Original subtitles

Neil Armstrong: I can see everything quite clear! Y.

It has a stark beauty all its own.

♪ See me when I float like a dove

♪ skies above...

B uzz a ld ri n: Magnificent desolation.

- Beautiful view. - Armstro n g: Isn't that something?

♪ Take me away ♪

Brian Cox: 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 home town... ”

Or ”m y city, ” or ”m y 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 re vealed 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.

Only now 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,

and how, 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 neighbouring large galaxy is 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.

Cox: One mission, more than any other,

has deepened our understanding of the galaxy,

a spacecraft bearing the name of an ancient Greek goddess...

Man: Everything functioning beautifully.

Cox: Gaia...

Man: Coming up on separation of the boosters.

Cox: Ancestral mother of all life on earth.

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

Cox: Ga ia'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 7, 000th of 7%,

over 7.5 million stars every hour.

Almost two billion in total so far.

To create a map like nothing ever seen before.

Cox: The gaia data is by far

the most detailed star map e ver produced,

a revolution in our understanding of the milky way.

This is the data, and it looks like a,

you know, 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 is 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 e very 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 its own gravity,

and for the stars to ignite.

Cox: New stars formed in their billions,

bound together by their mutual gra vitational 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

remained a subjective 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.

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 gra vita tional pull drew them together,

forming what we now know as the local group of galaxies,

our home archipelago.

Six billion years before the earth formed

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.

Like, over there,

close to the bright star, vega, is kepler-444.

The faint ancient star and planets orbiting around it

that's witnessed pretty much the entire history

of the milky way galaxy.

And then, maybe swinging around in the sky,

just close to the plough constellation that everybody can recognise,

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 got 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 burn bright 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,

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."

You know, that phrase puts images of Hollywood disaster movies into the mind,

of 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.

But 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 the ga ia 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.

If you 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 itsjourney to your eye

from the most distant stars when the pharaohs ruled Egypt.

And then, if you look out to the milky way,

to the shores of our galaxy,

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

I mean, that light began itsjourney

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 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 star birth.

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

This radio telescope is pointing towards the milky way,

as she'sjust 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 has been 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.

And 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.

As 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 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.

The data is not 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, a 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.

I mean, you, 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 are accidental b y-products 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.

And 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 neighbourhood, 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 consolations 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.

Man: Three, two, one, and lift-off of space shuttle Atlantis,

on a final visit to enhance the vision of hubble

into the deepest grandeur of our universe.

Woman: Standing by for srb separation.

Cox: 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,

2.5 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 a ndromeda's stars,

but also the motion of the galaxy itself.

And we now 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.

Cox: 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 of the 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 gra vity.

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 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.

Vasily belokurov: 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.

Das: 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.

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.

Gilmore: 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.

Das: 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 is just mind-blowing.

Gilmore: It was a beautiful launch. Really spectacular.

And 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.

Man: There's the good news.

Gilmore: 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 halfway around the sun.

And so, its two eyes are twice the radius of the earth's orbit apart.

And that's how we do parallax. All it is is a big version of your head.

Das: The last data released 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.

Michelle Collins: Most of the stars in the disk 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 disk, 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.

Cox: Alien stars from galaxies

that, long ago, shared our own corner of the universe.

Das: 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.

We're sort of playing a dance out there.

Cox: 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 touch it. We can't feel it.

Gilmore: 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.

Cox: The team have pieced together how dark matter orchestra Ted

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, and with what intensity

galaxies come together when they collide.

In many ways, it determines how galaxies end up after a collision.

Gilmore: 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 any more. It's quantitative science.

Rosario: The galaxy is a dynamic thing.

It's a living organism, if you want.

It is breathing. It is changing. It is transforming.

Collins: 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.