Original subtitles

I can see everything quite clearly.

it has a stark beauty all its own.

magnificent desolation.

beautiful. beautiful.

ain't that something?

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've got a particular wide perspective, you might say,

'I come from planet earth.'

but what is the largest structure that we could

legitimate call home?

well I would argue it's that.

that faint arc of light that stretches

across the sky from horizon to horizon.

It's 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 disc...

..that's existed since the universe was young.

Only now are we able to explore its history.

low rumbling

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 paths 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 in large galaxies

is over two million light years away.

so it certainly feel 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, are 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 1%.

over 1.5 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 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.

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 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 detail 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 earl 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 its had an interesting history

can be seen in this animation.

you can 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 fling all over the place.

and that tell 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 gravitational 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.

earl 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,

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 swing around in the sky...

..just close to the planet 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. Its 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 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 -

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, if you imagine that our sun...

..let's 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 hill.

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 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.

think about that - the light began its journey 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 its journey when there were Neanderthal

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, its 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,

it was torn apart in the impact.

Sagittarius dwarf brought fresh supplies of the vital ingredient

for star birth.

that is the sound for the most common element in the universe.

this radio telescope is pointing towards the milky way,

which has just risen above the horizon over there

behind the clouds.

and what you're listening to this 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.

I 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.

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 date 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 under way.

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 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 for 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, 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 feel 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 by- 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 south-west,

the remnants of Sagittarius dwarf, the debris from that collision

is 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 rival our own in size.

the 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 the 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 lift-off of space shuttle Atlantis

on a final visit to enhance the vision of Hubble...

... into the deepest grandeur of our universe.

standing by for srb separation.

the Hubble space telescope

is in its fourth decade of operation.

It’s 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 Andromeda stars,

but also the motion of the galaxy itself.

and we now know that the entire galaxy is heading towards us.

that's 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 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 gravity.

but Hubble has allowed us to see even further into the future.

It looks out far beyond the local group

towards the edge of the observable universe,

and seeing 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.

this story of our galaxy, the milky way, how it started,

how it was formed

and how it 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. the biggest camera ever put in space, actually.

it can observe the positions of stars so accurately

that you could see the edge of an euro coin on the moon from earth.

and that's just mind - boggling.

it was a beautiful launch, really spectacular.

then they 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.

applause continues

Gaia works by measuring parallax.

this is exactly the same way your eyes and brain work,

so that you can tell how far away something is

because of the slight difference and 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 round 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 this is a big version of your head.

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.

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 there are 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 touch 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 the 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.

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.

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.

next time - we explore our galaxy's supermassive black hole.

a monster that can destroy worlds...

..stop time

and is forcing us to reassess our understanding of reality itself.

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