Original subtitles

If I were to ask you, where do you come from, what would you say?

What story would you tell?

You might say, well, I come from my hometown, or my city, or my country.

If you have a particularly wide perspective, you might say, I come from planet Earth.

But what is the largest structure that we could legitimately call home?

Well, I would argue it's that.

That faint arc of light that stretches across the sky from horizon to horizon is

an outer spiral arm of our galaxy, the Milky

Way, our home island of 400 billion stars.

The Milky Way takes its name from the dense band of stars

that sweeps across the sky on the clearest of nights.

From our vantage point here on Earth, we see the galaxy from within.

But if we could travel outside the galaxy,

we would see the entire structure.

The Milky Way revealed as an island of light surrounded by darkness.

Hundreds of billions of stars in a single disk

that's existed since the universe was young.

Are we able to explore its history?

How it was born?

How, through a series of remarkable events,

it grew to become the galaxy we inhabit today?

Eventually, it will end.

We've discovered our own part in this story, too.

Living, as we do, inside the Milky Way.

Just over halfway along one of its magnificent arms.

Around a small but familiar star.

The Milky Way is an island, in a sense.

Every star you can see in the night sky is a part of our galaxy.

Our nearest neighbour is the Milky Way.

In large galaxies over two million light years away.

So it certainly feels as if we are isolated and alone, adrift in an ocean of dark.

And that is true, to a point.

There is no conceivable technology that will ever allow us to leave our island,

physically.

But science allows us to leave the Milky Way in our imaginations, to view our

galaxy from impossible perspectives in both space and time.

And to tell its story.

One

mission more than any other has deepened our understanding of the galaxy.

A spacecraft bearing the name of an ancient Greek goddess.

Everything functioning beautifully.

Gaia.

Coming up on separation of the boosters.

Ancestral mother of all life on Earth.

The four boosters, the four points of light, falling away.

Gaia's mission?

To map the locations of billions of stars in the Milky Way.

Nearly all of them for the first time.

Gaia spins on its axis.

Its sensors scanning the galaxy in all directions.

Every star is mapped an average of 70 times.

Allowing Gaia to calculate the speed and direction of each one.

Pinpointing their locations with accuracies up to one thousandth of one percent.

Half a million stars every hour.

Almost two billion in total so far.

To create a map.

Like nothing ever seen before.

The Gaia data is by far the most detailed star map ever produced.

A revolution in our understanding of the Milky Way.

This is the data and it looks like an artist's impression of a galaxy.

Something from science fiction.

But this is a high precision 3D map of our home.

Of our island of stars.

And we can even fly through it.

Such as the precision of the mapping of the position.

All these points of light are stars.

Some of them as far as 30,000 light years out from the solar system.

The map allows us to journey through the galaxy at impossible speeds.

Bringing distant stars within reach.

But this is also a journey through time.

The extraordinary thing.

About this map.

Is that it's alive in a sense.

I mean Gaia didn't just measure the positions of these stars.

It measured their velocities.

And that means we can tell where those stars are going.

What the galaxy is going to be like in the future.

But also we can tell where they came from.

So what the galaxy was like in the past.

By reversing the direction of every star.

We can rewind their histories.

Travelling backwards in time through billions of years.

Gaia has initiated a new science.

A science of galactic archaeology.

Where we can ask questions about the origins of our galaxy itself.

The first galaxies emerged just a few hundred million years after the big bang.

The universe was criss-crossed by a vast structure known as the cosmic web.

Great filaments of dark matter.

Along which gravity attracted ever denser concentrations of gas.

Separated by immense tracts of empty space.

The first stars were born where the filaments crossed.

Where the gas was dense enough to collapse under water.

Under its own gravity.

And for the stars to ignite.

New stars formed in their billions.

Bound together by their mutual gravitational pull.

These were the first galaxies.

Amongst them, the Milky Way.

In its embryonic form.

Far smaller and more irregular in structure.

Than the mature spiral galaxy we inhabit today.

The exact details of the Milky Way's birth remain the subject of research.

But thanks to modern day observations.

The story of how our galaxy grew from those early beginnings.

Is coming into much sharper relief.

The Gaia data allows us to see how the Milky Way evolved throughout its history.

And one of the clues.

That it's had an interesting history.

Can be seen in this animation.

You see that most of the stars orbit in very regular orbits.

Around the centre of the Milky Way.

That's exactly what you'd expect.

But you can see here that some of the stars have very different orbits indeed.

They seem to be flying all over the place.

And that tells us that something dramatic happened at some point.

As our galaxy made its way through the universe.

The Milky Way.

The Milky Way.

Across the universe.

Hundreds of billions of galaxies were forming.

Some.

Just a few dozen.

Were born close enough to the Milky Way.

That their mutual gravitational pull drew them together.

Forming what we now know as the Local Group of Galaxies.

Our home archipelago.

Our home archipelago.

Some of the Milky Way stars already had their own planets.

Early worlds.

That were about to witness the transformation of the galaxy.

The wonderful thing about astronomy.

Is that you can look up into the sky.

And even if you can't see worlds.

You can imagine them.

And you can imagine their stories.

Over there.

Close to the bright star Vega.

Is Kepler 444.

That faint ancient star.

With planets orbiting around it.

That's witnessed pretty much the entire history of the Milky Way galaxy.

And then maybe swing around in the sky.

Just close to the planet.

Constellation that everybody can recognize.

And follow it down.

There's a really faint star there.

You can't see it with the naked eye.

It's so nondescript.

It doesn't even have a name.

It's got a number.

It's called HD 73394.

But that star is an alien star.

It was born in another galaxy.

And it entered the Milky Way in a galactic collision with a smaller galaxy.

And Kepler 444.

Over there.

Witnessed it all.

And witnessed the Milky Way being thrown into chaos.

Kepler 444 was orbited by five planets.

And something new had appeared in their skies.

A smaller galaxy was approaching the Milky Way.

With stars that burned bright blue.

And stars that burned blue.

Gaia Enceladus.

A member of the local group.

Roughly a quarter of the size of our own galaxy.

Over hundreds of millions of years, the galaxies collided.

The stars of Gaia Enceladus penetrating deep into the Milky Way's heart.

But our galaxy held its ground.

But our galaxy held its ground.

Capturing billions of incoming stars.

An entire galaxy.

Swallowed whole.

These alien stars remain in our galaxy to this day.

The Gaia data tell us that collisions are the driving force of galactic evolution.

Some galaxies cease to exist as independent islands of stars.

While others grow.

And prosper.

The survival of the fittest.

Writ large.

When galaxies collide, that phrase puts images

of Hollywood disaster movies into the mind.

Stars getting ripped apart.

But that's not what happens at all.

I mean, you imagine that our sun were, say

the size of a small pebble or a grain of sand.

The nearest neighbouring star in this region of the galaxy.

Will be somewhere over by those hills.

The distances between stars is immense.

The stars don't collide.

So when galaxies interact, the stars get scattered.

The shape of the galaxy changes.

But nothing gets destroyed.

And in fact, sometimes galactic collisions can be engines of creation.

Gaia Enceladus, the alien galaxy, had brought

with it fresh supplies of interstellar gas.

The raw material of star formation.

For a time, this gas heightened the rate at

which the Milky Way could produce new stars.

Helping it to grow.

Long

before our star was born, the Gaia Enceladus collision era drew to a close.

What triggered the formation of the sun has long remained a puzzle.

But it's not over yet.

But the Gaia Telescope has discovered new clues to its origin.

In the events that followed billions of years later.

As our island of stars continued to evolve.

On the distant shores of the Milky Way, Gaia has

investigated a structure of epic proportions.

A stream of stars.

Winding their way around the galaxy.

This stream of stars is enormous.

It's almost unimaginable in scale.

Look up into the night sky.

Those stars that you can see are at most a few thousand light years away.

You think about that, the light began its

journey to your eye from the most distant stars.

When the pharaohs ruled Egypt.

Then if you look out to the Milky Way.

To the shores.

The shores of our galaxy.

You see light from a few tens of thousands of light years away.

I mean, that light began its journey when there were Neanderthals here in Europe.

But this stream of stars wraps around the galaxy.

It's hundreds of thousands of light years in extent.

A structure that large demands an explanation.

The stream is wreckage.

It's footprints, if you like.

Of a very violent event.

Gaia has confirmed the origins of this immense structure.

Through the telescope's unique ability to help us travel through time.

Backwards.

The data tell a story.

Of a new age of star birth.

Of the truth.

Of a transformation of the Milky Way.

Triggered by another galactic collision.

It was another galaxy from our local group.

Sagittarius Dwarf.

Perhaps 20 times smaller than the Milky Way.

Was torn apart in the impact.

Sagittarius Dwarf brought fresh supplies of the vital ingredient for the Milky Way.

A star birth.

That is the sound of the most common element in the universe.

This radio telescope is pointing towards the Milky Way.

Which had just risen above the horizon over there behind the clouds.

And what you're listening to is hydrogen gas.

The radio telescope is detecting the faint signal of hydrogen from across the galaxy.

Hydrogen is found throughout the Milky Way.

Sometimes in the form of towering clouds.

Light years high.

These regions are star factories.

Where the dense clouds of hydrogen gas collapse under gravity.

To forge new stars.

Hydrogen atoms radiate.

Radio waves.

At a very particular wavelength.

21 centimetres.

And as I speak.

That radiation is being captured by that radio telescope.

Imagine.

There are atoms over there.

And by over there I mean what?

Thousands.

Tens of thousands of light years away.

At some point.

Way way back in the past.

Out came the radiation.

And we can listen to it.

So we're listening to the lifeblood.

Of our galaxy.

A Sagittarius dwarf.

Passed through the Milky Way.

It brought fresh gas.

And fresh energy.

The impact sent ripples.

Across the Milky Way.

Triggering another spectacular era.

Of star formation.

And.

In the outer regions.

Of the galaxy.

Our own star.

And in the outer regions.

Was born.

The sun was soon joined.

By the earth.

And together.

They set out on their journey.

Through the galaxy.

We were born.

In the Milky Way.

But we may have been.

Conceived.

In a collision.

Now we can't say for certain.

That the collision with Sagittarius dwarf.

Caused the formation of our sun.

But data's not proven.

Precise enough.

And our understanding is not deep enough for that.

But what we can say.

Is that the birth of the sun.

Coincided with enhanced rates of star formation.

In the Milky Way.

Caused by that collision.

But that's not quite the end of the story.

Because in a very real sense.

The collision is still underway.

The remains of Sagittarius dwarf.

Are still orbiting.

On the fringes of the Milky Way.

Over the last five billion years.

The galaxy has crossed our path.

Two more times.

Each interaction.

Triggering a new generation of star birth.

A fresh sprinkling of light.

Inside our galaxy's spiral arms.

The finishing touches.

On a masterpiece.

Of galactic creation.

The poet John Donne.

Famously wrote.

No man is an island.

Entire of itself.

Every man is a piece of the continent.

Part of the main.

By which he meant that no human being.

Can isolate themselves from the rest of humanity.

Because our origins and our fates.

Are so deeply intertwined.

And therefore.

We must care deeply for each other.

And the same is true for galaxies.

No galaxy is an island entire of itself.

And the history of the Milky Way.

Stretches back 13 billion years or more.

That's pretty much the entire history of the universe.

And its story.

Is a story of collisions and interactions.

Between galaxies.

Of rivers and flows.

And streams of stars.

Stirring up the void and triggering the formation.

Of worlds like ours.

You and me.

Everyone.

Can trace our origins back to a collision.

Between galaxies.

You may be small.

But you are a consequence of grand events.

And those grand events.

Haven't stopped.

It just feels like it.

Because we don't perceive events that play out over billions of years.

Involving billions of stars.

But the unique thing.

About this time in history.

Is that we can speak with some confidence.

Not only about our galaxy's past.

But also.

About our galaxy's future.

And just as inexorably.

As those great islands of stars.

Drift through the universe.

Change will come again.

We move into the future.

With a new understanding.

Of our place.

In the galaxy.

We are inhabitants.

Of a small planet.

Orbiting around an ordinary star.

Where something extraordinary.

Has happened.

But although the galaxy made us.

It wasn't made for us.

We were born.

We are accidental byproducts.

Of its history.

And we will be.

Passive witnesses.

To its ongoing evolution.

The Milky Way is the great survivor.

And the echoes of its turbulent history.

Are literally written across the sky.

Over there.

In the southwest.

The remnants of Sagittarius Dwarf.

The debris from that collision.

Still wandering around somewhere on the fringes.

Of the Milky Way.

In that direction.

As Sirius rises in the east.

In the constellation of Canis Major.

There are the remains of another dwarf galaxy.

That we think collided with us long ago.

So the Milky Way.

Pretty much devours anything.

That comes into this region of space.

Because it's the largest galaxy.

In the neighborhood.

Except for one.

The local group.

Is home to another galaxy.

That rivals our own.

In size.

A galaxy.

That's been hiding.

In plain sight.

Right up there.

Just between the constellations of Cassiopeia.

And the square of Pegasus.

Is a faint misty patch of light.

In the sky.

About twice the diameter of a full moon.

So you can certainly see it with binoculars.

And even in the city.

I can take a photograph of it.

With a camera like this.

And there it is.

That object.

Is the Andromeda Galaxy.

And you see.

That it's a spiral shape.

You can see it even in this photograph.

In many ways.

Andromeda.

Is our twin.

And it's a twin.

That we've been able to explore.

In incredible detail.

Three.

Two.

One.

And liftoff.

Of Space Shuttle 11.

A private visit.

Against the vision of Hubble.

Standing by for SRB separation.

The Hubble Space Telescope.

Is in its fourth decade.

Of operation.

Its ongoing mission.

Has given us some of the most detailed images.

Of the universe ever seen.

Over the years.

Hubble has frequently.

Turned its attention.

To Andromeda.

Two and a half million light years.

From Earth.

It's mapped a spiral structure.

Similar to that of the Milky Way.

With such fine precision.

That we've been able to calculate.

Not only the motion.

Of Andromeda's stars.

But also.

The motion of the galaxy itself.

We know

that the entire galaxy.

Is heading towards us.

At over 400,000.

Kilometres per hour.

Now you may think.

Well what's one more collision.

I mean the Milky Way has survived.

All these collisions.

For pretty much the entire history of the universe.

Well this one will be different.

Because Andromeda.

Is bigger than us.

The Milky Way.

As we know it today.

Will not be immortal.

And the Earth.

Will witness its demise.

Two galaxies.

In a single sky.

Gradually.

But inexorably.

Merging into one.

In the impact.

There will be a last.

Colossal burst.

Of star formation.

But this will be very different.

To previous collisions.

This time.

Our galaxy.

Will meet its match.

The great galaxies.

Will distort each other.

Spiral arms.

Stars.

Will be scattered.

Until no traces.

Of the original structures remain.

The Milky Way's fate.

Is sealed.

Andromeda.

Will be the first of a series of mergers.

As the remaining galaxies.

In our local group converge.

Drawn together.

By gravity.

But Hubble.

Has allowed us to see.

Even further into the future.

It's looked out.

Far beyond the local group.

Towards the edge.

Of the observable universe.

And seen.

That every distant galaxy.

Is receding from us.

In a final twist.

These retreating galaxies.

Tell us.

Something profound.

About the nature.

Of the universe.

Itself.

We live in an expanding universe.

In fact we live in a universe.

That's accelerating.

In its expansion.

So all the galaxies.

Are rushing away from each other.

And in the far future.

They'll be rushing away from each other.

So fast.

That even if we sent a beam of light.

Right out to the galaxies.

It would never catch them.

Billions of years from now.

The remnants of the Milky Way.

Will form part of a single.

Gigantic collection of stars.

The merged remains.

Of the local group.

Alone.

As every other galaxy.

Recedes.

Into the distance.

Eventually.

All the galaxies.

Will fade from view.

And our galaxy.

Will stand.

At last.

In perfect.

Isolation.

An island.

Unto itself.

I think we live.

At a fortunate time.

In the history of the universe.

Because we can look into the sky.

And see the galaxies.

The astronomers of the far future.

Might imagine.

That they live in a universe.

Populated by countless billions of islands.

Of billions of stars.

But they won't be able to prove it.

They won't be able to see.

The true scale.

And majesty.

Of the universe.

We've been trying to understand.

The band of stars.

That stretches across the night sky.

Since the time of the ancient Greeks.

The story of our galaxy.

The Milky Way.

How it started.

How it was formed.

And how it's transformed.

Is really the story of us.

Inside the Milky Way.

You always have a slightly skewed perspective.

Of the way the Milky Way looks.

So we're in it.

And so what we would like to do is go above it.

And look down and see what it's like.

Now you can't do that.

Unless you could travel at millions of times the speed of light.

And we can't.

So the only way we can do it.

Is by working out accurately where all the stars are.

How far away they are from us.

In particular.

Gaia is a European space agency spacecraft.

Which is in principle.

A very simple little thing.

It's two telescopes collecting the light.

Putting it down onto one giant camera.

Biggest camera ever put in space actually.

It can observe the positions of stars.

So accurately.

That you could see the edge of a euro coin.

On the moon from earth.

And that's just mind-boggling.

It was a beautiful launch.

Really spectacular.

Then it got into this critical state.

Where they had to open up the sun shields.

It was critical that this opened up.

And protect the payload.

From the sun.

And that was the do or die moment.

There's the good news.

Gaia works by measuring parallax.

This is exactly the same way.

Your eyes and brain work.

So you can tell how far away something is.

Because of the slight difference in angle.

From this eye to that eye.

And so what we do with Gaia.

Is have a picture in the summer.

And a picture in the winter.

And in that stage.

Gaia has gone half way around the sun.

And so it's two eyes.

Twice the radius of the earth's orbit apart.

And that's how we do parallax.

All of this is a big version of your head.

The last data release from Gaia.

Was in December 2020.

And what's been really exciting.

Is that we've been able to get the distances.

And the motions of the star.

To a much better level of accuracy.

Most of the stars in the disc of the Milky Way.

All move in the same direction.

Rotating clockwise.

Around the centre of the galaxy.

And one of the most exciting things.

That came out of the first data release.

Was that a large sample of stars.

Were found.

That seemed to be rotating in the opposite direction.

To the majority of stars in the Milky Way disc.

And that's really surprising.

They probably came from a different galaxy altogether.

So they're almost these alien stars.

That have been brought in.

Alien stars from galaxies.

That long ago.

Shared our own corner of the universe.

The important thing to know.

About our galactic neighbours.

Is that nothing's actually sitting still.

We're all moving towards or away from each other.

And we're sort of playing a dance.

Out there.

And driving the dance of the galaxies.

Is the universe's most elusive.

Form of matter.

Dark matter is something that has gravity.

But produces no light.

It surrounds us.

In fact it dominates the mass.

In our own galaxy.

And yet we don't know what it is.

We can't see it.

We can't feel it.

We were able to start measuring very accurately.

The way stars move.

From radial velocities.

That's just towards and away from us.

And this allowed us to measure accurately.

For the first time.

How the dark matter was distributed near us.

The team have pieced together.

How dark matter orchestrated.

A series of galactic collisions.

That spanned billions of years.

Dark matter is really important.

In galaxy collisions.

Because it's so abundant.

So it's really driving the gravitational interaction.

Between the galaxies.

It is dark matter that determines.

How violent a collision is.

How rapidly.

With what intensity galaxies.

Come together when they collide.

In many ways it determines.

How galaxies end up.

After a collision.

So the thing that Gaia showed us.

Is not that it's plausible.

That this happened.

It showed it did happen.

It happened in just this way.

So it's not speculation anymore.

It's quantitative science.

The galaxy is a dynamic thing.

It's a living organism.

If you want.

It is breathing.

It is changing.

It is transforming.

It's all coming together in the end.

To tell us about how we got here.

And what our place in the universe really is.