All language subtitles for How the Universe Works S09E07 - The next supernova

White dwarfs.

Small stars that pack a big punch.

When white dwarfs were first
discovered,

the astronomers' reaction was,

"No. No, no, no, no, no,
that can't be real."

What's going on inside these things
can only be described as seriously
weird.

They're the cooling corpses of stars
like our sun.

But new research proves white dwarfs
are one of the driving forces of our
universe.

They eat planets. They flare out
in high-energy light.

They can really explode.

And they can tell us literally about
the nature of the universe itself.

And there's a dirty secret at
the heart of white-dwarf science.

We see dead stars exploding
and we still don't understand
why they're doing it.

Have scientists finally discovered
how these small stars

could be such massive galactic
players?

December 2018.

Astronomers spot strange flares
coming from a galaxy

250 million light-years from Earth.

GSN 069.

We know that GSN 069 has
a supermassive black hole
in its centre,

equal to about half a million times
the mass of the sun.

That's a big black hole.

And it blasts out X-rays

in a very very steady pace,

every nine hours.

Why?

The flares are so energetic and
regular,

the supermassive black hole
must be eating

the mass of the planet Mercury

three times a day.

The big question is, what's feeding
this black hole such a huge dinner?

In March 2020, scientists found
the answer.

An unlucky star
at the end of its life

had wandered into the death zone of
the black hole.

A star getting too close to
a supermassive black hole

is like a glazed doughnut getting
too close to me!

That thing just is not
gonna make it.

Stars that get too close
to a black hole get torn apart.

They sort of get attacked by
the black hole.

Some of that material is also
getting launched off in very
powerful winds

and jets and streams getting out.

Somehow, the star survives
its close encounter

with the supermassive black hole.

Further investigation reveals it's a
small, compact star. A white dwarf.

So what makes this tiny star
almost indestructible?

The answer lies in how it's formed.

We get a clue if we look at
the life-cycle of a star.

It's burning hydrogen into helium.

That's causing nuclear fusion

and that causes a star
to stay stable.

There is this delicate balance
between radiation pressure

from that nuclear fusion
pushing out

and gravitational pressure
pulling in.

But when stars like our sun
near the end of their life,

they run out of hydrogen fuel.

The sun-like star makes more
and more helium, which builds up
in its centre.

Gradually, the immense weight
of the star's outer layers

crushes the helium core.

As the core ages,
it gets smaller and hotter,

which increases the rate of
nuclear reactions.

These nuclear fusion reactions
produce more energy,

which pushes the outer layer
or "envelope" outwards.

Because there's more energy flowing
through the envelope,

the envelope swells up.

The star expands to around 100 times
its original size.

The yellow star becomes a red giant.

Eventually, red giants shed
their outer layers,

forming stunning gas shells
called "planetary nebulas".

Planetary nebulae are the most
beautiful objects in space.

They're all spectacular.

A star that ends its life in one of
these planetary nebulas,

leaves behind a white dwarf at the
centre.

This white dwarf is, essentially,
a cinder, a stellar cinder.

It's what's left after nuclear
fusion is no longer possible for
that particular star.

All that remains is a glowing
white dwarf.

The leftover core of the dead star.

But in Galaxy GSN 069,

the supermassive black hole
turbocharged the process.

It stripped off the outer layers of
the red giant in a matter of days.

The black hole has almost eaten
all of the juicy parts,

all the easy to get at
parts of the star.

Leaving behind the sort of bone or
the leftovers of the white dwarf.

This white dwarf is just a fifth of
the mass of the sun.

How can such a small star survive,
being so close to a black hole?

You might think, that because
a white dwarf is small,

it's not gonna last very long, cos
there's not that much stuff there
to eat.

It turns out it's quite
the opposite.

The pocket-sized white dwarf is
packed full of matter.

If it were a normal star, it would
have been shredded long ago,

but because it's such a dense,
tight ball of matter, it survives.

Imagine taking the sun
and crushing it down

to just about the size of the Earth.

The same mass, but now packed
way more tightly.

So a basketball-worth of this stuff

would weigh as much as
35 blue whales.

The white dwarf's extreme density
protects it

from the gravitational onslaught of
the supermassive black hole.

Its orbit takes it near that
black hole every nine hours.

Every time it encounters the black
hole, some of its material gets
sipped off.

They're playing a game of
interstellar tug-of-war
with one another.

The black hole is bigger,
so it's going to win,

but the white dwarf is very dense,
so it's very tough.

It's able to hang in there
for quite a long time.

It's gonna stay in orbit around
a supermassive black hole

for billions of years.

Talk about David and Goliath.

When astronomers first discovered
white dwarfs, they thought they
shouldn't exist.

How could something have such
an extreme density

and not collapse
under its own weight?

Quantum mechanics, the science of
atomic and sub-atomic particles,

has the answer.

We're used to the rules of physics
up here in the macroscopic world,

but when you zoom down into the
subatomic world, things get weird.

Here we have the electron.

One of the tiniest particles
in the universe.

And it's these little electrons

that are doing the work of
supporting an entire star.

Electrons really don't like being
squashed into a small space.

If you try to squash too many of
them, into too small a space,

they'll push back really hard and
this an effect called "degeneracy
pressure".

These degenerate electrons stop
white dwarfs from collapsing.

But they give these stars
strange qualities.

White dwarfs behave very differently
than normal matter.

Take planets and stars.

They become bigger
when they gain mass.

White dwarfs are the exact opposite.

As they gain mass, they get smaller.

The more massive a white dwarf,

the tighter the electrons squeeze
together

and the smaller and denser
the star gets.

The high-density means the white
dwarf's structure is also strange.

It has an extremely thin atmosphere
made of hydrogen

or, occasionally, helium gas.

If you were to take an Earth's
skyscraper

and put it on a white dwarf star,
if you climb to the top of

that skyscraper, you'd be outside of
the white dwarf's atmosphere,
you'd actually be in space.

Beneath the thin atmosphere
lies a surface of dense helium,
around 30 miles thick.

It surrounds an interior

made of super-heated
liquid carbon and oxygen.

A white dwarf at its surface can be
a half a million degrees.

It's even hotter in the interior.

And so, that kind of material,

it's not gonna behave
the way normal matter does.

Eventually, over billions of years,

the centre of the white dwarf
cools down into a solid.

As the carbon and oxygen atoms
cool down, they form a crystal.

Diamonds are actually
crystals of carbon.

So at the centre of these cool
white dwarfs could be a diamond
the size of the Earth.

White dwarfs gradually give off
their remaining energy,

until there's just a cold,
dead ball of matter.

A black dwarf.

We've never seen what we call
a black dwarf.

There's a simple reason for that.

It takes a tremendous
amount of time,

many tens of billions of years,

longer than the age of the universe,
to reach that point.

This is the dark destiny of most
mid-sized stars, including our sun.

This long, slow death may make
white dwarfs seem ordinary.

But these tiny stars could answer
some big questions about
our universe.

They might be small
and they might be dim,

but they are essential for
our understanding of physics.

New research into white dwarfs
may answer

one of the biggest questions of all.

Can life survive the death of
its star?

In the past, we've underestimated
white dwarfs.

But now they're causing a buzz
among astronomers.

One of the big questions over the
last decade is could a planet

survive around a white dwarf?

The logical answer would be no.

On their way to becoming white
dwarfs, stars evolve through
a red-giant phase.

They expand to become very huge.

So we figured any planets around
these stars might just get eaten.

In December 2019, evidence from
the constellation of Cancer

turned that idea on its head.

Astronomers spotted a strange-looking
white dwarf

about 1,500 light-years from Earth.

Subtle variations in light
from the star revealed a mystery.

The elements oxygen and sulphur

in amounts never before seen
on the surface of a white dwarf.

We know what the chemical signature
for a white dwarf is

and this stuck out
like a sore thumb.

Normally, hydrogen and helium make up
the outer layers of a white dwarf.

Oxygen and sulphur are heavier than
hydrogen and helium.

They should have sunk down,
but we still see them there.

So they must have gotten there
recently.

Using ESO's very large telescope
in Chile,

the astronomers took a closer look.

They discovered a small Earth-sized
white dwarf,

surrounded by a huge gas disc,

roughly ten times
the width of the sun.

The disc contained hydrogen, oxygen
and sulphur.

A system like this had never been
seen before.

So the next step was to look at
a profile of these elements

and figure out where we'd seen
something similar.

And the amazing thing is we have.

We've seen these elements in the
deeper layers of the ice giants

of our solar system,
Uranus and Neptune.

Hidden in the gas ring is a giant,
Neptune-like, icy planet.

It's twice as large as the star.

But the fierce 50,000 degree heat
from the white dwarf

is slowly evaporating
this orbiting planet.

The white dwarf is bombarding
the planet

with high-energy radiation,
X-rays, UV rays.

It's pulverising the ice molecules
in its atmosphere

and blowing them out into space.

The ice molecules are streaming
behind the planet, like the tail
of a comet.

The icy planet loses mass at a rate
of over 500,000 tons per second.

That's the equivalent of 300 aircraft
carriers every minute.

It sounds like that could be
curtains for the planet,

but, remember, the planet is large
and the star is cooling down.

As it cools, it will start blasting
the planet so intently and that
stream of gas will cease.

The planet will probably end up
losing only a few percent of
its total mass.

So the planet should survive and
continue orbiting the white dwarf.

But a mystery remains.

Why didn't the closely-orbiting
planet die,

when the star swelled to a red giant?

It had to have started farther out
and moved inwards.

Our best guess is that other
ice giants were probably lurking

somewhere in the outer regions
of the system

and knocked that planet inwards,

towards the white dwarf, some time
after the red-giant phase,

in some kind of cosmic pool game,
if you will.

This isn't the only white dwarf
with evidence of planets.

About 570 light-years from Earth,

there's a white dwarf star called
WD 1145+017.

After studying the star
for five years,

researchers report that
the white dwarf is ripping apart

and eating a mini rocky planet.

So, as the planet is being torn up,
we see this huge cloud of dust,

blocking out 50% of the light
of the star

and huge chunks of rock
passing in front of the star.

It's exciting to see

this planet being torn apart,

because it's not often
that we get to see an event,

we get to see something
in the process,

that we can observe
and we can learn from.

There's more and more evidence that
planetary systems can survive

the death of their star and
the formation of a white dwarf.

It just depends on the planet's
composition

and location.

The distance from the planet to
the star is a critical factor.

As you move farther and farther out
from a star, the intensity

of that solar radiation decreases.

So, the farther you go out,
the less heat you have,

the less high-energy particles are
reaching the surface of that planet.

Also, rocky planets can survive
better than gas giants.

Rocky planets can hold onto
their stuff better,

whereas gas can be blown away
much more easily.

These new discoveries raise questions
about habitability around stars.

Could white-dwarf systems
support life?

If we limit ourselves to only
looking for life on planets
orbiting stars, like our sun,

we would be doing ourselves
a huge disservice.

Far more important, is to look
for, around whatever star,

the habitable zone,
the Goldilocks Zone,

the region around a star
where a planet could support life.

When it comes to supporting life,

white dwarfs have some surprising
advantages.

Even though there's no fusion
happening,

they have all of this internal
energy stored up

that they release, that warms
the nearby planets.

Life might even prefer hanging out
around a white dwarf,

because it doesn't change much over
the course of billions of years.

With something like our sun, there
are flares and coronal-mass
ejections.

Eventually, it's gonna die
and we have to deal with that.

That's not a problem
with the white dwarf.

So, if life can gain a foothold,

it has a nice stable home.

We now think 25% to 50% of white
dwarfs have planetary systems.

Perhaps, one day, we'll find one
with an Earth-like planet

and maybe even life.

White dwarfs are the dead remains of
stars like the sun.

Most of these "zombie" stars

slowly cool down
over billions of years.

Most, but not all.

Some go out in a spectacular
explosion

known as a Type Ia supernova.

A Type Ia supernova is one of the
most violent, powerful, energetic
events in the universe.

We are talking about a star
exploding.

They can outshine entire galaxies.

They can create devastation over
hundreds and hundreds of
light-years.

They're a big deal.

We'd seen the aftermath of these
cosmic fireworks,

but, for over 60 years,

we had little direct evidence
they came from white dwarfs.

Then, students from University
College, London, UK, got lucky.

While taking routine photographs,
they spotted a supernova explosion

in our own cosmic neighbourhood.

M82, the Cigar Galaxy, is actually
really close to us on cosmic terms.

It's only about 12 million
light-years away.

This makes it one of the closest
galaxies in the sky.

- The blast called Supernova 2014
- J was
the closest Type Ia supernova

for over 20 years.

Its proximity allowed us
to look for the signature

of a white dwarf supernova.

A blast of gamma rays.

Gamma rays are a type of light
that's incredibly energetic.

They're the most energetic type of
rays or photons on the
electromagnetic spectrum.

White dwarfs should release
gamma rays when they explode.

But dust in interstellar space
soaks up the rays.

So unless an explosion is close by,
they're hard to detect.

For years, astronomers had been
looking for the gamma rays

that should be emitted by a Type Ia
supernova, but no-one had found
them.

Now, scientists had their chance

and the technology
to see the elusive rays.

Using ESA's INTEGRAL satellite,

they sifted through the shockwaves
sent out by the explosion in M82.

It was tough, but finally
they got a reading.

The tell-tale signal of gamma rays.

It's the best evidence yet

for white dwarfs exploding in
Type Ia supernovas.

- The reason Supernova 2014
- J was
so cool is that this observation

gave scientists the evidence
white dwarfs would explode

to create this specific type of
supernova.

So, which white dwarfs fade out?

And which ones go out with a bang?

A survey of stars revealed around
30% of white dwarfs live in
binary systems.

But white dwarfs are not good
neighbours.

A white dwarf in a binary system,
it's like a zombie.

It's the corpse of a star
that used to be alive,

but now it is eating the material
from a star that is still alive.

They very literally
suck the material

and suck the life out of that star,

by swallowing up
all of its outer layers.

The white dwarf zombie tendencies
can backfire.

Adding mass
to a white dwarf is like this.

We keep adding mass

from that companion star.

A little bit of hydrogen at a time,

building up that atmosphere
and, for a long time,

everything's fine.

Until you add too much mass

and you reach that critical
threshold and then...

The real-world consequences of
reaching the threshold are
devastating.

The extra weight of gas stolen
from the companion star

compresses carbon deep in the core
of the white dwarf.

When the white dwarf reaches
1.4 times the mass of our sun,

it hits a tipping point known as
"the Chandrasekhar Limit".

You add up the mass,
little by little by little,

till you get to that
Chandrasekhar Limit

and then blam, there's a supernova.

In a flash, carbon undergoes
nuclear fusion,

releasing a tremendous amount
of energy.

If the white dwarf explodes at
the Chandrasekhar Limit,

it's a little bit like fireworks

that all have the same amount of
gunpowder.

They'll all go off in the same way,
they'll be equally loud.

Well, the supernovas will be
equally bright.

This equal brightness
of all Type Ia supernovas

is vital to our understanding
of space.

Type Ias are known as
"standard candles"

and are useful tools for calculating
vast cosmic distances.

They were the key to the Nobel
Prize-winning discovery that

the expansion of our universe is
accelerating.

But what kind of companion star
triggers Type Ia supernovas?

For decades, the number one suspect
was red-giant stars.

A red giant is a good candidate,
because it's a very big, puffy star.

That material becomes easy pickings
for the white dwarf to siphon off,

until it gets big enough to explode.

To prove the theory, we needed
to find evidence in the debris

left behind after a supernova.

Stars are surprisingly hardy
objects.

They can survive an explosion
of a nearby star.

Some of these companion stars should
still be there.

A lot of 'em will be worse for wear,
but they'll still exist.

Scientists search through the remains
of 70 Type Ia supernovas.

Only one blast zone contained
the glowing remains of a red giant.

The fact that we've only found
maybe this one example
suggests that, actually,

they're not quite
the serial killers we thought.

It's probably likely that this is
the minority of these types of
supernova explosions.

Indeed, we now think that only
a small fraction

of these white dwarf supernovas
involve a red giant.

Despite the fact that, in the
standard textbooks for decades,

that was the preferred explanation.

If red giants don't cause
the majority of Type Ia supernovas,
what does?

New evidence suggests
colliding white dwarfs.

Star mergers that could exceed
the Chandrasekhar Limit,

producing explosions
with different brightness,

but if the explosions
vary in brightness,

can they still be used
as standard candles?

If we don't really know
what a Type Ia supernova is,

then when we use them to map out
the universe and the way
the universe is expanding,

we just can't be sure any longer
what it is we're looking at.

If we're wrong about that,
then we're wrong about
so many other things.

Our whole model of the universe
falls apart.

Is our understanding of the cosmos
completely wrong?

White dwarfs explode in spectacular
Type Ia supernovas.

They're a crucial tool for measuring
the universe,

but there is a problem.

The standard model says that
white dwarfs gradually steal mass
from a red-giant star.

Until they reach a tipping point
called the Chandrasekhar Limit.

But recent observations prove
this doesn't explain

how most Type Ia supernovas occur.

The majority of Type Ia explosions
remain a mystery.

We call the explosions from
white dwarfs "standard candles",

but they're really not that
standard, there's different types of
explosions.

It may be imperative to our
understanding of the entire universe
that we get this straight,

because the reason we think the
expansion rate of the universe is
accelerating,

it's based on the brightness of
Type I supernovas all being
the same.

Maybe that's not the case.

Researchers suspect that
a theoretical type of merger

could be responsible
for more Type Ia supernovas.

The result of two white dwarfs
crashing together.

But this messes with the math.

The Chandrasekhar Limit says
white dwarfs should explode

when they reach 1.4 times
the mass of our sun.

Two white dwarfs colliding
can exceed this mass.

More mass means a bigger bang

and a brighter explosion.

You're not adding gas little by
little, you're adding a whole other
white dwarf.

That will go off. It will look like
a Type I supernova,

but it won't be the standard candle,
it'll be brighter than we expect.

But no white dwarf mergers
have been found,

because detecting one after it
happens is virtually impossible.

If two white dwarfs merge together,
it's almost impossible to tell.

The DNA of the two systems is
all mixed together and it's all
identical.

You can't tell that there was
a separate companion
in the first place.

So, we can't just look
when there's a bright flash.

We have to go look for the ticking
time bombs in the galaxy.

Astronomers investigating
a strange-shaped cloud of gas

made a breakthrough.

Using ESO's very large telescope,
they focused in on

a planetary nebula
called Henize 2-428.

Planetary nebulas are normally
symmetric,

because red giants shed
their outer layers evenly,

as they become white dwarfs.

But this one is lopsided.

We think, in this case, there might
be the presence of a companion star,

that shapes and twists and sculpts
that planetary nebula.

Researchers peeled back
the gaseous layers

and discovered something shocking.

A two-star system,

made up of the most massive orbiting
white dwarf pair ever discovered.

Each star is 90% as massive
as our sun.

They're so close together, they take
just four hours to orbit each other

and they're getting closer.

If you've ever seen a car crash
about to happen,

you know that sense of
inevitability, as you witness that.

That's what we're seeing
in this system.

We see these two massive
white dwarfs

spiralling closer and closer
and closer and we know

that disaster is coming.

In around 700 million years, these
stars will merge and explode

in a Type Ia supernova.

Now, thanks to the discovery of
more systems like Henize 2-428,

we think white dwarf collisions
could be responsible

for the majority of
Type Ia supernovas.

Two white dwarfs can merge together

and if the sum of their masses is
greater than 1.4 solar masses,

then you can get
a Super-Chandra Type Ia.

We've now observed nine
Super-Chandra explosions.

And to complicate matters further,

we've spotted another form of
white dwarf supernovas.

Sub-Chandra Type Ias.

These mysterious white dwarfs
that we don't quite understand

die off much quicker than regular
white dwarf supernovas.

The explosions are less violent than
normal Type Ia supernovas

and fade away faster,

but we don't know why.

Maybe it has something to do with
the properties of the star
or the rotation,

but the Chandrasekhar Limit
may not be so exact.

It's kind of a Chandrasekhar Range.

The physics textbooks are now being
sort of rewritten or at least
modified.

We know that not all Type Ia
supernovas come from Chandra-mass
white dwarfs.

There's actually a variety of
Type Ia supernovas,

a variety of white dwarf masses and
configurations that can explode.

These new discoveries mean
researchers now study the chemistry

and duration of Type Ia supernovas,
not just their brightness.

But mysteries remain.

We still don't know what triggers
the actual explosion.

Something has to set off
a supernova.

It's like a sort of tinderbox, this
star is ready to be set alight,

but something has actually got to
set it alight in the first place.

And the question is,
what exactly is doing that?

Our best hope of understanding
white dwarf supernovas

is to run computer simulations,

experimenting with ways to trigger
fusion in the heart of the star.

It turns out you need a spark
to get that fusion going.

Otherwise, the usual failure in
computer models of a supernova is

oop, never saw anything.

We do not understand
what creates the explosion.

We don't know what the mechanism
for that is yet.

While computer simulations are
getting closer,

we still don't fully understand
Type Ia supernovas.

The deeper we investigate,
the more mysteries we uncover.

Like rogue white dwarfs streaking
across the galaxy.

And tiny stars that explode
over and over again.

Can these odd white dwarfs

shed more light on the mystery of
Type Ia supernovas?

White dwarfs are surprisingly
difficult to understand.

They behave in completely
unexpected ways.

But these oddballs may help answer

the remaining questions
about Type Ia supernovas.

These are white dwarfs,
but not as we know them.

2017.

Astronomers spot a rebellious star

raising hell in the Little Dipper
constellation.

It's like a zombie, but this isn't
one shambling down the road, it runs
like Usain Bolt.

This thing is screaming through
the galaxy at a much higher speed

than you'd expect
for a star like it.

The white dwarf called LP 40-365 is
moving incredibly fast

towards the edge of the Milky Way.

It's not the only star
behaving oddly.

In 2019, we spotted three more white
dwarfs racing across the galaxy.

Finding one white dwarf blasting its
way through space is weird enough.

To find three more, that's telling
you that something is going on

and whatever it is that's going on
happens a lot.

So what sent these renegades racing
across the galaxy?

LP 40-365 and these other weird
white dwarfs could be the result of
failed supernovas.

People have theorised that maybe
these things didn't finish
exploding.

If so, we should find some unburnt
fractions wandering around
the galaxy.

In the last 20 years, we have spotted
some unusually dim supernovas,

that could have sent LP 40-365
and friends flying.

So, what looks like happened is
that, in a binary pair,

there was stuff dumping onto
a white dwarf

and we were about to have
a Type I supernova.

But the Type I supernova didn't go
off symmetrically.

Some of it actually exploded
and some of it didn't.

That energy didn't go out
in all directions.

One of the things that occurred is
that these stars

got sent hurling across space
at these incredible speeds.

Stars could be sent flying so fast,
that they're no longer bound by

the gravitational pull
of their home galaxy.

LP 40-365 is known as
a hypervelocity star

and is moving so fast,

it's definitely headed out of
the Milky Way.

We call them Type Iax supernovas.

They could make up between 10% and
30% of Type Ia supernovas.

Many could throw out a runaway star.

But we still don't know why
the supernova fails.

The funny thing about science is
things that fail still teach you
what's going on.

Why are these ones different?
Were they not massive enough?

Too massive? Was the companion star
not feeding them the material
in the right way?

Something happened there to make
these stars not, basically, blow
themselves to bits.

That's telling us something about
the way Type Ias do explode.

It seems that life in a binary star
system can be rough for white dwarfs.

But for some lucky stars,
their lives can be more...

mellow.

Just because a white dwarf has
a normal star companion that it is
stealing material from,

it does not spell a death sentence
for that white dwarf.

February 2013.

Astronomers discover a star
in the Andromeda Galaxy

that flashes over and over
and over again.

With each flare, it shines a million
times brighter than our sun,

before dimming to its normal state.

- It's called M31
- N 2018-12a.

This is not a supernova.
It is its little sibling, a nova.

But what's weird about this one is
that it happens every year.

Astronomers have known for a long
time that there are these cases of

these nova that go off

somewhat regularly, every ten years,
every 100 years,

but finding one that goes off every
year is a remarkable discovery.

Much like supernovas, novas occur in
a close binary system,

where a white dwarf and another star
orbit each other.

The white dwarf pulls in hydrogen
from the companion star.

The gas falls onto its surface.

And so, as that hydrogen piles up,
eventually, it gets to the point

where it can fuse into helium
and goes bang!

In supernovas, fusion happens
deep inside the star's core.

But in novas, fusion only occurs
on the surface.

An explosion flares across
the white dwarf's exterior,

hurling unburned hydrogen
out into space.

The result?
An object called a remnant.

- The remnant from nova M31
- N is
400 light-years wide.

This particular remnant is much
bigger than even supernova remnants.

It's much larger, much denser
and brighter then most normal
remnants are.

But that makes sense, if the star
flares up so often.

Think about the star flaring away
for millions of years.

You build up a gigantic
nova remnant.

The repeating flares explain
the huge size of the remnant.

But why does the nova explode
so frequently?

Classically, we thought that when
a nova went off on the surface

of a white dwarf star,

that the white dwarf star's mass
didn't change very much.

Or maybe it got a little smaller.

Now we think that, after a nova,
the white dwarf gains a bit of mass.

- Recurrent novas, like M31
- N, steal
more mass from their companion star,

than they blow off in each explosion.

Some gain more and more mass,
exploding more frequently,

until they reach
the Chandrasekhar Limit

and go full-on supernova.

- M31
- N may very well be
the missing link that shows us

that some nova systems eventually
become supernova systems.

Working out how novas become
supernovas

and why some supernovas fail...

..might help us understand
what makes white dwarfs explode.

But just when we think
we get a break,

white dwarfs hit us
with another bombshell.

Death rays.

White dwarfs can explode
in violent supernovas.

But that's not their only
deadly trick.

They might also create
the most magnetic

and terrifying beast in the universe.

A magnetar.

Magnetars are scary, they just are.

I mean, it's even in the name.
The word "magnetar" sounds scary.

They are the reigning champion of
the largest magnetic field
in the universe.

The magnetic fields around magnetars
are so strong

that they can stretch
and distort individual atoms.

They can turn an atom into
a long, thin pencil-shape.

Once you start stretching atoms out
into this shape,

they can't bond together
in the usual ways any more.

So you can just throw out every
chemistry textbook in the world.

If an astronaut were unlucky enough
to get close to a magnetar,

say within 600, 700 miles,

the whole body of the astronaut
would be completely obliterated.

They would, more or less, dissolve.

The origin of these fearsome
creatures is a mystery,

but it must be something
very violent.

We think they send out a clue
as they form.

Powerful blasts of energy

shooting across the cosmos.

In the past few decades,
we've noticed these very odd,

very confusing and very brief

flashes of intense radio energy.

They're known as fast radio bursts
or FRBs.

Some FRBs don't repeat,
they're one and done.

You're talking about
an incredible amount of energy

released in less than a second,
then it's over.

Because these non-repeating FRBs are
so powerful, we think they could

come from a huge collision.

The heavier and denser
the objects colliding...

the bigger the bang.

New research suggests
a white dwarf star hitting

a dense, heavy neutron star

could be enough to birth a magnetar,

sending out FRBs in the process.

A neutron star is like
a white dwarf, even more so.

It is the leftover core
of a giant star.

They're effectively giant balls of
neutrons, squeezed together

into things
about the size of a city.

You have a neutron star.

An incredibly nasty, complicated,
exotic object.

And a white dwarf. An incredibly
nasty, ugly, complicated object.

Crashing headlong into each other.

As the two stars orbit more closely,

the neutron star strips gas
from the white dwarf.

This material spirals onto
the neutron star,

causing it to spin faster and faster.

The rapid rotation amplifies
its magnetic fields.

Until the two stars collide.

Creating a very magnetic monster.

A magnetar.

It's a turbulent situation.
You could think of it

as a newborn baby coming into
the world, kicking and screaming.

The turbulence produces a powerful
blast of electromagnetic radiation.

It races out of the collision site
at the speed of light,

until we detect it

as a fast radio burst.

We can hear the screams of agony
from millions of light-years away.

Those screams are
the fast radio bursts.

This could be the most difficult
childbirth in the cosmos.

Few suspected that white dwarfs
could create something as violent
as a magnetar.

But, in 2015,

astronomers found yet another strange
magnetic white dwarf,

in a binary system called AR Scorpii.

We don't really know why
white dwarfs become magnetic.

But what we do know is the ones that
tend to be the most magnetic are
often the heaviest.

One possibility is
there's a big star

that has come to the end of its life
and shrunk down into a white dwarf.

And if that star had its own
magnetic field, as it shrinks down,

it actually has a kind of
concentrating effect

and the magnetic field gets even
stronger as that happens,

so it's incredibly strong.

As well as being very magnetic,

AR Scorpii's white dwarf is spinning
very fast.

When you combine a fast spin
with a strong magnetic field,

what you get is some really crazy
physics.

The white dwarf starts to act
a bit like a lighthouse,

sending out intense radiation
into the cosmos.

As the white dwarf spins,
this beam of radiation

hits its red dwarf neighbour,
making it glow.

This is highly unusual.
No other system glows like this.

White dwarfs like this can help
reveal some of the mysteries of
magnetism.

When you have a magnetic field
that is this strong,

it's something like 100,000 times
stronger than any magnetic field

than we can create here on Earth,

it means that by studying what's
going on in this very remote system,

you're actually learning something
about physics

that you could never learn
here on Earth.

White dwarfs are emerging from out of
the shadows,

taking their rightful place as one of
the most fascinating objects in the
universe.

When we first observed white dwarfs,
they were weird,

they were curious, but just like
a sideshow.

But now white dwarfs are showing us
what they're truly capable of.

White dwarfs can sort of be seen as
these underdogs of the universe.

It's really become an exciting and
cutting-edge area of research.

Now we think these objects
may have

a lot of exciting science
to deliver.

Will the universe expand forever?
What is the ultimate fate
of the universe?

All of that may be waiting for
us inside a white dwarf.

Discount these things at your own
risk, because they're one

of the driving forces
in the universe.

Just because it's little,
it don't mean it ain't bad.

Don't underestimate
a white dwarf.

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