How the Universe Works

White dwarfs,

small stars that pack a big punch.

When white dwarfs were first discovered,

astronomers' reaction was no, 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 center, 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 to get too close to a black hole

get torn apart.

They sort of get attacked by the black hole,

and 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's 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 center.

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 center,

and 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, 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 the juicy parts, all the easy-to-get-at parts

of star, leaving behind the sort of

bone or the leftovers of the white dwarf.

This white dwarf is just 1/5

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,

because there's not that much stuff there to eat,

but 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.

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,

and 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,

and 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 subatomic 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 is 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 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 superheated 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 center 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 center 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,

and 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 midsized 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?