All language subtitles for How.The.Universe.Works.S09E08.The.Next.Supernova.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

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.

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.

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.

It could be anywhere.

It is nearly impossible to predict
where and when

the next supernova will happen.

The hunt is on
to find the next supernova,

before it finds us.

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.

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 gonna go boom.

Betelgeuse is on our list
of supernova candidates

because of this massive size.

The bigger star they are,
actually, the shorter the lifespan.

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.

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 star will live."

Turns out it's exactly opposite.

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.

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.

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 start builds up
that iron core,

it's doomed.

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.

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.

A huge amount of energy
is suddenly released

which forces the collapsing layers
back out.

The result? An enormous explosion

we call a supernova.

The shock wave 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.

A supernova could devastate
life on Earth.

And the evidence can be found

at the bottom of our oceans.

There are layers and layers of silt
that have built up,

and there seemed to be a layer,

about 2.6 million years ago

that was enriched
in a very strange chemical element,

something called iron-60.

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.

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,

the 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?

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.

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.

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's a direct impact
to life on Earth and us.

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.

Betelgeuse varies quite a lot
over the years.

There are some cycles,

and sometimes,
these cycles come together

and you get a deep minimum.

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 measured 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.

So, if Betelgeuse
wasn't cooling much,

what was making it dim?

To take a closer look,
we used the 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.

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.

As heavier material, like silicon,

emerges from the surface
of Betelgeuse,

it cools and condenses.

It's like stickin' the hose
in the wrong end
of your vacuum cleaner.

Instead of pullin' stuff in,
it blows all this dust
out into space.

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 baited breath.

It's gonna 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.

With luck, if Betelgeuse blows,

all we'll see
is a beautiful light show.

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?

The main star isn't alone.

It has a more dangerous accomplice.

There's another star there,
orbiting the larger star.

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 gonna be

the way they are now,
and sometime in the future,

things are going to change a lot.

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.

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 gonna run out of fuel,

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

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 onto the surface.

As the white dwarf pulls material

from its bloated
red giant neighbour,

it gets more and more massive.

It's gravitational pull increases

so it feeds even faster.

The thing about a white dwarf
is that on its surface,

the gravity can be 100,000 times

the gravity of Earth.

So, it's very intense.

The material from the star
is mostly hydrogen,

and this can pile up on the surface,

and it's getting squeezed very hard.

But that's how you fuse hydrogen,

that's how you make a hydrogen bomb.

It reaches
an incredible temperature.

36 million degrees.

It can actually become hot enough
to ignite nuclear fusion reactions.

But right on the surface
of the white dwarf,

you get this bright flash.

A nova.

A nova is a big explosion,

but not big enough
to destroy the white dwarf.

So, once things calm down,

the white dwarf starts feeding again
until there's another nova.

And another, and another.

Each time,
the white dwarf gets heavier.

Eventually, it can no longer
support its own weight.

The core of a 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.

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.

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.

In our hunt for the next supernova,

our leading suspects
are massive stars.

And they don't come much bigger
than P Cygni.

It's about 30 times the mass
of our Sun.

Double the mass of Betelgeuse.

So big, we don't call it a giant
or even a super-giant.

It's a hyper-giant.

And it's behaving suspiciously.

It was first spotted back in 1600,

but just a couple of decades
after that it disappeared.

And then, reappeared again,
and then, disappeared again,

and then, like, 100 years later,
re-appeared.

P Cygni is a luminous blue variable,

the rarest kind of stars
in the universe.

This star is so unusual,

we think,
because the core is producing

an enormous amount of energy

that's right on the tipping point

of being able to rip apart
the stars itself.

Stellar adolescence,
like P Cygni, are erratic.

They burn fiercely,
and they die young.

They have some of the shortest lives
of any star,

just a few million years,

which is why we see so few of them.

It looks like,

P Cygni is going to go supernova
any day now.

And for all we know,
it could blow up tonight.

At a distance of 550 light years,

P Cygni is too far
to threaten Earth.

But we could learn a lot from it

if only we could spot any signs

that it's about to blow up.

As an astronomer, and an astronomer

who has studied supernovas
professionally,

havin' 'em 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.

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?

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.

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.

Forcing protons
and electrons together

releases a huge amount of energy

in the form of tiny, illusive

sub-atomic 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.

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.

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.

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.

Because it was the first supernova
spotted that year,

it was called Supernova 1987A.

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

Essentially, a supernova went off
in our own backyard.

It was very close to us,

occurring in a neighbour galaxy
of the Milky Way,

and so, it was the brightest thing

seen in our skies
since the invention
of the telescope.

Supernova 1987A

blazed with the power
of a hundred 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.

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 the supernova blast

and they slip through the atmosphere
of the star

before it goes boom.

Neutrinos can escape
in as little as ten 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 wanna spot
the next supernova explosion,

we've gotta be paying attention
to the neutrinos.

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 gonna actually happen.

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.

They become really huge

and bloated and swollen,

and they're prone to huge outbursts.

These unstable stars
are called Wolf-Rayet stars.

They're very rare
and so hot and bright,

they emit more radiation
than a million Sun-like 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.

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 gonna blow hard.

When a star goes supernova,
it's 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 disc.

As the material falls onto the core,

it compresses and it speeds up.

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 funnelling material

around and up and out

in the form of huge beams
of radiation.

So the energy
from the supernova collapse,

instead of being emitted 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.

This is a gamma ray burst.

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.

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.

A gamma ray burst from Apep
might last only ten 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 our ozone layer,

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

And the acid rain
would destroy our crops.

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.

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.

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.

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.

Apep is on the edge
of an enormous explosion.

It's 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 noticed that one star
that they thought

would become a supernova,
instead blinked out.

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.

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?

How can 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?

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

onto that black hole,
and it's powering
a little bit of light,

a little bit of the infrared light
still gets out.

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 gonna
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 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.

As fusion turns hydrogen to helium

and then to carbon,
convection mixes the carbon

so it burns up.

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.

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 this 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.

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.

Anytime 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.

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.

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.

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

but the hunt continues.

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.

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.

This is almost literally
a cosmic cycle of life.

And a 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 ionised atmosphere means that

now, it can conduct electricity.

So it will, probably, increase
lightning across the planet.

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.

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 levelled

forests and savannahs
creating grasslands.

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.

You've living in a savannah
where there's lions

and leopards and cheetahs.

The savannah 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.

Standing upright also triggered

the most important change
in our history.

Walking around on two feet
freed our hands

to be able to start doing things.

And you can imagine,
as you start doing things,
it drives your brain

to more complexity
as you try 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.

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.

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.

Supernova 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 could imagine my face lighting up,

I would really lose it.

I would love to see
a supernova up-close.

I mean, what a light show.

But there's no way I would wanna be
that close cos I don't wanna die.

Subtitles by Deluxe