All language subtitles for How the Universe Works s09e06 War of the Galaxies.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 going to 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
we're on similar

but very strange orbits.

Most stars of the Milky Way
are orbiting in a sort of

regular pattern, but these stars

at the center, they're in
these highly elongated orbits.

Coming in from very far,

swinging around the center of
our galaxy, and then going back

out again, a little bit
like a comet does.

This group of stars plunges

wildly through
the center 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 science-y,

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
information 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 10 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.

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 have stretched it out.

Gaia-Enceladus was
distorted but not defeated.

The battle was just beginning.

It would have 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.

Eventually, most of those
stars would have

then settled down into

the disk of the Milky Way
and become a part of it.

Little galaxy try
to take on the Milky Way...

You're 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 center of our galaxy,

we see a bulge that's 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

sub-categories 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 looked
like they might have collided

with one another 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 disk 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 disk 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
neighborhood 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.

A 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 a massive as predicted.

So its gravity is double
what we thought.

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

1/10 the mass of the Milky Way.

That's enough to make
a pretty big punch.

In about 2.5 billion years,

it will smash into our galaxy.

It's gonna plow through
the disk 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 can have dire consequences.

The gravitational clash between

the invader on 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
a face-on spiral galaxy.

That would be like
waking up to my face

every morning... spectacular.

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 is 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

zip code will take
a severe beating.

The Milky Way Galaxy
is bigger than

the Large Magellanic Cloud,

so we are gonna win,
[exhales heavily]

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?

For billions of years,

the Milky Way conquered
galaxy after galaxy,

tearing its smaller rivals
to pieces.

But our galaxy is about
to meet its match.

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...
It's heading right for us.

Data from the Hubble
Space Telescope shows

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 a galaxy of
comparable size.

This is two heavyweight prize
fighters 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.

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 spiraling 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 them 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 disk 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,
Andromeway.

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 neighborhood.

It's 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 it's honorable
discharge, Milkomeda

may produce one final,

devastating act of war.

Imagine World War II,
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.

The 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 center of
the colliding galaxies.

They produce more energy
in one second than

the sun will in its entire
10-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 centers.

The supermassive black holes
start to feed.

The gas that was in
those galaxies starts to

funnel toward the black hole
and then 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... 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
studied because

it had two supermassive
black holes in it.

Now the galaxy itself
looked like it

had been disturbed,
like something had happened.

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 center.

It's the first time we 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,

and that means they had
greater chance for their

gravitational interactions to

pull them together
and smash them together.

The early universe was at war.

Conflicts between galaxies
were common.

They collided frequently
and grew quickly.

But not every galaxy profited
from the carnage.

Some brave galaxies
took on the big guns

and nearly died as a result.

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 looked
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.

And 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 look 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 center 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,
it was 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 is 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...
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.

In the great game
of galactic warfare,

losing can be catastrophic.

For weak and small galaxies,

resistance is futile.

Pillaged for resource is 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-faced 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

center in a way
that strongly perturbed

the gas on one half
of the galaxy.

The two-faced
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...
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.

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

10 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 center 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 traveling
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 neighbors

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 that 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.

And 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 revolutionized

our understanding of how
galaxies live and die.

Ultimately, it's these galaxy
mergers that are one of

the great engines
of all structure 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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