All language subtitles for How the Universe Works s08e04 Death of the Last Stars.eng

?

Narrator: Imagine a universe
with no stars --

a dark, endless night.

This is not some
Sci-Fi nightmare.

This is our future.

There will definitely be a point
in the future when, you look up,

you will no longer
be able to see stars.

Thaller: Things really will get
darker and darker,

until there will be almost
no memory of light left.

Narrator:
For billions of years, stars brought life to the universe.

The fact that you exist at all
is because of stars.

Narrator: Now, they're dying out
in a star apocalypse.

Stricker: The effect
could be tremendous.

It can permeate
throughout the universe.

Narrator:
What's causing the die-off,

and what happens to life
when the lights go out?

Eventually, the whole
entire universe

starts to get
a little bit weird.

-- Captions by vitac --
www.vitac.com

captions paid for by
discovery communications

narrator:
For over 4.5 billion years,

the sun has bathed
our home planet with light.

Its bright, stable glow
helps life flourish,

but hidden in
the night sky,

other planetary systems
haven't been so lucky.

Thaller: Hanging right above
your head every night,

we see up there
these dead corpses of stars.

Narrator:
400 light-years from earth lies a system called sdssj1228.

A disk of debris orbits

the faintly glowing leftovers
of a dead star.

J1228 is a dead star.

It is a core of a star
that had aged,

blown off its outer layers,
revealed the core --

which is about
the size of the earth,

but has about half
the mass of the star in it.

And we call these
"white dwarfs."

Narrator: May, 2018.

Astronomers
investigated j1228

using the world's largest
optical telescope --

the gran telescopio canarias.

They discovered what appears
to be a ball of iron

orbiting the white dwarf.

The lump of metal,
less than 400 miles across,

could be the exposed core
of a destroyed planet.

It's a clue to
this system's past.

Thaller: It's always
a little poignant

when you see evidence
of a planet around a dead star.

You know, you think back of
when that star was shining,

and could there have been life
in that solar system?

Narrator: The j1228 system
is a cosmic graveyard.

It might look different
than our solar system,

but this is our future.

This discovery of a dead planet
orbiting a dead star

is like looking into
a crystal ball.

And is it the future
of our own solar system?

Yep.

For a glimpse into your future,
you know,

all you need to do
is look up.

Narrator: Just like j1228,
our sun will die,

killing off earth
in the process.

This terrifying fate
will play out across the galaxy

in a star apocalypse.

Our sun is a fairly common type
of star in the milky way,

and so, other stars
in the milky way

will undergo the same sort
of fate as the sun.

They will end up
as white dwarfs.

And so, any other planets out
there orbiting sun-like stars

will undergo
a similar fate.

Once the stars like our sun
have died out,

what's gonna happen?

Could life still survive
around white dwarfs?

Narrator: To understand the fate
of sun-like stars,

we have to look
inside them.

Buried within are clues to how
they live, and why they die.

Plait:
The core, the very center, that's where the action is.

That's where the star
is fusing light elements

into heavier elements.

And that works like
a hydrogen bomb.

It's the same thing.

If you compress hydrogen enough,
it gets very hot,

and the pressure
gets very high,

and if fuses into helium,
and generates energy -- heat.

And that's what's happening
in the core of every star.

Narrator:
Because of their enormous mass,

stars have huge
amounts of gravity.

This gravity pushes inwards,
trying to collapse the star,

but fusion energy from the core
stops that from happening.

It's really this sort of
very balanced dance

between gravity pushing in,
fusion energy pushing out.

You can think of a star
as losing energy,

continuously,
to the outside world/

and gravity is saying,
"yes, I'm gonna take over."

But, no, the nuclear reactions
inside a star

replenish the energy
that's lost,

and keep the star hot
and pressurized inside,

so that the pressure-gravity
balance can be maintained.

Narrator:
This balance keeps sun-like stars alive

for up to 10 billion years,

until the star's gas tank
runs dry.

Plait: It's gonna
run out of fuel.

And when that happens,
it's going to die.

But what is that
gonna look like?

How is this
gonna happen?

Narrator:
One hundred million years ago,

things in the j1228 system
started to get ugly.

First, the star grew large --
really large.

Straughn: Once the center starts
fusing heavier elements,

the outside will swell

into what will eventually be
a red giant star.

Narrator: J1228 transformed
into a red giant.

Its outer layers blew off,

extending out
over 40 million miles.

When stars like our sun die,
it's not a quiet affair.

It's very violent,
and ugly, and messy.

They turn into
red giants,

and they turn themselves
inside out,

and vomit
all over the solar system.

Narrator: When j1228 swelled
into a red giant,

nearby planets
were stuck in a kill zone.

The dying star engulfed them,
or fried them

with temperatures of over
1,200 degrees fahrenheit.

Atmospheres disappeared.

Oceans boiled away,

but one planet survived
j1228's death throes.

Here's a case where a planet
survived, in some sense,

the death of its own star,
and it's still hanging around,

still hanging on,
hoping for something new.

Narrator: The red giant's
expanding outer layers

separated from
the star's core.

With no active fusion, the core
collapsed into a white dwarf.

The white dwarf's
dense gravity

then went to work
on the one surviving planet.

Flippenko:
The planet that might've been orbiting the normal star

can gradually spiral in
toward the white dwarf,

and then, eventually, the
gravity of the white dwarf pulls

on the near side of the planet
more than on the far side,

and that tears it apart.

Plait: What we're seeing here
is a dead star

dining on
its own solar system.

That's what is in the future
for the sun.

Narrator: J1228 feasted on the
remains of its rocky worlds,

leaving behind a disk of debris
and the planetary core.

It's a glimpse
of earth's future.

What happened here
around this white dwarf

is gonna happen
to earth.

It's gonna be stripped
of its atmosphere,

its crust,
and its mantle,

and the only thing that
will remain will be the core.

Narrator: Fried and ripped apart
by a dying star --

not a good way to go.

Fortunately,
for life on earth,

our own sun
isn't dying just yet.

Plait:
The sun is middle-aged.

It's 4.5 billion years old,

and it's going to go on for
another 5 or 6 billion years.

Sutter: We've got
a little bit of time

before our sun pukes
all over the solar system.

Narrator: Our home planet
may be safe for now,

but systems like j1228
show us

that sun-like stars
are destined to die,

killing off any life
orbiting them.

But sun-like stars
aren't the only stars

dying across the cosmos.

There are others out there,
and they're all doomed.

There's a wonderful
rainbow of stars

out there,
of all different shapes,

all different sizes,
and all different colors.

Straughn: We're talking down to,
you know,

fractions of the mass
of the sun,

up to hundreds of times
the mass of the sun.

Narrator:
When it comes to the star apocalypse, size matters.

The bigger and brighter
the star, the faster it dies.

?

Narrator:
Our universe is a vast expanse of death and destruction.

All of the stars are destined
to die, but not all at once.

There's not going to be
one particular point

where all the lights turn off
at the same time.

It's more like
a power outage,

where different grids go off
at different times, until,

like, there's the one last
light bulb that'll just go off.

Narrator: This is because stars
come in different sizes.

The way a star dies
has everything to do with

the amount of mass
it started life with.

It carries that all the way
through its lifetime.

Narrator: The sun is
a medium-sized star

living a stable existence
for billions of years.

Giant stars
are different.

They live fast,
and die young.

A star like the sun,
which is a medium-sized star,

it lives about
10 billion years.

The really massive stars,

they live maybe
10 million years.

Narrator:
Massive stars can be tens or even hundreds of times

more massive than the sun.

When it comes to life span,
that's a problem.

Flippenko:
A massive star has more fuel to burn, in a nuclear sense.

So, you might naively think
that it lasts longer,

but it's the
exact opposite.

Narrator:
Massive stars can only access hydrogen fuel in their core.

The rest is trapped
in the outer layers,

and can't be used as fuel.

Plait: If there's hydrogen
in the core, you're good.

If there's hydrogen outside
of the core, it can't be used.

If it's not in your fuel tank,
it's not doing you any good.

Narrator:
Massive stars also have more gravity than smaller stars,

so they have to burn
their hydrogen fuel faster

to prevent the star
from collapsing.

They burn their candle
on both ends.

Because of
their incredible mass,

their fusion reactions
in the core

happen at an
incredible rate.

Stricker: Giant stars are
kind of fast and furious.

They are bright.

They live their life,
and they die very quickly.

Narrator: When a giant star's
fuel runs out,

the core collapses
catastrophically

under the overwhelming
force of gravity.

And then, boom,
supernova.

Narrator:
The death of a giant star

triggers one of the biggest
bangs in the universe.

The blast would instantly
vaporize nearby planets.

But these star deaths
are also critical for life.

When massive stars die,
they release heavy elements

they've been making through
the course of their lives.

And sometimes,
they even make new ones.

And it's these heavier elements
that are essential for life.

We owe our existence
to stars

that formed
billions of years ago.

Narrator:
In may of 2018, we spotted evidence of ancient stars

creating the stuff of life.

We picked up
an infrared light signal

from a distant galaxy
named macs1149-jd1.

The signal was
ionized oxygen.

It's been traveling
for 13.3 billion years,

so the oxygen formed when
the universe was very young --

just 500 million years
after the big bang.

This oxygen formed
in the hearts of massive stars.

Hopkins: The presence
of oxygen tells us

that there needed to be massive
stars in the early universe

in order to synthesize
hydrogen and helium

into heavier elements,
like oxygen,

and then explode
to eject that oxygen

back into the interstellar
and intergalactic medium.

Narrator:
Extreme pressure in the cores of the stars produces oxygen...

...and other elements,
like carbon and nitrogen.

Supernova blasts spread these
elements across the universe,

helping to create
new generations of stars,

and, most importantly, us.

If there is one single fact
that you should care about

in all of science --
and this is my favorite fact --

is that you and I
are a consequence of star death.

Bullock: Before you can have
life, you need to have

the kind of elements
out of which life forms.

You need carbon.
You need nitrogen.

You need oxygen.

You need the elements
that are the backbone

to the biology
that makes us possible.

Where did those elements
come from?

Well, they came from stars.

They came from stars that formed
in the early universe,

before even
the sun existed.

Narrator:
The huge size of massive stars

quickly signs
their death warrants.

Their explosive ends help create
new stars, and even life.

The fact that you exist at all
is because of stars.

Narrator: But, probing galaxies
across the universe,

we've discovered
something else.

The star apocalypse
isn't just killing stars.

It's stopping them
from ever being born.

Thaller:
Star formation is dying.

And in fact,
it's dying rather quickly.

The universe,
right before our eyes,

is becoming
a darker place.

It's running out of fuel.

And eventually, no more stars
will be made at all.

?

?

Narrator: Life on earth follows
a series of regular patterns.

Day after day,
the sun rises...

And sets,

and stars light up the darkness
of the night sky.

The reason I got into astronomy
to begin with

was because I grew up
in a rural part of the country,

and the sky
was beautiful and dark.

You go outside at night,
and you look up,

and you could see
thousands of stars.

But it won't be
that way forever.

Narrator: 2016, a network
of telescopes across the world

measured the energy outputs
of over 200,000 galaxies.

?

They discovered that
in the past 2 billion years,

the universe has lost
half its brightness.

The night sky is getting darker

as stars flicker
out of existence.

About 10 billion years ago,

the universe
kind of hit its peak,

and lots of stars were shining.

It was an incredibly
bright place,

but in the last
couple billion years,

it's really, overall,
become a less bright place.

The darkening universe
isn't just a sign

that stars are dying.

It seems there's a problem
with star birth as well.

Mingarelli: When we look
into the universe's past,

what we find is that long ago,

stars were forming
at a much higher rate.

Right now what we see is
that really,

stars are dying off faster
than they're being born.

A milky way-type galaxy, today,

produces about
seven stars per year.

However, 11 billion years ago,

a galaxy like our own would've
produced 10 times more stars.

Narrator: In the early universe,
old stars died,

and new ones formed
in their place

from the material left over.

It was a cycle that kept
the cosmos bright.

Not anymore.

Plait: It kind of sucks for us.
We like a bright universe.

We like all this energy and life

that's vibrating
through the universe,

but that's just not always
going to be the case.

The universe is already
winding down.

?

Straughn: One of the biggest
mysteries in galaxy evolution

is figuring out how galaxies
stop forming their stars.

And we really don't know
the answer yet,

and it's really important
for us to figure out why

because in the end,
stars really equal life.

?

Narrator: To find out what is
shutting off the stars,

we study galaxy clusters.

?

These giant regions of space

contain hundreds of galaxies
bound together by gravity.

Slowly, the clusters
pull new galaxies into them,

causing something
strange to happen.

What we see happening when
a galaxy falls into a cluster

is that its star formation
is quenched.

It's shut off.

Narrator: The cause of this
quenching effect

has baffled scientists
for decades.

Then in October of 2018,

an international team
of astronomers

investigated this
long-standing mystery.

?

They tracked variations
in quenching

across 14 galaxy clusters

and found
a possible explanation.

Thaller: The ability a galaxy
has to make new stars

is related to the larger
environment it finds itself in.

In clusters of galaxies
where many galaxies

are orbiting around each other,

we see interactions that strip
gas and dust away from galaxies.

The stuff that makes up stars

literally just thrown off
into space.

Narrator: Stars formed from
dense parcels of cold gas,

something galaxies
are filled with.

But when a galaxy is dragged
into a cluster,

everything changes.

Clusters of galaxies contain
a lot of hot gas,

whereas you need cold gas
inside of a galaxy

in order to form stars,

and when a galaxy is moving
through this hot gas,

then the cold gas inside
is stripped away.

Narrator:
If this new study is right,

and galaxy clusters are
stripping away star-forming gas,

new starlight will become rare.

Looking over the history
of the universe

and how much gas was out there
and how much is still left,

I think it's fair to say
that most of the stars

that will ever be made
already have been made.

They've already been born.

Narrator: Thanks to the shortage
of star-forming gas,

stars won't just be dying
in the universe.

They'll go extinct,

and the first to go
will be the largest.

Plait:
As the universe runs out of gas

and fewer of these stars
are being made,

eventually sometime
in the future,

all the high-mass and even
medium-mass stars like the sun,

they'll be gone.

What does that mean for life?

?

Narrator:
Some of the brightest stars

will disappear forever,

begging the question,
can life survive

the monsters
that dead stars leave behind?

The long-term fate of the
universe is not a pretty sight.

Some very interesting creatures
can start to appear.

?

?

Narrator:
In the star apocalypse,

the first stars to fade away
will be the brightest --

the giant stars,
followed by the mid-sized suns.

The universe will become
unrecognizable.

The far future will be
a very dim universe,

especially for creatures
like us.

If there's no more gas,
no more new stars, it gets dark.

Narrator:
Scared of the dark?

You will be...

Because 100 billion years
from now,

in the shadows
of this new universe,

monsters will come out to play.

Now we find ourselves in the era
of stars and starlight.

What comes after
you can think of

as the era of the dead corpses
of old stars.

Narrator: We already see
the corpses of dead stars

scattered
throughout the cosmos --

black holes,
pulsars, white dwarfs.

What happens when more stars die
out and the dead take over?

Can life survive?

Thaller:
It's actually possible that life in the universe will survive,

but we're going to have
to get more creative.

Narrator: January 2019.

The gaia satellite studied
15,000 white dwarfs

within 300 light-years of earth.

These are the corpses
of sunlight stars.

Plait: White dwarfs are
the remnants, the cores,

of stars like the sun
after they die.

There's no more fusion going on
inside of a white dwarf.

So it's just kind of
sitting there cooling off,

but it turns out
there's a slight reprieve.

Narrator: The white dwarf
corpses usually cool off and dim

over tens of billions of years.

Gaia's data showed something
different,

something
we've never seen before.

Some of the older dead stars
aren't dimming at all.

O'dowd: We used to think
that white dwarfs

could really only dim over time.

After all, there's no source
of fusion,

no source of energy
in their interiors,

but new studies with
the gaia satellite have shown

that there must be
some other energy source

keeping those older
white dwarfs shining bright.

?

Something is giving these
white dwarf corpses a spark,

bringing them back from the dead
as zombies.

The leading contender

is that the insides of
white dwarfs

actually crystallize.

Narrator: Up to 6 billion years
after dying,

the hot carbon and oxygen matter
inside the white dwarf

cools and crystallizes,
becoming solid,

giving the dead star a lifeline.

This actually releases energy.

As the star cools,
it winds up releasing

a little bit more energy
than it otherwise would.

Narrator:
This unusual heat source could warm up a nearby frozen planet,

giving life a second chance.

Thaller:
There will be some extra energy available from these objects.

So this is the time that we have

to cuddle up
close to the zombies.

Narrator: Crystallization
can rejuvenate old white dwarfs,

and the process
could even provide

a spectacular setting
for an orbiting planet.

We have a special name

for cooled-down crystallized
carbon and oxygen.

We call them diamonds.

The long-term fate
of our universe

will be sprinkled with
all these glittering diamonds.

A zombie that comes to life
and shines like a diamond

might be pretty to look at,

but it's still no guarantee
that life could survive here.

You can kind of think
of these white dwarfs

as maybe making a little
more energy for the universe,

but even that's going
to eventually run out.

The whole thing becomes
a gigantic crystal and, again,

it's just going to
start cooling and fading away.

Narrator: The zombie fizzles out
into a dark cinder,

giving off almost
no light at all,

but there's another monster
lurking in the cosmos.

Thaller: When a star that's much
more massive than the sun dies,

it explodes violently,

and during that explosion,
the core collapses

and becomes an incredibly dense,
small object,

one of the most wonderful
real monsters in the universe.

This is a pulsar...

Psr b0329+54,

3,000 light-years away from us.

The pulsar has the mass
of the sun,

but is just 12 miles across.

Its rapid spin generates beams
of radiation from its poles,

bringing the zombie to life.

Now, we've discovered an alien
world orbiting this zombie star.

Mingarelli:
In 2017, a new planet

was discovered around a pulsar.

They're about twice
the mass of the earth,

and that's really incredible.

Narrator: The pulsar planet
sounds intriguing,

but the prospects
for life aren't good.

Orbiting a pulsar would be
a brutal environment for life.

Mingarelli: It's highly unlikely
that there's life

because the radiation from this
system would be overwhelming

and likely blow away
the atmosphere.

Narrator: As for sustaining life
in the universe,

none of these options
is what you'd call a safe bet.

?

These are momentary reprieves
from the inevitable.

No matter what you do,
eventually,

you're going to run out
of these gimmes.

You're going to run out of the
get-of-jail-free cards.

Inevitably, everything is going
to cool and fade away.

?

Narrator: This might be
game over for stars

and even for life.

But there is still a glimmer
of hope hidden in the cosmos,

a star that isn't dying.

It appears blessed
with eternal life,

and its color is red.

Red dwarfs -- we are literally
surrounded by them,

but they are largely
invisible to us.

?

Narrator: Illuminating
every corner of our night sky

is the light of stars...

But what we see with a naked eye
doesn't tell the whole story.

?

Thaller:
The stars that you're seeing are mainly stars like the sun

or even more massive
and even hotter than the sun.

They're bright. You can see them
from a distance,

but amazingly, the most
common form of star,

by far, are
the red dwarf stars.

They're up there right now
in the sky,

but they're just too small
and too faint to see.

Narrator: Red dwarfs are up to
10 times smaller than the sun,

and they burn less brightly.

Right now,
hidden in the night sky,

over three-quarters of the stars
in our galaxy are red dwarfs...

And while the larger stars
are dying out,

we've never seen
a red dwarf die,

making them
the best bet for life

to survive the star apocalypse.

When the most massive stars
eventually go out

and are not replaced,
what will be left

are much, much dimmer stars
like red dwarf stars.

Narrator: We've seen star death
across the universe,

so why not red dwarfs?

Turns out their size

gives them a crucial advantage
over larger stars.

Thaller: The more massive a star
is, the hotter it burns.

A red dwarf star burns
at a lower temperature.

So it doesn't burn through
it's fuel

quite as quickly
as a mid-mass star does.

These are like the economy
cars of the universe.

They're just sipping
on their nuclear fuel,

and they can coast along.

Narrator: Not only that,
despite being smaller,

they have access to more fuel.

Our mid-size sun
is split into three layers --

a core, a radiation zone,

and a convective layer.

The radiation zone prevents
hydrogen in the top layer

from ever becoming available
for the core to burn.

So the sun can only access

about 10 percent
of its total hydrogen fuel.

Once the hydrogen
in our sun's core runs out,

its days are numbered.

In some ways,
these mid-sized stars

end up starving themselves.

Narrator: The smaller red dwarfs
are different.

They can access all
the hydrogen they want.

Plait: In low-mass stars,
outside of the core,

this outer layer
is fully convective.

What that means is, stuff near
the core rises to the surface

and then drops back down
all the way to the core,

and that means if you have
hydrogen somewhere

outside of the core,
eventually, it's going to make

its way down there,
and it can be used for fuel.

Hopkins: The red dwarf has
access to everything

at the all-you-can-eat buffet.

It can grab stuff from
the distant regions

at the surface of the star
and bring it all the way

down the gullet
to the heart of the star.

Narrator: This all-you-can-eat
hydrogen buffet

extends the life span of red
dwarfs to incredible lengths.

The universe is over 13 billion
years old,

but any red dwarf
that age is a toddler.

A red dwarf, even if it was born

at the very beginning
of the universe

when red dwarfs
could first form,

even today, it's just
a tiny fraction of its lifespan.

They can last for trillions
of years,

thousands of times
the current age of the universe.

Sutter:
Thirteen billion years old --

that seems like a long time,

but a small red dwarf,

it's barely out of diapers.

?

Narrator:
Red dwarf stars will not die out

for 10 trillion years or more...

And we're discovering
they have another Trump card

that's good news for life.

?

February 2017.

NASA announced
the discovery of a system

in the aquarius constellation
called trappist-1

where seven earth-sized
planets orbit a red dwarf star.

Plait: It turns out that red
dwarfs, apparently,

are really good
at making planets,

including planets that are
roughly the size of the earth.

That's really cool because
these stars last a long time.

If they have planets
orbiting them with life,

they could outlast our solar
system by trillions of years.

Narrator: Sounds promising, but
red dwarfs have an ugly side.

In October 2018,

astronomers turned
the hubble space telescope

to a series of young
red dwarf stars

in the tucana-horologium
association.

They witnessed these infants
throwing daily stellar tantrums.

Thaller: Even though they're
the smallest stars,

they actually have
some of the strongest flares

and storms on them.

Narrator:
Red dwarfs can emit flares

10,000 times more powerful
than the sun.

These flares would cook
any nearby planets.

Oluseyi:
When a red dwarf star forms,

they're rotating very rapidly,

and this creates
a lot of magnetic activity

which creates flares
and mass ejections.

Narrator:
For life to exist,

it would have to wait for infant
red dwarfs to grow up.

Oluseyi:
As a red dwarf gets older,

there's drag between
the magnetic fields

in space as it rotates,
and that has the effect

of slowing down
its rate of rotation.

And so this means
the activity settles down.

So maybe later, in this life
of a red dwarf star,

they can support
planets with life.

Narrator: Red dwarf stars will
dominate the future universe

and may give life
a chance to survive.

These small red stars
are extremely long-lived,

but no star is immortal.

Even though they're really going
through their nuclear fuel

very slowly, there's just not
enough fuel to last forever.

Narrator: These little stars
will die out eventually.

Unlike their larger stellar
siblings, they'll go quietly.

Well, it actually just gets
hotter,

and the color of a star
depends on its temperature.

So as the red dwarf gets hotter,
it turns bluer.

So sometime
in the very distant future,

some of these red dwarfs

are actually going
to become blue dwarfs.

Narrator:
The universe isn't old enough for blue dwarfs to exist yet.

But trillions of years from now,

a dim blue glow will
complete the star apocalypse.

There will be a last star,
one last red dwarf,

maybe now turning blue
as it warms up,

but it too will eventually
cool off, fade away.

And there will be no more stars
in the universe.

It is inevitable.

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Narrator:
In this dark, starless universe,

prospects for life
seem impossible.

But will something else
take the place of stars?

Oluseyi: As we get to the end of
the universe,

things get really cold,
but they also get really weird.

Narrator: Trillions of years
from now, the star apocalypse

will leave the universe
empty and dark,

a never-ending night.

The universe at this time

is nothing like
the universe of today.

There's no light, and it's
really cold and very lonely.

When all of the stars die
and the light goes away,

anything that relies on the heat

and the processes from
these stars will start to die.

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Sutter:
Once all the lights go out,

the only things that will remain
will be the leftovers.

Narrator: With stars as we know
them long gone,

could something else
spark into existence

in this cosmic wasteland?

You'd think that's it,
no more star formation.

But the universe still
has a few tricks up its sleeve.

Narrator: Over the history
of the universe,

generations of stars
have lived and died.

They released heavy metal
elements into the universe,

building materials
for a new kind of star,

and stars born from these
new materials

can do things
their ancestors could not.

As you enrich the universe,
as more and more metals

get produced over time,

you can lower
the temperature needed

for fusion reactions in a star.

Narrator: With lower
temperatures needed for fusion,

stars have
become smaller and smaller.

O'dowd: Currently, the smallest
possible star

is a little under 10 percent
the sun's mass.

But eventually it may
be possible to form stars

that have around 4 percent
the sun's mass.

Narrator: Hundreds of trillions
of years in the future,

a new star may dominate
the universe,

built from scraps left over
from generations of dead stars,

a star so small

that it burns cold
instead of hot.

One of the weirdest types
of stars

that scientists hypothesize
might exist in the far future

is the frozen star.

Sutter: You can start forming
stars that are very, very small

and very cold,
where nuclear fusion

is happening in the core,
but the surfaces are cold.

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Narrator:
These small, cold objects

will be thousands of times
dimmer

than the faintest star
we see today.

So cold, the temperatures
on the surface

could reach just 32 degrees
fahrenheit...

And ice clouds may form
in the star's atmosphere.

They are so much cooler
than stars now.

They could actually have ice,
water ice, on their surface,

even though
they are technically stars.

Sutter: It's literal water-ice
covering the surface of a star,

the same ice that you can use
for ice-skating

or ice racing or curling.

You could do all of this

on the surface of a star
in the far future.

Narrator: It's hard to predict
if life could arise

on planets
orbiting frozen stars.

We won't know
until one appears...

And that
won't be for a very long time.

The universe is far too young

for even the first one
of these things

to even be a glimmer of an idea.

So if you want to wait,
you know, a quadrillion years,

then we can find out.

Narrator:
Stars helped create us,

building and spreading
the ingredients

for life to develop,

but the coming star apocalypse

may mean the end of life,

just not for a while.

Small red stars will continue
to illuminate the darkness...

Safe havens for life to survive
and even flourish.

As for us on earth,

we should be
most thankful for one star

because without it,
we simply wouldn't exist.

Thaller:
I really want you to never experience a sunny day again

and not think about this.

The sun, someday, will burn out,

and so will all of
the other stars.

We are in this wonderful era
of light and warmth

coming out of the sky,

and everything is going to go
dark, absolutely everything,

everywhere in the universe.

So for the time being, you know,
enjoy the light.

Step outside, enjoy the sun,

and think about how lucky we are
to live in this time.

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