All language subtitles for How.The.Universe.Works.S09E01.War.Of.The.Galaxies.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

Our universe is at war.

The universe is a very violent
and deadly place.

Entire galaxies fight to the death.

Talk about clash of the titans.

It doesn't get more titanic
than this.

It's a slaughter.
It's a massacre.

Only the strongest survive.

If a galaxy wants to stay alive,
it has to feed on other galaxies.

Our own galaxy
also fights for survival.

We are facing
the ultimate destruction

of the Milky Way galaxy.

These battles are how galaxies live,
grow and die.

These collisions got us
to where we are today,

and they're gonna determine
the future of the universe.

In 2018, astronomers used
the Gaia space telescope

to map our Milky Way galaxy.

They tracked the movements of
a billion stars.

And they found that some behave
very strangely.

When astronomers were mapping stars
in our galaxy,

they found a whole bunch that were
on similar but very strange orbits.

Most stars in the Milky Way

are orbiting in a sort of
regular pattern,

but these stars at the centre,

they're in these highly
elongated orbits.

Coming in from very far,

swinging around the centre of
our galaxy,

and then going back out again,
a little bit like a comet does.

This group of stars plunges wildly
through the centre of our galaxy.

When you track their direction
and speed on a chart,

you get a shape
that looks a bit like a sausage.

This doesn't sound very sciency,

but this sausage is really
what the stars look like

if you look at the shapes of
their orbits

in a certain configuration.

What sent so many stars
on such a strange path?

It must have been a huge event.

We think these stars are the result
of a past cosmic collision.

They are casualties
from an enormous battle

between the Milky Way
and a foreign galactic army.

They don't move like stars
in the Milky Way

because they're not from
the Milky Way.

These stars are actually
alien stars.

They're invaders
from outer, outer space

The attackers left their mark
on the Milky Way.

We find similar battle scars
on galaxies across the universe.

Our models of galaxy formation
are still pretty uncertain.

We still don't really understand
how galaxies got to where they are,

how we go from the Big Bang
to the Milky Way.

Wars between galaxies
have profound consequences.

For the winners, the losers
and for us.

What we're learning
is that these galactic battles

have had a huge impact on what
the universe looks like today.

Our understanding of galaxies

has changed entirely
in the last few decades.

We understand now that
every big galaxy like the Milky Way

started from many smaller things
colliding,

changing each other as they went.

Nearly ten billion years ago,

the sausage stars were part of
a foreign galaxy.

It was on a collision course
with our home, the Milky Way.

We call this invading army
the Sausage Galaxy,

or Gaia-Enceladus.

The galaxy that we fought
probably had about 50 billion stars,

so we're talking about something

that is a significant fraction of
the size of the Milky Way.

Gaia-Enceladus was a tough opponent.

But the Milky Way was 20 times
its mass,

and that makes a huge difference.

When galaxies interact
with each other,

size definitely matters.

The bigger galaxies are gonna
dominate over the smaller ones,

ripping them apart
and essentially consuming them.

Galaxy interactions
are all about bullies.

The bigger you are,
the badder you are.

When two galaxies collide,

it's like two massive armies
marching towards each other.

And these galaxies aren't fighting
with knives or spears or guns,

or even nuclear bombs,

they're fighting with something
much more powerful.

Gravity itself.

Each galaxy contains
billions of stars and planets.

And a supermassive black hole,

millions of times
the mass of the sun.

That's a lot of
gravitational firepower.

As these galaxies approach
each other,

you can get tidal effects.

The same way that the moon
can raise tides

on one side of the Earth
and the opposite side,

one galaxy can stretch
another galaxy

along a certain direction.

As Gaia-Enceladus
advanced towards us,

our galaxy's superior gravity
grabbed hold of the smaller galaxy.

As it approached,

the gravity from the Milky Way
would've stretched it out.

Gaia-Enceladus was distorted,
but not defeated.

The battle was just beginning.

It would've passed through
our galaxy,

maybe orbiting a couple of times,

before being torn apart
by our gravity.

The Milky Way's gravitational power
ripped Gaia-Enceladus apart

and captured billions of its stars.

And eventually most of those stars
would have then settled down

into the disc of the Milky Way
and become a part of it.

Little galaxy try to take on
the Milky Way,

you gonna get what's coming to you.

Despite winning the battle,

the Milky Way suffered
serious damage.

The collision with
the Sausage Galaxy

left a scar on the Milky Way,

and when we look
near the centre of our galaxy,

we see a bulge just left over
from that collision.

The Milky Way isn't the only galaxy
scarred by war.

Across the universe, rival armies
made up of billions of stars

slug it out,

leaving behind distorted and damaged
casualties of war.

There's a million different
subcategories of them.

There's tadpole galaxies
that have long tails,

longer than our own galaxy.

There are things like
Arp-Madore 2026,

where you see
this eerie glowing face,

two big eyes looking right at you
from across the universe.

There are galaxies that look like

they might've collided
and blown holes through each other.

These battle scars give us
important clues

about one of the biggest mysteries
in astronomy.

How galaxies develop and grow.

But there's a problem.

We can't watch these battles
in real time.

The scale of galaxies is huge.

They're hundreds of thousands of
light years across.

It's going to take them millions or
billions of years to come together.

So it's like looking at one frame

from a really energetic fight scene
in a movie.

By piecing these snapshots together,

astronomers can build up
a detailed picture

of past conflicts,

and discover how these battles
transformed galaxies

over billions of years.

We have pictures of
isolated galaxies,

we have pictures of
interacting galaxies,

and we have pictures of
aftermath galaxies.

And that's helped us discover
something alarming.

The Milky Way faces
yet another attack.

From an enemy armed with
an enormous secret weapon.

Will our solar system
survive the onslaught?

Across the universe,
galaxies are at war.

Their main weapon?
Gravity.

It tears the combatants
into weird and wonderful shapes.

Our galaxy didn't escape the mayhem.
It's peppered with battle scars.

The overall shape of the Milky Way
is a flat disc of stars and gas.

Except recently we have found out
that at the edges,

it's actually warped a little bit
like the brim of a fedora.

The stars actually dip down
below the plane on one side,

and dip above it on the other.

We think the attacker
was one of our satellites.

A galaxy that orbits the Milky Way
like the moon orbits the Earth.

It's called
the Sagittarius Dwarf Galaxy.

From looking at how the stars move
in the Milky Way,

we suspect that
the Sagittarius Dwarf Galaxy

has actually crashed through
the Milky Way a few times

on its course of its orbit
around the galaxy.

It came in
about six billion years ago,

hit the disc hard
about two billion years ago,

and crashed again
about a billion years ago.

And our gravity has pulled it out

into a gigantic looping
stream of stars

that is moving in and out of
our Milky Way.

The war is not over.

The insurgent galaxy will return.

When galaxies interact,

often they're caught in
this huge cosmic dance

where they revolve around each other
a few times,

or they even crash through
each other

and then come back around.

The Sagittarius Dwarf Galaxy
looks like it's crashing in

with ever-increasing frequency.

A new skirmish could take place
in the next 100 million years.

So, should we be worried
about these attacks?

Because the Sagittarius Dwarf Galaxy

is so small
compared to the Milky Way,

it will do some damage
at the beginning.

But because we're so massive,
we can absorb the impact.

I mean, this galaxy,
it's looking for a fight,

but it's also 10,000 times
smaller than us,

so this is gonna be no sweat at all.

So far, the Milky Way
has been victorious.

But the danger isn't over.

We are surrounded by enemies.

Our local neighbourhood of galaxies
has three major galaxies,

but up to 50 smaller ones.

All these galaxies
are potential troublemakers.

Each one of these could be armies
that rise up against us.

The two most famous galaxies
that orbit the Milky Way

are the Large and Small
Magellanic Clouds.

These are two independent
dwarf galaxies

that you can see in the night sky
from the southern hemisphere.

We thought
the Large Magellanic Cloud

orbited our galaxy
at a safe distance

of 160,000 light years.

We thought it would stay that way
and we thought it was harmless.

Now a new discovery shows
we were wrong on all counts.

The new factor that changed our view
of the Magellanic Cloud

is we found out it has
a lot more dark matter

than we thought.

Dark matter.

The most mysterious stuff
in the universe.

Dark matter is literally
what it sounds like.

It's matter that we cannot see.

But it has gravity

and can affect objects
that we can see.

Adding in this extra dark matter
makes the Large Magellanic Cloud

at least twice as massive
as predicted,

so its gravity is double
what we thought.

And it's secretly been gathering
allies,

has been gathering dark matter
on its side,

and now it's a much bigger threat
than we thought before.

So it's not just going to orbit us,

it's gonna collide with
the Milky Way.

Moving at nearly
a million miles an hour,

the Large Magellanic Cloud
will not swing past us,

it will attack.

The Large Magellanic Cloud

is one tenth the mass of
the Milky Way.

That's enough to make
a pretty big punch.

In about two-and-a-half billion
years,

it will smash into our galaxy.

It's gonna plough through the disc
of the Milky Way.

It's gonna blow a cavity.

It might even damage
our spiral arms.

Earth sits in one of
those spiral arms.

Could our planet become
collateral damage?

If the Large Magellanic Cloud

passes through
the plane of our galaxy

near our location,
that could have dire consequences.

The gravitational clash
between the invader

and the Milky Way

could hurl stars and planets
out of our galaxy.

Earth could be one of them.

Our planet's very close to
its own star,

so the odds are that you'll just get
ripped out along with your star,

so we'd be moving along
with the sun

even as the sun gets jettisoned
from our galaxy.

And it'll move off
out into intergalactic space.

And that's not terrible, I mean,
it's not gonna get destroyed,

but it's a little lonely.

Our view of the night sky
would radically change.

We'd be able to see
much more of the Milky Way,

especially if we got kicked up
above the plane of the galaxy.

We'd be able to see
the whole shebang.

Just look at any image of
a spiral galaxy,

they're gorgeous.

Now, imagine seeing your night sky

filled with
the face-on spiral galaxy.

That would be like waking up
to my face every morning.

Spectacular.

(LAUGHS)

If we were unlucky,

our home planet could have
a close encounter

with an invading star.

The odds are very low

that another star
will pass close by the sun.

But those odds aren't zero.

It could happen that another star
passes close enough

to affect the planets.

And if that were to happen,

it could upset the delicate balance
in the solar system.

We don't know where the Earth
could end up.

It might find its way into the sun.
You just don't know.

Or there might just be
a rain of comets

into our inner solar system.

Our own planet might be flung out,

in which case this would be
a death knell for all life on Earth.

I'm not someone who's like
a doom and gloom person,

but, like, that would be insane.

You don't know what's gonna happen,
but most of the options are bad.

All these nightmare scenarios
will extinguish life.

Earth might survive,

but our cosmic zipcode
will take a severe beating.

The Milky Way galaxy is bigger than
the Large Magellanic Cloud,

so we are gonna win,

but it's gonna hurt us
for a long time.

The Large Magellanic Cloud
will leave our galaxy battered,

bruised,
but ultimately undefeated.

But there's a far bigger threat
looming over the Milky Way.

It's gonna face an opponent
that it can't defeat.

Will this mega-collision
be the Milky Way's last stand?

In the not-too-distant future,
galactically speaking,

a much, much larger battle
is due for the Milky Way.

A battle with a local superpower,
the Andromeda Galaxy.

We thought this huge galaxy
might wound us in the future.

Now recent evidence reveals

it's going to make
a full-scale assault.

We've known for a long time

that Andromeda is heading
more or less toward us,

but we didn't know exactly
in what direction.

But in recent years, we've been able
to pinpoint this a lot better,

and, uh, yeah, it's heading
right for us.

Data from the Hubble Space Telescope

shows the two galaxies will collide
in about four billion years.

And it will be a monumental battle.

This collision that is coming,
and it is coming,

is not gonna be anything like
the Milky Way has experienced before

in its 10 or 12-billion-year
history.

This is galaxy of comparable size.

This is two heavyweight
prizefighters coming at it.

Warriors with the same
gravitational firepower.

Simulations suggest
a clash of the titans.

Each of them
with half a trillion stars in them.

Well, that sounds like
a pretty spectacular collision.

Fights between
equally-matched galaxies are rare

and messy.

When the battle kicks off,

there will be no good news
for either side.

When the Andromeda Galaxy
and the Milky Way galaxy

start to get close,

they're gonna start
affecting each other profoundly.

Tendrils of stars
are gonna be thrown out.

Gas is gonna be thrown out.

It won't be a single impact.

Gravity will send the two opponents
into a spiralling dance of death.

The first pass is actually
not a direct hit.

They're gonna swing past each other,
in fact.

And at this point,

their gravitational interaction
is gonna slow 'em down,

and they're gonna come back
toward each other.

The galaxies will collide
and fly apart again,

inflicting more and more damage
with each clash.

If you were to go outside
and look up,

you could see the disc of our galaxy
getting ripped apart

by tidal interactions
with Andromeda.

The two beautiful spiral galaxies
will tear each other apart,

leaving one vast, elliptical galaxy.

The fate of
- the Andromeda/
- Milky Way battle

is that they will merge.

This is going to be
one gigantic galaxy.

And that presents a problem.

What are we gonna call
this new galaxy?

Of course, my nerd colleagues
have come up with names

like Milkomeda, Andromoway,
whatever.

Those are corny.

We should just call it Hakeem.

With a trillion stars,

it will be one of the biggest
galaxies in the universe.

In the Hakeem galaxy,
things gonna be completely new.

First off, it's gonna be
a really good-looking galaxy,

let's get that straight
from the get-go.

Second, it's gonna be powerful.

And I'm talking powerful.

This may be
the most remarkable galaxy

in the history of the universe.

Milkomeda,
or Hakeem if you prefer,

will become the undisputed boss of
our cosmic neighbourhood.

Its calm appearance concealing
a history of violence.

It's the result of
a complete war zone of mergers

over the course of
billions of years.

Many galaxies crashing together,
fully reconfiguring each time.

And slowly you grow this smooth,
placid, big blob of stars.

After billions of years of warfare,
our galaxy will finally be peaceful.

But before its honourable discharge,

Milkomeda may produce
one final devastating act of war.

Imagine World War Two,

and then all of a sudden

one of the sides comes up with
the Death Star.

That's what we're talking about
here.

A weapon of cosmic destruction.

When giant galaxies clash,

the battles are spectacular
and destructive.

The victors steal
huge numbers of stars

and vast amounts of gas

as fuel for
the ultimate super weapon.

This special weapon
that these monster galaxies have

is a giant death ray,

a jet of material racing across
thousands of light years.

These huge outbursts of energy

blast out of the centre of
the colliding galaxies.

They produce more energy
in one second

than the sun will in its entire
ten-billion-year lifetime.

We call them jets.

These incredibly powerful jets
aren't just brief features.

They can be sustained
for millions of years.

And they can maintain
their structure

for thousands of light years.

It's like turning on a garden hose
in Chicago

and using it to water a garden
in London.

Exactly what triggered these jets
was a mystery.

Then, in June 2018,

astronomers in Hawaii
captured something stunning,

a jet forming
during a galactic collision.

The team found something
really incredible.

They found two galaxies
that were in a cosmic collision,

and actually found an active jet
in one of these galaxies.

It was the first time anything
like this has been discovered.

When galaxies collide,

the clash drives huge clouds of gas
and dust

towards their centres.

The supermassive black holes
start to feed.

The gas that was in those galaxies

starts to funnel toward
the black hole

and will fall upon it.

Not all this gas ends up inside
the supermassive black hole,

powerful magnetic fields
carry some of this matter

to the poles,

and blast it out
in tight, narrow jets.

A super weapon is born.

This discovery helps us understand
how giant elliptical galaxies form.

Knowing that mergers of
spiral galaxies can cause these jets

helps us put together
a complete picture

of how these huge
elliptical galaxies might be formed.

The discovery doesn't answer
all our questions.

There's another mystery.

How did the super giant galaxies
that dwarf the Milky Way get so big?

Our Milky Way galaxy is big-ish.
It's slightly bigger than average.

But IC 1101, for example,

is more than 50 times larger than
our home galaxy,

and has more than a trillion,
with a T,

a trillion stars in it.

The biggest galaxies
make the Milky Way look like an ant.

These galactic giants
pose a problem.

There hasn't been enough time
since the birth of the universe

for them to become so large.

Even by conquering smaller galaxies.

When we look into
the distant universe,

we see something very strange
that we don't quite understand.

We see enormous galaxies

that existed just a billion years
after the Big Bang.

And even though
these cosmic collisions

help explain
how galaxies get bigger,

they don't quite explain everything
about how galaxies grow over time.

So we still have a big mystery
on our hands here.

So, in 2019,

an international team investigated
a very large galaxy

over 300 million light years away.

We call it NGC 6240.

NGC 6240 was being steadied

because it had two supermassive
black holes in it.

Now, the galaxy itself
looked like it had been disturbed,

like something had happened,

so they thought that potentially
it had had a recent merger.

They were expecting to see
two supermassive black holes

in the galaxy's heart.

As the researchers peered through
the layers of gas and dust,

they discovered something
surprising.

What we found was staggering.

We found not two, but three
supermassive black holes

lurking in the centre.

It's the first time
we've found a galaxy

with three supermassive black holes.

Evidence of a three galaxy pile up.

This galaxy is an active battlefield
of not two,

but three armies colliding at once.

And because there are three armies
involved,

there are three galaxies involved
with three times as much mass,

three times as many stars,
three times as much material,

and three times as much violence.

This three-way battle may explain
how the largest galaxies

got so big so fast.

It could be that galaxy mergers
are more frequent

than what we thought previously.

And therefore,
galaxies become more massive

faster than previously expected.

In the past,

galaxies may have battled
and collided more often than today.

Back then, galaxies were
more densely packed together.

Our universe is expanding
as it ages,

which means in the past
all the galaxies in the universe

were closer together.

That means they've had
greater chance

for their gravitational interactions

to pull them together
and smash them together.

When galaxies fight,
the big get bigger.

More mass means more gravity.
The vital ingredient for victory.

But galactic conflict
doesn't always result in growth.

A strange new astronomical object
had scientists confused.

They just look like stars
from the ground.

However, with the advent of Hubble

and beautiful
space-based telescopes,

it was possible to look at
these "stars" again

and actually discover that
they were galaxies.

They're kind of crazy.
They're a huge number of stars,

but crammed into
an incredibly tiny space

on an astrophysical scale,

something 500 times smaller than
our Milky Way galaxy.

We call them
ultra-compact dwarf galaxies,

or UCDs.

You might imagine
the difference between

the Milky Way galaxy and a UCD

as the difference between a cloud
and a rock,

where the rock is just
the same kind of material,

but compressed to just
incredibly high densities

compared to some fluffy,
gaseous thing.

What are these strange galaxies?
They seem to break all the rules.

To find out, astronomers zoomed in

to a particularly dense
ultra-compact dwarf galaxy

called M60-UCD1.

M60-UCD1 is 300 light years across.

It's tiny. It's a pinpoint
compared to our enormous galaxy.

Our galaxy has 200 or more
billion stars in it.

M60-UCD1 only has 140 million,

but they're packed into
this incredibly tight volume.

The night sky inside the galaxy

would look very different
from our own.

On Earth,
when you look at the night sky,

you see a few thousand stars.

But if you were in M60-UCD1,

you wouldn't just see
a few thousand stars,

you would see hundreds of thousands
of stars in the night sky.

That would be amazing.

As the astronomers looked deeper
into the heart of this tiny galaxy,

things got even weirder.

They found a supermassive black hole
much bigger than expected.

It actually has a black hole
that's bigger, five times bigger,

than the black hole at the centre of
our Milky Way galaxy.

When we see supermassive
black holes inside of galaxies,

they tend to scale with the size of
the galaxy itself.

A bigger galaxy has a bigger
supermassive black hole.

Why does such a tiny little object

have such an oversized
central black hole?

The only possible explanation?

This tiny galaxy was once
much larger.

These galaxies might have begun
their lives

as, in fact, much bigger galaxies.

And that what we see today is really
just the very central densest part

of a much larger galaxy.

Based on the size of
its supermassive black hole,

M60-UCD1 may once have contained
many billions of stars.

Something captured them,

and we don't have to look far
to find the aggressor.

A nearby super galaxy

with lots of gravitational
firepower, M60.

M60's a monster.
It has a trillion stars in it.

It's bigger than the Milky Way
and we're pretty big.

The battle was not
a full-on frontal assault.

M60 raided its smaller opponent,
capturing its troops.

This is more of a stealthy,
guerrilla hit and run,

where we're gonna move in,
pick off some of your troops

and then get out
before you even notice.

All that's left from one of
these drive-by galaxy interactions

is this supermassive black hole

with a fraction of
its original stars.

The conflict devastated M60-UCD1.

Over 98% of its stellar army
were captured

and became prisoners of war.

It used to be a big galaxy,

but it suffered one too many defeats
and now it's a fallen empire.

We can frame this battle
between M60 and M60-UCD1

as just a battle,

but, in fact, it's a slaughter,
it's a massacre.

These small galaxies
get all their troops removed,

but the HQ,

the supermassive black hole,
remains,

but it doesn't have any troops left.

Eventually, M60 will conquer
its battered opponent,

destroying what's left of
the compact galaxy.

It'll get ripped apart
even further,

and more and more stars
will be consumed

by the bigger galaxy.

So chances are this little dwarf is
eventually going to be pulled apart

and become a part of M60.

But while this little galaxy
is down,

it is most certainly not yet out.

It's likely to stage
a counterattack.

Its final mission? A kamikaze charge
that will leave M60 reeling.

It's absolutely chaotic.

Stars are sent off flying
in every direction,

and they're moving
at really high velocities.

And when stars are stripped
out of galaxies,

they become what we call
rogue stars.

These ejected, runaway stars
will leave the galaxy

at two million miles an hour.

They'll hurtle through
the emptiness of space,

never to be seen again.

But even this dramatic assault
can't change the inevitable outcome.

M60-UCD1 is doomed.

In the great game of
galactic warfare,

losing can be catastrophic.

For weak and small galaxies,
resistance is futile.

Pillaged for resources
by their more powerful opponents,

they slowly become burnt-out wrecks.

But some peaceful galaxies
face an equally terrible fate.

They starve to death.

Cosmic wars are vicious.
They destroy many galaxies.

But violent conflicts
can also give galaxies new life.

Case in point, galaxy NGC 4485.

NGC 4485 has a nickname of
the Two-Face galaxy,

like the Batman villain,

because it has two different halves
of the galaxy

doing completely different things.

Half of the galaxy is sort of old
and calm and relatively quiescent,

whereas half of it
appears to be undergoing

a sort of fireworks display of
new star formation.

Why are new stars only born
in one half of this galaxy?

We found a clue
on the edge of a photo

taken by the Hubble Space Telescope.

It was evidence of an attack
by another galaxy.

We think that another galaxy
passed through it just off centre

in a way that strongly perturbed
the gas

on one half of the galaxy.

The Two-Face galaxy skirmish
gave it a gravitational jolt,

forcing clouds of gas together.

When we think of galaxies,
we think of stars,

and of course
galaxies are made of stars,

but of course gas is the stuff
that stars are made of.

When two galaxies collide,

the gravitational duel can trigger
a huge burst of star formation.

You need something to give a galaxy
a push,

and that's exactly
what a galaxy collision does.

And when gas clouds collide,
they compress.

And when they compress,
you get knots in them

that can compress more
and form stars.

So you can think of these collisions
as very violent events,

but ultimately it can breathe
new life into a galaxy.

But the spoils of war
don't last long.

In the short term, the victor galaxy
can come out glorious

with so many new stars.

But this celebration is short lived
because that round of star formation

quickly uses up
the material available.

If a galaxy wants to stay alive,
it has to feed on other galaxies.

But what happens when opponents
are closely matched

in gravitational firepower?

When two equal mass galaxies meet,
when two armies of equal size meet,

they can exhaust all their fuel,
they can kill all their troops,

so at the end of it
there's nobody left to fight.

The galaxies themselves
can be stripped of gas and dust,

all the things you need to actually
make star formation happen.

Once the gas is used up,

the galaxy can't create
any new stars.

It can only get more gas
by looting other galaxies.

We can sometimes see a gas stream
coming from one galaxy

and joining to the other,

and then the gas from that galaxy
is now free

and it starts to make stars again.

You're bringing in new ingredients
to form stars,

new gas, new dust to become part of
this larger galaxy.

All sorts of weird and wonderful
things can happen in galaxy mergers.

A star is born

when a huge cloud of gas
collapses under gravity,

triggering nuclear fusion.

The star ignites
and begins to shine.

It's a glorious sight

and often a sign
that the war is finally ending.

When two galaxies come together,
boom,

all of a sudden, stars light up
all over the place.

In this moment, as you catch
this system getting excited,

the star formation rate's ramping up

and it's sort of
a cosmic fireworks display,

as if they're celebrating
the end of this hostile encounter

with the other galaxy,
this war.

But it's also a sign
that the victorious galaxy

is using up its gas supply.

So galaxies constantly need to raid
new targets

and that raises
an important question.

What happens if there's a galaxy
just alone in space,

nothing else is colliding with it,
sort of a pacifist galaxy.

The poster child
for these peace-loving galaxies

is NGC 1277.

NGC 1277 is a very peculiar galaxy.

It's pretty big
and its stars are extremely old.

It basically hasn't formed new stars
in the last ten billion years,

so it's kind of the veterans home
of galaxies.

NGC 1277 lives in a rough part of
the cosmos

called the Perseus Cluster.

Thousands of other galaxies
surround NGC 1277

and they are all ready for a fight.

So, you might ask why hasn't it had
encounters with other galaxies

that might rejuvenate it?

The answer, once again, is gravity.

NGC 1277 sits inside
this massive galaxy cluster

that has a ton of mass.

And if you look at its position,

it's fairly near the centre of
the cluster.

The combined gravity of
thousands of galaxies

pulls on NGC 1277,

accelerating it
to two million miles an hour.

And so it has spent
the last few billion years

travelling faster and faster,

until now it's almost at
its fastest pace.

It's very hard for gravity
to catch it,

or catch one of its neighbours,

and bring them together
to merge with each other.

NGC 1277 has no chance of grabbing
new gas to make new stars.

It's dying.

All it has left are old red stars.

When it comes to galaxies,
red is dead.

No new stars means no big stars,
no blue stars,

just small, dim, red dwarfs.

Galaxies that don't fight
just fade away.

At that point,

the history of the universe
becomes really kind of boring.

All the stars
will simply start to die out.

Eventually there will be
the last star

formed in the Milky Way,

with no new galaxy
bringing fresh material.

Without galaxy collisions,
the universe dies.

Galactic battles mix things up
and replenish gas supplies.

And our own galaxy
has reaped the benefits.

Our Milky Way galaxy
fought a massive battle,

but that battle
may have been necessary

to build solar systems
like the one we live in right now.

Clashes with
the Sagittarius Dwarf Galaxy

occurred at the same time
the sun formed.

It's possible that we owe
our very existence

to the collision with
the Sagittarius Dwarf Galaxy.

Maybe the gas

that ultimately gave rise to
the birth of our solar system

once came from another galaxy
entirely.

So, galactic wars are both creative
and destructive.

Galaxies are built from collisions.

Galaxies survive from collisions.

And galaxies can also die
from collisions.

Far from being destructive events,

colliding galaxies may be the reason
that you and I are here.

Intergalactic warfare

has revolutionised our understanding
of how galaxies live and die.

Ultimately,
it's these galaxy mergers

that are one of the great engines

of all structured growth
in the universe.

These collisions got us
to where we are today,

and they're gonna determine
the future of all the universe.

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