Original subtitles

MIKE ROWE: Our universe is at war.

The universe is a very violent and deadly place.

ROWE: Entire galaxies fight to the death.

OLUSEYI: Talk about clash of the Titans.

It doesn't get more titanic than this.

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

ROWE: Only the strongest survive.

If a galaxy wants to stay alive,

it has to feed on other galaxies.

ROWE: Our own galaxy also fights for survival.

We are facing the ultimate destruction

of the Milky Way Galaxy.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: They are casualties from an enormous battle between

the Milky Way and a foreign galactic army.

BULLOCK: 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.

ROWE: 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.

ROWE: 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.

THALLER: 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.

ROWE: 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.

PLAIT: 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.

ROWE: 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.

SUTTER: 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.

ROWE: 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.

SUTTER: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

HOPKINS: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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?

ROWE: 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.

ROWE: 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.

BULLOCK: 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.

PLAIT: And our gravity has pulled it out into a gigantic,

looping stream of stars that is moving in and out

of our Milky Way.

ROWE: 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.

ROWE: A new skirmish could take place in the next

100 million years.

So should we be worried about these attacks?

SUTTER: 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.

ROWE: So far, the Milky Way has been victorious.

But the danger isn't over.

We are surrounded by enemies.

SUTTER: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: In about 2.5 billion years,

it will smash into our galaxy.

SUTTER: It's gonna plow through the disk of the Milky Way,

it's gonna blow a cavity.

It might even damage our spiral arms.

ROWE: 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.

ROWE: 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.

HOPKINS: 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.

PLAIT: 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.

ROWE: Our view of the night sky

would radically change.

OLUSEYI: 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.

[scoffs] That would be like waking up to my face

every morning -- spectacular.

[laughs]

ROWE: If we were unlucky,

our home planet could have

a close encounter with an invading star.

PLAIT: 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.

OLUSEYI: 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.

MINGARELLI: 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.

OLUSEYI: You don't know what's gonna happen,

but most of the options are bad.

ROWE: All these nightmare scenarios

will extinguish life.

Earth might survive, but our cosmic

zip code will take a severe beating.

SUTTER: 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.

ROWE: 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.

ROWE: Will this mega collision be

the Milky Way's last stand?

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: Warriors with the same gravitational firepower.

Simulations suggest a clash of the Titans.

BASRI: Each of them with half a trillion stars in them.

That sounds like a pretty spectacular collision.

ROWE: Fights between equally matched galaxies

are rare and messy.

When the battle kicks off,

there will be no good news for either side.

PLAIT: 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.

ROWE: It won't be a single impact.

Gravity will send the two opponents

into a spiraling dance of death.

PLAIT: 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.

ROWE: 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.

ROWE: The two beautiful spiral galaxies

will tear each other apart,

leaving one vast elliptical galaxy.

PLAIT: The fate of the Andromeda, Milky Way

battle is that they will merge.

This is going to be one gigantic galaxy.

OLUSEYI: 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.

ROWE: With a trillion stars, it will be

one of the biggest galaxies in the universe.

OLUSEYI: 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.

ROWE: 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.

ROWE: 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.

ROWE: A weapon of cosmic destruction.

♪♪

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

ROWE: So in 2019, an international team

investigated a very large galaxy

over 300 million light-years away.

We call it NGC 6240.

MINGARELLI: 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.

ROWE: 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.

ROWE: 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.

ROWE: This three-way battle may explain how

the largest galaxies got so big so fast.

MINGARELLI: It could be that galaxy mergers

are more frequent than what we thought previously,

and therefore, galaxies become more massive

faster than previously expected.

ROWE: 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.

ROWE: 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.

ROWE: 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.

MINGARELLI: 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.

HOPKINS: 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.

ROWE: We call them ultra-compact dwarf galaxies,

or UCDs.

HOPKINS: 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.

ROWE: 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.

ROWE: The night sky inside the galaxy

would look very different from our own.

PLAIT: 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.

ROWE: 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.

PLAIT: 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?

ROWE: 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

ROWE: 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.

ROWE: The battle was not a full-on frontal assault.

M60 raided its smaller opponent, capturing its troops.

SUTTER: 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.

MINGARELLI: All that's left from one of these drive-by

galaxy interactions

is this supermassive black hole with a fraction

of its original stars.

ROWE: The conflict devastated M60-UCD1.

Over 98% of its stellar army were captured

and became prisoners of war.

SUTTER: It used to be a big galaxy,

but it suffered one too many defeats.

And now it's a -- It's a fallen empire.

PLAIT: 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.

SUTTER: These small galaxies get all their troops removed,

but the HQ,

the supermassive black hole remains,

but it doesn't have any troops left.

ROWE: Eventually, M60 will conquer its battered opponent,

destroying what's left of the compact galaxy.

PLAIT: 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.

ROWE: 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.

ROWE: Cosmic wars are vicious.

They destroy many galaxies,

but violent conflicts can also give galaxies new life.

Case in point,

galaxy NGC 4485.

HOPKINS: 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.

ROWE: 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.

ROWE: 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.

ROWE: 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.

BASRI: 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.

ROWE: But the spoils of war don't last long.

SUTTER: 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.

ROWE: 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.

ROWE: The poster child for these peace-loving galaxies

is NGC 1277.

STRAUGHN: NGC 1277 is a very peculiar galaxy.

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

HOPKINS: It basically hasn't formed new stars in the last

10 billion years,

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

ROWE: 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?

ROWE: 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.

ROWE: The combined gravity of thousands of galaxies

pulls on NGC 1277,

accelerating it to two million miles an hour.

STRAUGHN: And so it has spent the last

few billion years traveling faster and faster.

Until now, it's almost at its fastest pace.

HOPKINS: It's very hard for gravity

to catch it or catch one of its neighbors

and bring them together to merge with each other.

ROWE: NGC 1277 has no chance of

grabbing new gas to make new stars.

It's dying. All that has left are old red stars.

SUTTER: When it comes to galaxies, red is dead.

No new stars means no big stars,

no blue stars, just small dim red dwarfs.

ROWE: Galaxies that don't fight just fade away.

THALLER: 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.

♪♪

ROWE: 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.

ROWE: Clashes with the Sagittarius Dwarf Galaxy

occurred at the same time the sun formed.

BULLOCK: It's possible that we owe our very

existence to the collision with the Sagittarius Dwarf Galaxy.

TREMBLAY: Maybe the gas that ultimately gave rise

to the birth of our solar system

once came from another galaxy entirely.

ROWE: 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.

ROWE: 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.

OLUSEYI: These collisions got us to where we are today,

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