
first detection of a visitor from outside our solar system. It's a giant chunk of rock
that we've named Oumuamua. Supreme84X asks, what would
happen if this thing hit us? Well, based on its
brightness and variations in that brightness,
the body is estimated to be 180 meters long
and maybe 30 meters wide. Now, assuming an albedo
or reflectivity of 0.1, that gives it a volume of
around 130,000 cubic meters. A typical asteroidal
density is 2,000 kilograms per cubic meter. So that gives it a mass
of around a quarter billion kilograms. It was moving much faster than
most solar system objects, at around 50
kilometers per second, at its closest
approach to Earth. So its kinetic energy, half
mv squared, was around 3 by 10 to the power of 17
joules, or 70 megatons of TNT. And that's greater than
the yield of Tsar Bomba, the greatest hydrogen
bomb ever detonated. Assuming Oumuamua
is entirely rocky and hit the atmosphere
pretty much head on, it might actually
reach the ground before disintegrating
to deliver that energy. It would become what
we call a city killer. The global effects,
however, would be limited. PixelatedDonkey asks
whether there's a chance that some stars/solar
systems will be ejected from the galaxy
when the Milky Way collides with Andromeda. In fact, yes. Recent studies suggest
that there's a 3% chance that the Sun will jump galaxies
on Andromeda's first fly by. And there's some
smaller chance that it will miss that jump,
coming up short, and end up in
intergalactic space. And some stars will be
sling-shotted out of the galaxy by the two supermassive
black holes of Andromeda and the Milky Way as
they fall together. And what will the residents
of one of those exiled systems see? Well, a very dark night sky. Dark everywhere except for
the gigantic elliptical galaxy Milkdromeda, the final
result of the merger. But don't worry. We have a few billion years
to come up with a better name for that galaxy. Timothy Judge points out a
significant potential error in the paper by Portegies
Zwart and collaborators. Mob nagh translates
from the Klingon more accurately to the stone
is lonely, not lonely rock. Let's hope the referee caught
that one before publication. Dogh bachHa'.
Always nice to add the cheerful thoughts of unpreventable extinction possibilities to the joyous holidays.