How the Universe Works

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.