Original subtitles

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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