All language subtitles for How the Universe Works (2021) - S09E07 - The Next Supernova_track3_[eng]

[music playing]

NARRATOR: There's
a killer lurking

in our galaxy, a star ready
to explode into a supernova.

These are the most
visually stunning

events in the universe.

NARRATOR: Seen from Earth, it
would have a terrible beauty.

But for us, it could be fatal.

In a few seconds, it
can release as much energy

as the sun will over
its entire lifetime.

NARRATOR: We're trying to hunt
it down, but it's lying low.

We haven't seen a
supernova in the Milky Way

in over 400 years.

NARRATOR: It could be anywhere.

It is nearly impossible
to predict where and when

the next supernova will happen.

NARRATOR: The hunt is on
to find the next supernova

before it finds us.

[booming]

October 2019, one of the
brightest stars in the sky

looks dangerously unstable.

If you look at the
constellation of Orion,

one of the shoulders of Orion
is a star that is obviously red.

This is Betelgeuse.

I could go into my
backyard and see it.

You could clearly see that
it was getting dimmer.

NARRATOR: Is this a warning?

Is Betelgeuse about to die in
a massive cosmic explosion,

a supernova?

We've been studying this
star for hundreds of years.

And one thing we're sure about
is that it's big, very big.

Betelgeuse is a massive
star, maybe 15 or 20 times

the mass of our sun.

And it's near the
end of its life.

It is a massive,
enormous, luminous star.

And one day, it's
going to go boom.

NARRATOR: Betelgeuse is on our
list of supernova candidates

because of this massive size.

The bigger star
they are, actually the

shorter the lifespan.

NARRATOR: The lifespan
of a star depends

on a delicate balance between
two competing forces--

Gravity pulling in and heat
and pressure pushing out.

Stars exist because
they're held up.

They're not held up by pillars.

They're held up by energy
flowing out of the core

toward the surface of the star.

That stops the
gravitational contraction.

MICHELLE THALLER:
Stars get their energy

from nuclear fusion
reactions right in the core.

And the most basic one is
taking two hydrogen atoms

and slamming them together
to form a helium atom.

And you might think, OK,
the more hydrogen you have,

the more stuff you have, maybe
the longer the start will live.

Turns out it's exactly opposite.

NARRATOR: The reason-- gravity.

The more mass a star
has, the stronger

its gravity, gravity
that crushes its hydrogen

atoms closer together.

As you crush
things more and more,

the temperature gets hotter
and hotter and hotter.

And the nuclear fusion
reactions burn faster.

So bigger stars burn their
fuel very, very quickly

and live short lives.

Smaller stars burn their
fuel much more slowly

and live long, protracted lives.

So when you are a
big star, you live fast

and you die young.

NARRATOR: Betelgeuse burns
brighter than 125,000 suns.

But now it's running out
of its hydrogen fuel.

So it's burning whatever it
has left just to stay alive.

Stars are basically
factories for burning

hydrogen into helium.

And then, once the
helium is burned,

they start burning heavier and
heavier elements, like carbon

and nitrogen and oxygen.

It's a little like, you
burn something, you get ash.

But then if you
crush the ash enough,

you could burn it again.

And then you crush it some more,
and you can burn it yet again.

NARRATOR: But this process
can't go on forever.

As the size of the atomic nuclei
being fused together grows,

the amount of energy
released falls.

The fuel the star needs to
resist the pull of gravity

is running out.

Unfortunately,
the amount of energy

you can extract by putting
two nuclei together

gets smaller and smaller the
bigger the nuclei are until you

come to making iron,
and iron, it turns out,

is the last thing you
can make that way.

The problem with iron
is, when you fuse it,

it doesn't make energy.

It takes it away.

So when the star builds up
that iron core, it's doomed.

HAKEEM OLUSEYI: It can no
longer create energy in its core

to flow out toward the
surface strong enough

to keep it from collapsing.

So collapse is what they do.

NARRATOR: In a
fraction of a second,

the star's core collapses
down from the size of a planet

to about the size
of a small city.

And when that happens,
all hell breaks loose.

NARRATOR: A huge
amount of energy

is suddenly released,
which forces

the collapsing layers back out.

The result-- an enormous
explosion we call a supernova.

JAMES BULLOCK: The
shockwave from a supernova

rips out at thousands
of miles per second.

And for a brief period
of time, they're

brighter than an entire galaxy.

NARRATOR: A supernova could
devastate life on Earth.

And the evidence can be found
at the bottom of our oceans.

MICHELLE THALLER: There
are layers and layers

of silt that have built up.

And there seem to be a layer,
about 2.6 million years ago,

that was enriched in a very
strange chemical element,

something called iron-60.

PHIL PLAIT: Iron-60 is a
radioactive isotope of iron,

and it doesn't last very long,
just a few million years.

And the only place that we
know of that can make iron-60

is a supernova in
an exploding star.

That means there must have
been a supernova close enough

to the Earth within the
past couple of million years

to have physically deposited
material on our planet.

That freaks me out.

NARRATOR: The sign of this
shocking assault on our planet

is a thin layer of this
very rare type of iron.

We find it in the mud
of every ocean floor

and always at the same depth.

This interstellar dust
must have drenched

our world in one enormous
burst 2.6 million years ago.

It was a terrible time.

A third of large animal species
in the sea suddenly died out.

There were some
pretty amazing fish.

Probably the most
amazing is the megalodon,

a giant shark-- teeth the size
of dinner plates and so on.

But they went extinct
2.6 million years ago

at the end of the Pliocene.

What happened?

PHIL PLAIT: A lot of
sea creatures died.

And a lot of them were
in shallow waters,

whereas deep-water
animals tended to survive.

That sounds kind of
like a supernova.

That can do things that
would affect our atmosphere,

would affect shallow water,
but not deeper water.

NARRATOR: Supernovas create
huge amounts of cosmic rays.

When they crash
into other atoms,

they break up and produce
showers of dangerous shrapnel

called muons.

These charged particles
are similar to electrons,

only 200 times heavier.

So they penetrate more
deeply and cause more damage.

They can pierce
through our atmosphere,

pierce through our
skin, get into a cell,

and disrupt the DNA.

They'll go right through
a mouse but deposit

in the body of a larger animal.

So the impact on an animal
the size of a megalodon,

say, could be pretty extreme.

NARRATOR: Muons can shatter DNA,
causing mutations and cancer.

But their power
weakens as they travel

through water, which
may be why only

deep sea creatures survived.

The extinction really
tells us that we're not

separate and apart from
the universe and the goings

on up there, right?

Supernova going off
and things like that--

OK, it's a pretty light show.

No.

It is a direct impact
to life on Earth and us.

NARRATOR: So are we in
danger of extinction?

Is Betelgeuse about to explode?

When stars explode
as supernovas,

they can devastate planets
hundreds of light years away.

Betelgeuse is about 550
light years from Earth.

So, when it dramatically
dimmed in 2019,

scientists were concerned.

But Betelgeuse
has dimmed before.

ALEX FILIPPENKO:
Betelgeuse varies

quite a lot over the years.

There are some
cycles, and sometimes

these cycles come together,
and you get a deep minimum.

NARRATOR: So dimming is part
of the star's natural cycle

as it nears the end of its life.

But to get a full picture, we
took Betelgeuse's temperature.

If the star was dimming, that
would mean that the surface

was cooling over time.

We actually made measurements
of the temperature of Betelgeuse

and found out that
wasn't happening.

It hardly cooled at all.

It cooled, like,
50 or 100 degrees.

You might expect
a much, much more

dramatic change in the
surface temperature

if it were about to explode.

NARRATOR: So, if
Betelgeuse wasn't cooling

much, what was making it dim?

To take a closer look, we
used a very large telescope

and an exoplanet
hunting instrument

called SPHERE and came up
with an extraordinary image.

When I first saw this image
of Betelgeuse, it blew me away.

I almost gasped.

I may have said a
word I can't say on TV.

That was very exciting.

NARRATOR: The image reveals
that, while the upper part

of Betelgeuse was
still bright, the lower

part was noticeably dimmer.

We had images of
Betelgeuse from before,

and we were able to compare
the new ones with it.

And so you could see
that half of Betelgeuse

looked pretty much the same.

But the other half was
significantly dimmer.

And what could make a
star dim that quickly?

And remember how
big this star is.

Nothing happens on
Betelgeuse quickly.

So this must be something
happening right on the surface.

NARRATOR: As heavier
material like silicone

emerges from the
surface of Betelgeuse,

it cools and condenses.

It's kind of like sticking
the hose in the wrong end

of your vacuum cleaner.

Instead of pulling stuff
in, [imitates explosion]

it blows all this
dust out into space.

NARRATOR: Betelgeuse
has cosmic indigestion

and is belching dust, which
makes the star seem dim.

But it's not over.

All through 2020,
Betelgeuse first brightened

and then dimmed again.

So astronomers are watching this
massive star with bated breath.

It's going to explode.

The question is, when?

It's probably sometime
in the next 100,000 years.

But it could be tomorrow.

It could have already
exploded and we're

just waiting to see the light.

NARRATOR: With luck, if
Betelgeuse blows, all we'll see

is a beautiful light show.

At a distance of
550 light years,

it's probably too far
to do serious damage.

But is there another star
we should worry about?

A closer star, just 150
light years from Earth,

could do us some major
damage, a star like IK Pegasi.

But it isn't this star which
we can see in our night

sky that's the threat.

The main star is only about
1.6 times the mass of the sun.

That's nowhere near enough
mass to go supernova.

And yet, we think it is the
progenitor for a supernova.

How can that be?

NARRATOR: The main
star isn't alone.

It has a more
dangerous accomplice.

There's another star there
orbiting the larger star.

PHIL PLAIT: And this is what
we call a binary system--

Two stars orbiting each other.

Right now, the system is stable.

But things aren't always going
to be the way they are now,

and sometime in
the future, things

are going to change a lot.

NARRATOR: IK Pegasi
is really made up

of IK Pegasi A, a large white
star, and its accomplice,

a white dwarf called IK
Pegasi B. This tiny star

is the real threat to Earth.

You can think of a
white dwarf as a zombie.

You know, it's a dead star,
and they can eat living stars.

If there's a normal star like
the sun near a white dwarf,

the white dwarf has very,
very intense gravity.

It can literally pull
material off that normal star,

and that material will
then pile up on the surface

of the white dwarf.

So it really is
eating a living star.

NARRATOR: These stars
orbit each other just

18.5 million miles apart.

That's closer than
Mercury is to our sun.

But they're not interacting
with each other, yet.

The problem is,
sometime in the future,

that normal star is
going to run out of fuel.

And when it does, it's going
to expand into a red giant.

NARRATOR: When it gets
to the end of its life,

IK Pegasi A will cool and
swell up to become a red giant.

And that's it, no big explosion.

It won't become a supernova.

But that's just when it's
accomplice, IK Pegasi B,

will start to feed.

A lot of that material
will gravitationally

be attracted to the white dwarf
and fall under the surface.

NARRATOR: As the white
dwarf pulls material

from its bloated
red giant neighbor,

it gets more and more massive.

It's gravitational
pull increases,

so it feeds even faster.

Eventually, it can no longer
support its own weight.

The core of the star
is actually very dense.

In fact, if you had, like,
a teaspoon of material,

it would weigh about as
much as an 18-wheel truck.

And it's basically right at
the limit of normal matter

being able to hold
up at that density.

You dump more and
more stuff onto it,

and eventually there's
a limit that's reached.

And it either collapses
or, more generally, blows up.

NARRATOR: When, this
happens IK Pegasi will

be brighter than the
full moon in our sky

because it's only
150 light years away.

Having a supernova 150 light
years sounds like a bad idea,

and it is.

That's close enough
that it might

have some physical
effects on the Earth.

NARRATOR: Right now, IK
Pegasi is about as far

from Earth as the
supernova suspected

of killing off the megalodon.

So how worried should we be?

The good news is
the IK Peg system

is moving away from
the sun and the Earth

right now at a decent clip.

So if it's not going to blow
up for a while, that means

it could be on the other side of
the galaxy by the time it does.

By that time, we'll
be completely safe.

As an astronomer and an
astronomer who has studied

supernovas professionally,
having them

far away is fine with me, close
enough that we can study them

well but not so close
that I can study

them personally on a physical
level on my own body.

Yeah, no.

NARRATOR: A close supernova
would be devastating for life

on Earth.

Will there be any
warning signs before one

of our prime suspects
is about to blow?

[booming]

To find a supernova
warning signal,

we need to know what's
happening deep inside the core

of an exploding star.

At the very beginning
of a supernova explosion,

the core of a massive
star is collapsing.

There's no more nuclear
fusion going on,

and it is compressing to
higher and higher densities.

NARRATOR: The star's
gravity crushes

protons and electrons
so close together

they merge to form neutrons.

The star's core becomes one
of the densest materials

in the universe.

It's like a gigantic
atomic nucleus--

Roughly half a million Earths
compressed into the volume,

the size of a city.

That's really,
really dense stuff.

If you had about a
teaspoon full of material,

that would be about as
much mass as Mount Everest.

NARRATOR: Forcing protons
and electrons together

releases a huge amount
of energy in the form

of tiny, elusive, subatomic
particles called neutrinos.

Neutrinos are one
of the most abundant

particles in the universe.

But they don't interact with
things very much at all.

HAKEEM OLUSEYI: Neutrinos are
often called ghost particles

because they do what ghosts do.

They walk through walls.

But neutrinos walk through us.

They walk through the planet.

They walk through stars.

They're super ghosts.

[whooshing]

NARRATOR: At first, these
neutrinos can fly straight

out of the core of the star.

But, as the star
collapses, it gets so dense

that some neutrinos
get trapped and

their energy turned into heat.

And that creates a shockwave
that rips the star apart.

And the ensuing explosion is
brighter than billions of stars

all put together.

NARRATOR: This light show may
be spectacular, but it's only 1%

of the energy released
in a supernova.

The rest is in the form of a
massive burst of neutrinos.

So neutrinos could act as a
supernova early warning system.

At least that's the idea.

On February 24th, 1987,
that idea was tested.

An astronomer was doing a
routine survey of a dwarf

galaxy close to ours.

He was taking pictures of
it, develops the pictures,

and says, hey,
there's a star here

that wasn't there yesterday.

He basically got up, walked
outside, and looked and went,

oh, there's that star.

And it turns out he had
discovered a supernova.

NARRATOR: Because it
was the first supernova

spotted that year, it was
called Supernova 1987A.

1987A a was an amazing event
in the world of astronomy.

Essentially, a supernova
went off in our own backyard.

PAUL SUTTER: It was
very close to us,

occurring in a neighbor
galaxy of the Milky Way.

And so it was the brightest
thing seen in our skies

since the invention
of the telescope.

NARRATOR: Supernova 1987A
blazed with the power

of 100 million suns.

But that wasn't the
most exciting part.

For the first time, we
received an early warning

that a supernova was about
to appear three hours

before it lit up our night sky.

Neutrino observatories
around the world

saw a sudden surge in neutrinos
from the same direction

on the sky.

NARRATOR: Neutrinos' ability
to zip across the galaxy,

slipping through stars
and planets like ghosts,

gives them an unbeatable head
start during a supernova.

The neutrinos are released
in the very earliest moments

of this supernova blast.

And they slip through the
atmosphere of the star

before it goes boom.

NARRATOR: Neutrinos can escape
in as little as 10 seconds.

But it can take hours
for the shockwave

to travel right through the star
and blast off the outer layers,

revealing the light.

The result is that
we see neutrinos

from a supernova explosion
before we see the actual light.

So if we want to spot the
next supernova explosion,

we've got to be paying
attention to the neutrinos.

NARRATOR: Astronomers set
up the SuperNova Early

Warning System, a network
of neutrino detectors

all around the world.

It should give astronomers
several hours notice

of an impending supernova.

But, so far, nothing.

No supernovas have
occurred near enough

for the system to detect.

Neutrinos are like the
friend that never comes.

We're sitting here
waiting for him.

But we don't know when it's
going to actually happen.

NARRATOR: But when they do
come, we might be in trouble

because some supernovas are
armed with the most powerful

weapon in the universe--

Gamma rays.

[booming]

Our hunt for the Milky
Way's next supernova

has identified some
potential suspects--

Very massive, lonely stars and
stars with smaller sidekicks.

In 2018, astronomers
found a system called Apep

8,000 light years away with two
very massive stars, each one

about as massive as Betelgeuse.

These are giant stars
nearing the end of their lives

with massive outer layers of
gas that continually contract

and heat up again and again.

HAKEEM OLUSEYI:
They become really

huge and bloated and
swollen, and they're

prone to huge outbursts.

NARRATOR: These unstable stars
are called Wolf-Rayet stars.

They're very rare and so hot and
bright they emit more radiation

than a million sunlike stars.

This intense energy is
blasting their outer layers off

into space.

Mass loss has been
occurring from the star,

so much so that you've
actually lost all the hydrogen

that wasn't burned into helium.

So now you have a star
that's made entirely

of helium and heavier elements.

NARRATOR: With no hydrogen
left, these massive stars

are running low on usable fuel.

They're like ticking
time bombs, made

even more dangerous because
they're spinning so fast.

It's spinning so
quickly, it's on the verge

of ripping itself apart.

And this means that,
when this thing blows,

it's going to blow hard.

NARRATOR: When a star goes
supernova, its core collapses.

The smaller it gets,
the faster it spins.

Some cores collapse into
fast, spinning neutron stars.

Heavier ones, like Apep,
collapse into even denser

and more mysterious objects--

Black holes.

The immense gravity within
Apep's collapsing core

will drag back some of the gas
and dust into a spinning disk.

As the material
falls on to the core,

it compresses and it speeds up.

NARRATOR: The dying
star spins faster

and faster as it collapses.

And this incredible
rotation drives the creation

of massive magnetic fields
that are capable of funneling

material around and
up and out in the form

of huge beams of radiation.

So the energy from
the supernova collapse,

instead of being admitted
spherically in every direction,

comes at us in a
tightly focused beam.

Like a laser from
the Death Star,

it is pointed in one direction.

NARRATOR: This is
a gamma ray burst.

PHIL PLAIT: It is the
single scariest thing

the universe has to offer.

This is an explosion so
powerful that, in a few seconds

or minutes, it can
release as much energy

as the sun will over
its entire lifetime.

You do not want to get
caught in a gamma ray burst.

Let's just put it that way.

NARRATOR: The impact
of a nearby gamma ray

burst on our home
planet is almost

too terrible to think about.

It would be a very
bad day for Earth.

Earth's atmosphere could
be partly blown away,

and there could be
chemical reactions

in the atmosphere
that would form

all kinds of noxious products.

NARRATOR: A gamma
ray burst from Apep

might last only 10
seconds, but its impact

would last for decades.

The generation of nitrogen
oxide from a gamma ray burst

would be disastrous.

In the upper
atmosphere, it would

eat away at our ozone layer.

In the lower atmosphere, it
would come out as acid rain.

And the acid rain would
destroy our crops.

NARRATOR: Nitrogen dioxide
also filters out sunlight,

turning the skies dark and
cooling the Earth enough

to trigger a new Ice Age.

Any life on the land, in
the shallow parts of the sea,

or that live near the sea
surface would be done.

In fact, it would ultimately
result in extinction.

NARRATOR: Blasted by ultraviolet
radiation from our sun,

freezing cold and
hungry, humanity's future

would be bleak.

So we really need to know,
when Apep goes supernova

and produces its deadly
beam of gamma rays,

are we in its line of fire?

The good news is
that we are probably

not right in the direct
firing line of Apep.

PAUL SUTTER: The axis of
rotation of the Apep system

is pointed 30
degrees away from us.

So if it does blow, it's
likely that the jets

are going to miss us.

It makes me feel
better that this gamma

ray burst isn't pointing at us.

But, of course, there are
many other cosmic catastrophes

potentially waiting to get us.

NARRATOR: Apep is on the edge
of an enormous explosion.

Its huge gravity
and incredible spin

should produce a
spectacular supernova.

But what if some stars
are too big to blow?

Galaxy NGC 6946-- a local galaxy
just 20 million light years

away and well known to
supernova detectives.

It's the fireworks galaxy
because it has produced so many

supernovas in the past century.

And they notice that
one star that they

thought would become a
supernova instead blinked out.

NARRATOR: The star
under investigation

is N6946-BH1, a
cosmic heavyweight 25

times the mass of our sun.

That's way more than the
eight solar masses we

thought guaranteed a supernova.

This is a very
massive, very luminous

star, the prototype
of what you expect

to explode as a supernova.

MICHELLE THALLER: And over
the last couple of years,

its brightness
has been changing.

Maybe the star was beginning
to go a bit unstable.

But then, right in
front of our eyes,

this star just
completely disappeared.

This is a huge mystery.

Why didn't this thing blow up?

NARRATOR: How could a
star just disappear?

There had to be
something left behind.

So astronomers began
a search for evidence

and found a crucial clue.

When you look in
the infrared, you

can still see some light there.

So there was something
happening there.

But what?

NARRATOR: We think the infrared
light is heat coming off

the debris of the dead star.

Something is pulling
it inwards, something

powerful but also invisible--

A black hole.

The outer stuff
from the star is still

falling on to that
black hole, and it's

powering a little bit of light.

A little bit of the infrared
light still gets out.

NARRATOR: How can
a giant star become

a black hole without exploding
into a supernova first?

The answer lies in how
dying stars burn their fuel.

For stars that are about, say,
20 times the mass of the sun,

you're actually going to
burn things convectively.

That means the gases inside
the core are moving around.

A good analogy is water
in a boiling pot of water.

You've got your
potatoes up here.

You're trying to boil them.

You've got convective cells
of water that are heated.

Bring the heat up to the top.

Get the potatoes hot.

And then those blobs of water
cool down, become denser,

and settle down to
the bottom again

where they're heated once more.

NARRATOR: As fusion
turns hydrogen

to helium and then to
carbon, convection mixes

the carbon so it burns up.

PAUL SUTTER: Convection
cells work inside

of a star like
massive elevators that

take hot gas from
the central regions,

bring it up to the surface,
allow it to cool, and then

pull that material back down.

They're constantly churning
back and forth inside of a star.

NARRATOR: But stars more
massive than roughly 20 times

the mass of the
sun, like N6946-BH1,

don't burn carbon this way.

Instead of mixing,
the heavier atoms

created by the fusion reactions
just start to pile up.

That means there's
a layer of very

dense material building up on
just the surface of the core.

All of the stuff is
just ready to collapse.

It's possible that, if
you have enough mass sitting

around, the collapse
is so powerful

that it actually collapses
into a black hole

before any supernova goes off.

That, then, is a
failed supernova.

It's a star that
pretty much directly

collapses to form a black hole.

NARRATOR: If many of the massive
stars we expect to go supernova

won't, that's a problem.

We used to think we had the
basics of supernovas cracked.

Any time you have a star
more massive than eight times

the mass of the
sun, it was destined

to explode as a supernova.

And then along comes a star
that screws everything up.

NARRATOR: To make things
worse, we found no clear

distinction between
stars that go out with

a bang and those that don't.

As many as 30% of massive stars
could die without exploding.

Our search for the
next killer supernova

is getting even harder.

PHIL PLAIT: Stars blow up
when we don't expect them to.

They don't blow up
when we expect them to.

They can have several
stars orbiting each other,

and the one that blows up
isn't necessarily the one

you think it will.

NARRATOR: So right now we
can't identify a prime suspect,

but the hunt continues.

PAUL SUTTER: As far
as we know, there

are no life-threatening
stars out there,

but we haven't done
a complete survey.

So please keep funding astronomy
so we can keep looking.

NARRATOR: Supernovas destroy.

But can they also create?

Did a supernova spark
humanity's rise to dominate

our world and our solar system?

Supernovas are spectacular,
devastating, and frightening.

But without them,
we wouldn't exist.

The iron in your blood and
the calcium in your bones

was literally forged inside of
a star that exploded billions

of years ago as a supernova.

And I think this is one of the
most beautiful and the most

profound things that we've
learned in astronomy,

that we're literally viscerally
connected to the cosmos

and the cosmos is
connected to us.

With every breath,
we are inhaling

oxygen that was created
in a supernova explosion.

[booming]

This is almost literally
a cosmic cycle of life.

[booming]

NARRATOR: And the
supernova may even

be responsible for the
dawning of our intelligence

by causing lightning.

It might sound
rather incredible,

but a supernova might actually
influence, directly, weather

right here on the Earth.

The cosmic rays from a
supernova will create charges

in the lower atmosphere.

That energy will
break apart molecules,

excite atoms and molecules,
and it will ionize them.

And an ionized
atmosphere means that now

it can conduct electricity.

So it probably increased
lightning across the planet.

NARRATOR: It's possible the
same gamma ray burst that caused

a mass extinction
2.6 million years ago

also affected
Earth's atmosphere,

triggering tremendous
bursts of lightning,

which caused forest fires.

HAKEEM OLUSEYI: We have
evidence of widespread fires

at this time.

So it could be that
lightning was increased,

and that created more fires.

And those fires could have
leveled forests and savannas,

creating grasslands.

NARRATOR: So how could this
change/boost our intelligence?

With their forest homes burnt,
our ancestors, early hominids,

had to adapt to life out in the
open, which meant standing up.

DAN DURDA: You're
living in a savanna

where there's lions and
leopards and cheetahs,

and the savanna is
mostly grassland.

It's a lot more efficient,
perhaps, on two feet.

You can run.

And moving on two
feet might have

been the survival mechanism.

NARRATOR: Standing upright also
triggered the most important

change in our history.

DAN DURDA: Walking
around on two feet

freed our hands to be able
to start doing things.

And as you-- you know, of
course, you can imagine that,

as you start doing things,
that drives your brain to more

complexity as you're
trying to figure

out how to manipulate things.

And this is perhaps the biggest

evolutionary leap,
because, without it, we

don't get tool use.

We don't get fire.

We don't get intelligence.

NARRATOR: As our ancient
ancestors adapted

to their new habitat, they
took their first steps

toward world domination.

At least, that's the theory.

The idea presented here
is this would be the dawn

of modern humans as we see it.

And we would owe
that to lightning

created from a gamma ray burst.

That's nuts.

NARRATOR: Supernovas
are extraordinary.

They launched our
journey into the cosmos.

And in time, a
supernova may end it.

We're searching hard to
spot which one it could be.

But, for now, the only
way we'll know for sure

is when it lights up our sky.

While a supernova might appear
to be the death of a star,

the beauty of it is
that it's really a story

about beginnings, as well.

Supernovae giveth,
and they taketh away.

Without supernovae, the
Earth wouldn't exist

and we wouldn't exist.

I actually do imagine standing
out on a nice winter night,

looking up at
Betelgeuse, and actually

seeing the thing explode.

There would be
this bright light.

I can imagine my
face lighting up.

I would really lose it.

I would love to see
a supernova up close,

DAN DURDA: Right?

I mean, what a light show.

But there's no way I would
want to be that close

because I don't want to die.