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.
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.
Captions paid for by discovery communications
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 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."
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 know, you think back of 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 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.
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?
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.
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.
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.
This balance keeps sun-like stars alive
for up to 10 billion years,
until the star's gas tank runs dry.
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?
One hundred million years ago,
things in the j1228 system started to get ugly.
First, the star grew large... really large.
Once the center 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 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.
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...
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 5 or 6 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 colors.
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 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.
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.
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 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.
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 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.
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.
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.
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.
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, and 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 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.
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 anymore.
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 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.
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 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.
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,
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.
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.
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 sunlight 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.
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.
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.
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.
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.
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.
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.
The 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.
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.
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.
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.
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.
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?
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 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.
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.
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,
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.
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.
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.
Thirteen 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 planets 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
very 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 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.
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?
As we get to the end of the universe,
things get really cold, but they also get really weird.
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.
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 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.
With lower temperatures needed for fusion,
stars have become smaller and smaller.
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.
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.
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 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.
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.