All language subtitles for Universe S01E03 The Milky Way - Island of Light

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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