All language subtitles for Universe 03.eng

I can see everything quite clearly.

It has a stark beauty all its own.

See me when I float like a dove.

Skies above...

Magnificent desolation.

- Beautiful view.
- Isn't that something?

Take me away.

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 "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.

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.

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 7, 000th of 7%,

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 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 every star,

we can rewind their histories,

travelling backwards in time
through billions of years.

Gala 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 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 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,

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.

Gala-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.

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 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 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's 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 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 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 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.

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.

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 Andromeda'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.

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'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.

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.

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 is just mind-blowing.

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.

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
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.

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 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.

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.

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 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.

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.