Original subtitles

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.

-== [ www.OpenSubtitles.org ] ==-

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 going to 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 start 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 star 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 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.

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 seem 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,

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?

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 is 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 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.

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.

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 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,

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 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.

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?

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 going to 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 going to 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 under the surface.

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.

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.

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.

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, 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.

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.

1987A 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 neighbor 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 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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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 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.

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.

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.

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.

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 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?

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.

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.

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 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.

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.

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 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.

This is almost literally a cosmic cycle of life.

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.

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 leveled forests and savannas,

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'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.

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 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.

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.

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,

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.

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