All language subtitles for How.The.Universe.Works.S08E04.Death.Of.The.Last.Stars.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[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 and you will
no longer be able to see stars.

Things really will
get darker and darker

until there will be almost
no memory of light left.

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

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

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

The effect could be tremendous.

It can permeate throughout
the universe.

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.

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.

Hanging right above your head
every night,

we see up there
these dead corpses of stars.

400 light-years from Earth
lies the SDSS J1228 system.

A disc of debris

orbits the faintly glowing leftovers
of a dead star.

J1228 is a dead star.

It is the 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".

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.

It's always a little poignant
when you see evidence of a planet

around a dead star.

You think back to when
that star was shining

and could there have been life
in that solar system?

The J1228 system
is a cosmic graveyard.

It might look different
to our own 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,
all you need to do is look up.

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.

It 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?

Can life still survive
around the white dwarfs?

July 2018: heliophysicists tried
something a little different.

They didn't look at a star,
they listened.

In a sense, stars are noisy.

There is gas
moving around inside of them.

There's turbulence. There are huge
parcels of gas moving up and down.

It's not really an orchestra
playing coherently

this beautiful music.

It's more of a cacophony.
It's more like

just a bunch people
shouting for attention.

But all of this comes together
to make the surface vibrate.

And this is telling us what's
going on deep inside the star,

which we cannot
see directly otherwise.

Just as the seismic waves
from earthquakes

tell us about our planet's interior,

this cosmic performance grants us
special access into our Sun.

It provides a window into the
physics of the interior of stars.

What are they doing inside?

To understand the fate
of Sun-like stars,

we must look inside them.

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

The core, the very centre,
that's where the action is.

That's where the star is fusing

light elements
into heavier elements.

It 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 it fuses into helium

and generates energy, heat.

That's what's happening
in the core of every star.

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 happening.

It's really this, sort of,
very balanced dance

between gravity pushing in
and 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 going to take over." But, no.

The nuclear reactions
inside a star replenish

the energy that's lost

and keep the star hot
and pressurised inside,

so that the pressure-gravity balance
can be maintained.

This balance keeps Sun-like stars
alive for up to ten billion years

until the star's gas tank runs dry.

It's going to run out of fuel
and when that happens,

it's going to die.

But what is that going to look like?
How is this going to happen?

100 million years ago,
things in the J1228 system

started to get ugly.

First, the star grew large,
really large.

Once the centre
starts fusing heavier elements

the outside will swell into what
will eventually be a red giant star.

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.

When J1228 swelled into a red giant

nearby planets were
stuck in the kill zone.

The dying star engulfed them

or fried them with temperatures
of over 1200 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.

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.

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.

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.

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.

Fried and ripped apart by a dying
star is not a good way to go.

Fortunately for life on Earth,
our own Sun isn't dying just yet.

The Sun is middle-aged.
It's 4.5 billion years old,

and it's going to go on for
another five or six billion years.

We've got a little bit of time
before our Sun

pukes all over the solar system.

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 colours.

We're talking down to, you know,
fractions of the mass of the Sun,

up to hundreds of times
the mass of the Sun.

When it comes to the star apocalypse,
size matters.

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

Our universe is a vast expanse
of death and destruction.

All 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.

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.

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.

(CLOCK TICKING)

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.

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.

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.

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.

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.

Massive stars also have more gravity
than smaller stars.

So, they have to burn
their hydrogen fuel faster

to prevent the star collapsing.

They burn their candle on both ends.

Because of their incredible mass,
their fusion reactions in the core

happen at an incredible rate.

Giant stars
are kind of fast and furious.

They are bright. They live their
life, and they die very quickly.

When a giant star's fuel runs out,

the core collapses catastrophically

under the overwhelming
force of gravity.

And then - boom, supernova.

The death of a giant star

triggers one of the biggest bangs
in the universe.

The blast would instantly
vaporise 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.

In May, 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 ionised oxygen.

It's been travelling
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.

The presence of oxygen tells us
that there needed to be

massive stars in the early universe

in order to synthesise
hydrogen and helium

into heavier elements like oxygen,

and then explode
to eject that oxygen

back into the interstellar
and intergalactic medium.

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 favourite fact,

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

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.

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.

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.

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.

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,
you look up and you could see
thousands of stars.

But it won't be that way forever.

2016: a network of telescopes
across the world

measured the energy output
of over 200,000 galaxies.

They discovered that
in the last two 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.

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.

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 any more.

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.

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.

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.

The cause of this quenching effect
has baffled scientists for decades.

Then, in October, 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.

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.

Stars form 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.

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.

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.

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?

Some of the brightest stars
will disappear forever.

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.

(LOW PULSING)

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 unrecognisable.

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.

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.

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?

It's actually possible that
life in the universe will survive.

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

January, 2019.

The Gaia satellite studied
15,000 white dwarfs

within 300 light-years of Earth.

These are the corpses
of Sun-like stars.

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.

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.

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 crystallise.

Up to six billion years after dying,

the hot carbon and oxygen matter
inside the white dwarf

cools and crystallises,
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.

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

giving life a second chance.

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.

Crystallisation 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 crystallised 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.

The zombie fizzles out
into a dark cinder,

giving off almost no light at all.

But there's another monster
lurking in the cosmos.

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.

In 2017, a new planet
was discovered around a pulsar.

They're about twice
the mass of the Earth,

and that's really incredible.

This pulsar planet sounds intriguing,

but the prospects for life
aren't good.

Orbiting a pulsar would be
a brutal environment for life.

It's highly unlikely
that there's life

because the radiation from this
system would be overwhelming

and likely blow away the atmosphere.

Perhaps it is time
to think outside the box

by looking at one of the most
mysterious objects in the universe.

A brown dwarf.

A brown dwarf is basically
a failed star.

It is a star that wasn't
quite big enough

or massive enough to cause
hydrogen fusion in its centre.

Brown dwarfs are much smaller
than the Sun.

But they do generate some heat...

..and they have enough gravity
to hold planets in orbit.

But compared to a fully-fledged star,
they're small fry.

You could imagine a scenario

where a brown dwarf forms
with a family of planets,

but these planets
won't get a lot of heat and light

from that brown dwarf.

Planets orbiting a brown dwarf
would freeze very quickly.

But these almost-stars
do have a trick up their sleeve.

Brown dwarfs are incredibly common

and they often form binary pairs
with each other.

What if you had two brown dwarfs
that were right at that limit?

Just not quite enough material
to actually start fusion.

Over time, if they were
in a binary star system,

they could spiral together,
maybe even

collide and combine
into a single star.

Then these two failed stars
would join together

and finally begin to shine.

Two failed stars collide
creating a new Franken-star.

You could potentially form
an actual, legitimate star.

A star can start shining
and warm up those planets.

Maybe there is a chance
for life, after all.

As for sustaining life
in the universe,

none of these options
is what you 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 out of jail free" cards.

Inevitably, everything is going
to cool and fade away.

This might be game over
for stars and even life.

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

A star that's not dying.

It appears blessed with eternal life.

And its colour is red.

Red dwarfs - we are literally
surrounded by them,

but they are largely
invisible to us.

Illuminating every corner of our
night sky is the light of stars.

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

The stars that you are 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.

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.

We've seen star death
across the universe,

so why not red dwarfs?

It turns out their size

gives them a crucial advantage
over larger stars.

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
its fuel 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.

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

ever becoming available
to the core to burn.

So the Sun can only access

about 10 per cent
of its total hydrogen fuel.

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

its days are numbered.

In some ways, these mid-size stars
end up starving themselves.

The smaller red dwarfs are different.

They can access
all the hydrogen they want.

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.

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.

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 can bring it
all the way down the gullet

to the heart of the star.

This all-you-can-eat hydrogen buffet

extends the lifespan 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.

13 billion years old,
that seems like a long time

but a small red dwarf,
it's barely out of diapers.

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 plan
orbit a red dwarf star.

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.

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.

Even though
they're the smallest stars,

they actually have some of the
strongest flares and storms on them.

Red dwarfs can emit flares

10,000 times more powerful
than the Sun.

These flares would cook
any nearby planets.

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.

For life to exist,

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

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.

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 slowly,

there's just not enough fuel
to last forever.

These little stars
will die out eventually.

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

Well, it actually just gets hotter,

and the colour 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.

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.

In this dark, starless universe,

prospects for life seem impossible.

But will something else
take the place of stars?

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.

Once all the lights go out,

the only things that will remain
will be the leftovers.

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.

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 couldn't.

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.

With lower temperatures needed
for fusion,

stars have become
smaller and smaller.

Currently,
the smallest possible star

is a little under 10 per cent
the Sun's mass.

But eventually
it may be possible to form stars

that have around 4 per cent
the Sun's mass.

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 hypothesise

might exist in the far future
is the frozen star.

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.

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.

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.

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 long, 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.

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.

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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