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The Milky Way, our home,

formed not long after
the Big Bang.

One of trillions in the
universe.

This is our galaxy.

Billions of planets orbiting
billions of stars.

We are only just beginning
to understand its true place

in the universe.

It was only a hundred years ago,

people thought our Milky Way
was the entire universe.

If we really want to
understand where we come from

and how the galaxy was formed,

we can't just look in our cosmic
sort of backyard.

We need to look much further
afield.

And when we do,

we discover a universe
in turmoil.

Our history
is made up of multiple collisions

and interactions
with our neighbors.

Our Milky Way is not static.

It is dynamic and it has

such a rich, dynamic history.

And our place in it

far from secure.

A collision
can change the structure of a galaxy,

reorders the stars,
so you end up with something

that looks different,
that behaves differently.

Now we can see our
galaxy's future and its inevitable end.

The Andromeda
galaxy is actually heading towards us

at about 250,000 miles per hour.

It will be a
really nice sight, actually.

You know, just watch it coming.

I mean, there's nothing
you can do about it

except sit back
and enjoy the view.

It's all coming
together to tell us about how we got here

and what our place
in the universe really is.

"The Milky
Way," right now, on "NOVA."

Above us in the night sky,

visible all around the world,

the Milky Way wraps its arms
across the sky,

a band of stars like no other.

When
the Milky Way is up overhead,

the skies are so brilliantly
bright that I swear

the, the band of the Milky Way,
the disk of our own galaxy,

quite literally casts a shadow.

Our Milky Way is this
really incredibly beautiful place.

It's this wonderful collection

of beautiful stars, gas,
and dust that all kind of

swirls together,

almost like an abstract
painting.

We've been trying

to understand the band of stars
that stretches across

the night sky since the time
of the ancient Greeks.

Humans have been looking up

at the night sky
since the dawn of time

because we want to know
what's out there.

Because the story of our galaxy

is the story of every one of us.

How does it all fit together?

What are we part of?

Can we understand it?

The Milky Way
galaxy takes its name from the dense

band of stars that we see
from Earth,

when in fact, it's a structure
that entirely surrounds us.

Every star in the sky is part of
it, including our sun.

When looking into the night sky,

you would see this band of stars
stretched across it,

which actually corresponds
to the disk of the Milky Way.

So we actually live
inside the Milky Way.

Our galaxy is a spiral galaxy.

And we can build up
this picture,

which we have been doing for

hundreds of years so far,
since the first astronomers,

like Galileo, to, to kind of
build up this beautiful picture

of our Milky Way.

Right in the
center, you have a bulge.

Then you have a pancake-like
structure.

That's the disk,
and that's where we are.

And then further out,
you have a faint halo of stars

that goes quite far beyond
the disk.

It's this
beautiful spiral structure

of hundreds of billions of stars
all orbiting around

a supermassive black hole right
at the center of the galaxy.

The Milky Way's
complex structure has taken

billions of years to evolve.

And yet, it's one of the most
familiar forms in nature.

So let's start
at the very center,

and in the center, there is
a very old bulge,

contains most of the old stars.

And this is the remnants
of the first stars that formed

in our part of the universe.

Right at the very heart of it,

there is a
supermassive black hole.

That is the core of the
Milky Way as we know it.

And then around that's
the bulge,

and then there's this
big bar structure,

mostly old stars,

and that's what drives
the spiral arms.

And so we can then say,
"Where are we in all of this?"

We know pretty well where
the sun is.

And hey presto, one sun,
and it'll be about there,

roughly halfway from the center
to the outer

spiral arm structures, and this
is where the sun lives today.

The Milky Way's elegant spirals

are the signature
of its dynamic history.

The challenge is how to observe
it and tease out that history

from our position on the inside.

One of the problems
of trying to study the Milky Way

from our position here on Earth
is that it's really hard

to get a sense of what
the galaxy looks like overall.

So, if we really want to
understand where we come from

and how the galaxy was formed,
we can't just look in

our cosmic sort of backyard.

We need to look much further
afield.

Clues to how the Milky Way

formed and evolved emerged

in the 1990s, with the launch of

the most ambitious
space telescope at the time.

Five, four, three, two, one.

And lift-off of
the space shuttle Discovery

with the Hubble Space Telescope.

Our window on the universe.

Standing by for SRB separation.

Both solid rocket boosters
have separated.

The Hubble Space Telescope
was one of the greatest feats

in space missions
of human history.

This 2.4-meter piece of glass,

we've turned it on our universe

and it has enabled
untold advances.

Images from Hubble

transformed astronomy...
transformed science.

Hubble isn't
just focused on the Milky Way.

It also looks beyond,
much deeper into space.

The data from Hubble
is unsurpassed.

It gives us the sharpest views
of galaxies

and the distant universe.

Hubble's a little
bit like a time machine.

It's able to pick up light
from galaxies

that come from very far away.

And because they've come from
very far away,

we're looking at them
a completely different time,

far back in time.

To look far back
in time, Hubble trains its gaze

on one tiny blank patch of sky

for over 11 days.

What appeared was pretty
incredible.

We were able to
see galaxies in this ultra-deep field

that is farther away
than we've ever, ever looked.

So it's really given us an idea

of how many galaxies
there are out there

and the variety of galaxies
out there.

It's a very
hard number to estimate,

but it is absolutely
in the trillions.

Their morphology
can be incredibly complex:

big train wreck mergers

or absolutely, stunningly,
beautifully round

grand design spirals

and everything in between.

There are starburst galaxies

that are generating new stars
at prodigious rates

and there are small galaxies,

which are my favorite.

We call them dwarf galaxies.

And they may be thousands
of times less massive

than the Milky Way,

but they're actually the most
common galaxy in the universe.

Hubble tells
us there are trillions of galaxies

in the universe.

And by focusing on the ones
that are the farthest away,

it looks deep back in time,

giving us a picture
of what galaxies look like

in their infancy.

And they started forming in an
era of immense cosmic activity.

Not long after the universe
began.

Before the Milky Way forms,

space is filled
with a vast structure

known as the cosmic web.

Hydrogen and helium gas collect
along the web's filaments.

But the web itself is made from
something more mysterious.

It's called dark 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, we can't touch it,
we can't feel it.

Galaxies really need
dark matter because it's kind of like

the glue that binds them all
together.

You can almost say it's like
the seed of galaxy formation.

It creates these huge structures
into which ordinary matter falls

and then that matter
all gets compressed

and can turn into stars.

And that really is, then,
what seeds galaxy formation

as a whole.

The first stars
are born where the filaments cross

and dark matter is at
its densest,

drawing large amounts
of gas together

until it collapses under
its own gravity.

Causing stars to ignite.

New stars in their billions
are bound together by gravity,

orbiting a common center.

These are the first galaxies.

Among them, the Milky Way,
in its embryonic form.

A whirling disk of gas and stars

surrounded by an invisible halo
of dark matter.

Across the universe,

hundreds of billions of galaxies
are forming.

Some... a few dozen...
are born very close

to our own Milky Way.

Over time, gravity draws
these galaxies ever closer

to form what we know as
the local group.

Our local group is a
set of galaxies that lies in a volume

of the universe that we believe

is gravitationally bound
together.

Meaning that these galaxies are
close enough that at some point

they might all combine together
or collide together

to form one big, large galaxy.

The galaxies within
the local group can all feel

one another's gravity,

so they're all sort of slowly
moving together with time.

Just three billion
years after the Milky Way began,

it rises in the night sky
of its first planets,

but with only half the stars
and a more irregular structure

than the mature galaxy
we see today.

So how did our galaxy
get its spirals?

To answer the question,
a new spacecraft is built.

Gaia will look directly
at the Milky Way itself.

Its designers are determined
to overcome an age-old problem:

how to measure the true distance
between stars.

Being able to
determine the distance to objects

is one of the most
fundamental things

you need to do to understand
the structure of our universe.

To measure
the distances accurately,

Gaia's engineers must devise
an orbit for the craft

big enough that it can measure
the same star from two points

very far apart,
called a parallax measurement.

Gaia will need to travel

almost a million miles
from Earth.

Attention
pour la décompte finale.

Dix, neuf, huit,

sept, six,

cinq, quatre, trois,

deux, un, top... décollage.

I've been involved in Gaia since
the very beginning of it.

It was a beautiful launch,
really spectacular.

The spacecraft shares the name

of the ancient Greek
Earth goddess, Gaia.

It took four minutes.

You could see the flame
of the rocket

and you could see the individual
stages popping off.

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.

Gaia's mission
is to map the true positions

of a billion stars
in our Milky Way...

nearly all of them
for the first time.

Before Gaia, we just looked at

the images of our galaxy.

We were missing half of
the information.

Gaia is the first-ever

precision distance
measuring machine

that mankind has ever had.

So how is it possible
for Gaia to map the Milky Way

so accurately from within?

First, it travels to its
distant vantage point

called L2,
a gravitational sweet spot.

It can hold here
with minimal fuel use

as it follows the Earth in its
extensive orbit around the sun.

Astronomy has always
been at the forefront of technology,

but the kind of technology
we work with right now

is absolutely amazing.

With just a whisper
of nitrogen to help Gaia's telescopes

sweep smoothly
through 360 degrees

four times a day,

it makes over one-and-a-half
million observations an hour.

After four months,

it has looked at the whole sky
at least once.

Gaia gathers data
on the brightest stars

across the whole sky...

stars within the disk
of the galaxy,

from the center to the halo
and beyond.

After it has traveled millions
of miles in its orbit,

it observes the same stars
from a different vantage point.

After nearly two years
of almost non-stop sky-scanning,

scientists can triangulate
the true position

of over a billion stars

for the most accurate map
of the galaxy ever created.

The Gaia map.

The Gaia data has allowed us

to see our own galaxy
like never before.

I think that Gaia opened up

a really new axis of information
to us

that we just have never imagined
it would do.

These are like having
completely, you know,

revolutionary cartographers

make an entirely new map
of our home galaxy.

Finally,
astronomers have their Holy Grail:

the Milky Way,
mapped in three dimensions.

This is our first-ever
honest 3D picture of the Milky Way.

It's not a simulation
from a computer

and it is not an, an attempt at
guessing the structure

from approximate data.

Every one of those stars
is individually measured

to high precisions.

So this means that we can
move ourselves around

through this and see, well, what
does this bit of the Milky Way

actually look like?

And you decide you want to look
at it from far away,

and you can do that.

Or you can zoom in close
and say, "I want to know

"how that star cluster works.

I'll go and sit inside it."

Gaia can tell the difference
between a star

that's at the front of
that cluster

and a star that's at the back
of that cluster,

even though the cluster itself
is 5,000 light years away.

Gaia is not only measuring where
things are

to delightful precision,

but equally,
you can see things moving.

And it's actually the moving
that's the critical bit.

In addition to mapping
stars in three-dimensional space,

Gaia captured another dimension,

the result of its repeated trips
around the sun:

time.

This data could help us
understand

how our galaxy evolved.

Gaia doesn't just tell
us where the stars are in the sky,

but also how fast they're moving
across the sky and towards us,

and that's an essential bit

of information to understand how
things change over time.

Once scientists
know how a star is moving,

they can use Newtonian mechanics

to calculate where it is going.

And using the same calculations,
they can reverse the motion

of the star to uncover
where it has been.

This new data is revolutionizing
a field of science

known as galactic archaeology.

Galactic archaeology
is the process of identifying

the history and the motion
of stars

so you can figure out
where stars come from,

how old they are, and how their
motions change over time.

What's been really incredible
about Gaia is, if we couple it

with spectra that we're
observing back on Earth,

we're able to date the stars
and really use them

as the fossils that they're
supposed to be.

So this means we can work out
what the fossils tell us about

the evolutionary events that
happened in the Milky Way's past

and then date them.

So put them in
chronological order.

So we combine everything
together in order to get

a really clear understanding
of how the Milky Way came to be.

This new data from
Gaia has helped scientists spot a pattern

between the Milky Way

and our neighborhood cluster
of galaxies, the local group.

The important thing to
know about our galactic neighbors

in the local group
is that nothing's actually

sitting still.

Gravity means that we're all
moving towards or away

from each other and we're

sort of playing a dance
out there.

Gravity is

the great cosmic attractor.

This dance of the galaxy
and its neighbors

have been going on
for billions of years.

Gaia is only
just now revealing the steps

to this intricate
intergalactic dance.

When the Gaia satellite
started producing its data,

and astronomers started
analyzing this data,

there was something rather
curious.

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 unusual.

And it was really surprising.

So that
means that not all of the stars

that make up our galaxy,
the Milky Way,

were actually born here.

They probably came
from a different galaxy altogether.

So they're almost these alien
stars that have been brought in.

Gaia's data led scientists

to make an astonishing
discovery.

So the most mind-blowing thing

is that those stars
are the remnants

of a humongous collision,

and they actually come from
another galaxy.

If we could travel
back in time ten billion years

and land on one of the earliest
planets within our Milky Way,

we'd see something spectacular
in the night sky.

Billions of stars coming into
view heading towards us.

The Milky Way is about
to collide

with another galaxy from
our local group.

Called Gaia-Enceladus.

A quarter of the size
of our galaxy,

Gaia-Enceladus is drawn
into the Milky Way,

bringing disorder to its
flat disk.

When you look at
a galaxy merger,

it looks like an incredibly
violent process,

but it's actually something
that's incredibly elegant,

and that is because galaxies
are, ultimately,

mostly empty space.

And so when galaxies collide
or crash together,

they pass through one another
like ghosts.

The chance for a star-star
collision in a galaxy merger

is actually exquisitely low.

It's really quite
a beautiful process,

because the way in which
the mutual gravity

of these two galaxies actually
interact with one another

causes one to start
sort of spiraling around.

Once it, you know, plunges in,
it spirals around it,

and then comes back and returns,
so it's kind of like, you know,

two objects in a sort of
celestial ballet

around one another.

A collision can
change the structure of a galaxy,

reorders the stars in the
galaxy, gives them new orbits,

moves the gas
into different places.

And so you end up with something
that looks different,

that behaves differently.

The invisible
driver of all these interactions

is the same stuff that formed
the galaxies in the first place:

dark matter.

Because it accounts
for most of the gravity in the galaxy,

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.

Just a few billion
years after the Milky Way formed,

already much more massive than
Gaia-Enceladus...

the Milky Way's gravity
overwhelms its neighbor.

Absorbing it entirely.

The Milky Way is bigger by
a billion stars.

For the first time ever,

we have seen how our Milky Way
has grown bigger.

What we've
learnt from this collision

is really about how much richer
our galaxy grew,

but it doesn't actually tell us
about us yet.

To find out
how our solar system got here,

scientists have been
tracing the history

of another unusual group
of stars.

They loop around our
galactic disk

in a spectacular trail called
the Sagittarius Stream.

So the Sagittarius
Stream is really interesting

because it might actually help
us understand

where we came from.

It is what's
known as a tidal stream,

which is a stream of stars

that have been stretched
across the night sky

due to the gravity of the
Milky Way.

The
Sagittarius Stream is so big,

it goes all the way up and even
all the way down,

so we can just carry
the Milky Way from its handle.

It's really, really large
stream.

The trail of stars we see today

is named after the galaxy that
they used to belong to,

Sagittarius Dwarf.

The Sagittarius galaxy

was discovered by a student
and myself in the '90s.

Most of the Sagittarius galaxy
is actually spread out

into two streams, one in the
front and out the back,

like giant comet tails wrapping
around the entire sky,

going out for maybe
100,000 light years away.

We could see these,

but it wasn't possible to
understand how they got there.

Now, with Gaia,
we have motions of these stars,

so we can see what direction
they're moving in,

which ones are going fast,
which ones are going slow.

For the first time ever,

it's been possible to say,
"Ah, this is what happened!"

The Sagittarius Stream
is essentially the tidal debris

that has been left over when
a dwarf galaxy,

the Sagittarius Dwarf, actually
plunged into the Milky Way.

By studying the stream of stars,

scientists have uncovered
the story

of a much more recent
galactic collision.

This time,
with a much smaller galaxy.

When the Sagittarius
galaxy orbited into the Milky Way,

it came, foolishly,
rather far in.

As it dives
towards the Milky Way,

the dwarf galaxy begins
to have its stars pulled off.

When it goes through the disk,

it punches a hole in the disk,
and the stars get

put in particular patterns.

And it's got stretched into
these two great long streams.

The much smaller
galaxy encroaches upon the Milky Way,

just like Gaia-Enceladus did,

but the timing is intriguing,

because this collision happens
just before

the birth of
our own solar system.

One of the most important
consequences of galaxy mergers,

like the destruction of the
Sagittarius Dwarf galaxy

by the Milky Way,

is a new, fresh injection
of gas into the galaxy, right?

And it is gas,
particularly cold gas,

that is the fuel from which
all stars are born.

For star formation to occur,

basically,
the colder, the better.

The most important
gas that the collisions bring

is made of one of the oldest

and most ubiquitous elements
in the universe.

So what I'm listening to here
is the lifeblood of our galaxy,

hydrogen.

We can detect it with
our radio telescopes,

like in this case,
pointing right at the Milky Way.

Hydrogen is the most common
element in the universe,

and it's in our own galaxy.

We don't see gas with our eyes,

and therefore we are
not used to the idea

of there being plenty of gas
in the Milky Way.

But if you use a radio
telescope, you can see it.

You can look at the radiation
coming from that gas,

and that's exactly what
we're doing right now.

This gas is connected
to stars deeply...

it's what stars form from.

If this gas wasn't there,
stars would never have formed.

Hydrogen was created
shortly after the birth of the universe,

and it has always been spread
throughout the Milky Way.

But not evenly.

It clumps together in
dense clouds

that, in this iconic image,

extend up to 30 trillion miles.

Scientists call them
stellar nurseries,

where temperatures are
low enough for gas to condense.

Stellar nurseries

are some
of the largest, coldest,

and certainly among the darkest
regions within any galaxy.

If you were to fly through
a stellar nursery,

it would be extremely cold,
and it's an extremely

turbulent and chaotic place,
pervaded by magnetic fields,

and, and charged particles
streaming throughout.

It might be glowing
a little bit,

and as you approach
closer and closer,

you would realize that
it's actually heating up a bit.

It's, it's actually becoming
warmer.

You would perhaps surmise that
this is where

a new group of stars is
being born.

Hydrogen can be thought of

as the lifeblood of galaxies,

because it's the first building
block of stars.

In the center of a star,
it's fusing hydrogen together

all the time to produce helium.

And that gives off energy,
which allows the stars

to light up.

When the
Sagittarius Dwarf galaxy

collides with our Milky Way,

it brings more hydrogen
to these clouds,

triggering a new era
of star birth.

When galaxies
interact with one another

and they collide with one
another,

what typically happens is that
you actually get

a big burst of star formation
occurring.

And that's primarily because

you are essentially bringing
in a new source

of star-forming fuel into the
Milky Way.

This era coincides
with the birth of our own sun,

4.6 billion years ago.

The jury's still out,
but we think that the sun

could have formed in that first
enhancement in star formation.

The timing of the
collision between the Milky Way galaxy

and the Sagittarius Dwarf galaxy

coincides with a peak in
star formation

that we see happen in our
Milky Way.

And we know that
the age of the gas

in which our solar system
was formed

lies very close to this spike
in star formation.

It is certainly possible, right?

That our own solar system

is anchored around a star

that was born from gas that
did not originate

in our home galaxy...
it was taken, it was pulled,

or consumed by the Milky Way

when it ripped apart
a satellite galaxy,

maybe even
the Sagittarius Dwarf.

For a small galaxy,
Sagittarius Dwarf has had a big impact,

and not just by triggering
star birth.

It plunges back and forth
through the Milky Way

as the galaxies become enmeshed,

which likely contributed to the
formation of the spiral arms.

But its influence is
fast fading.

The question as to
whether the Sagittarius Dwarf galaxy

is still around
kind of depends on

what you kind of end up thinking
of as being a galaxy,

after a certain point.

It is really a galaxy that is

in the process of being
totally disrupted.

And one day it will end up
merging

with the center of our galaxy.

So in some sense, it's only the

sort of memory of the galaxy
that is left behind.

When we
look up at the night sky,

it's easy to think of
the Milky Way as static.

But we now know it's evolved
through a turbulent history

of collisions and mergers.

I think that Gaia
opened up this whole new vision for us.

Our Milky Way is not static.

It is dynamic and it has such
a rich, dynamic history.

But none of it is random.

The force that causes galaxies
to form,

merge,

and evolve

is gravity.

The thing that ultimately
sculpts

how those galaxies look
is gravity.

It's not the collisions.

It's the stars within those
galaxies tugging on one another.

And it's the underlying
dark matter halos

of those galaxies
merging together.

So we're actually at a
really exciting time now in astronomy.

Because we can tell the story

not only of how our galaxy
came to be,

and how everything led up
to now,

but we can also start to peer
into the future

and see what's in store,
what's yet to come

for the evolution of our galaxy.

The more
we learn about the Milky Way

and its dynamic history,

the more incredible
it seems that we ourselves,

orbiting just one star
among billions,

have been able to figure out
our galaxy's story,

written in the stars.

And we are now poised
to map out its ultimate fate.

The Milky Way is no
stranger to galactic collisions.

As we look around the night sky,

we see evidence that our
Milky Way galaxy

has had these interactions
with galaxies before,

but what's coming next
is something

on an entirely different scale.

This faint smudge of light
that you see right there

in the center of the image,

it's not some condensation
on the lens

or a cloud in the sky above us.

This is an entire other galaxy,
a huge galaxy,

two-and-a-half million
light years away from us.

To put that into units

that humans can try to
understand,

this faint smudge of light
is about

15 billion billion miles away.

This galaxy is called Andromeda.

And is set to play a defining
role in our galaxy's future.

The Hubble Space Telescope

has taken extraordinary images
of Andromeda.

Compared to the disk of the
Milky Way,

Andromeda seems tiny,

when in fact, it's anything but.

It's our largest neighbor
in the local group.

With the same spiral structure
and the same long history

of feeding on smaller galaxies.

This image right
here is actually ridiculous

when you think about it.

It's an observation of part
of the Andromeda galaxy

taken with the
Hubble Space Telescope,

and the level of detail here
is incredible.

This image contains about
100 million stars

that we can see
in another galaxy.

It's just mind-blowing.

When we look at it,

we start to be able to
understand its structure.

And what strikes me immediately
is that it's kind of familiar.

If you zoom in on the
spiral arm,

it's exactly the same as what
we see

when we look into our own
Milky Way.

And when we look at the
Andromeda galaxy,

we see this history, we see that

it's been cannibalizing these
satellite galaxies

in a similar way to the
Milky Way,

growing into this beast,
this giant

that's a match
for our own galaxy.

We now have many
beautiful images of Andromeda.

We've studied it with a huge
range of telescopes,

and in many ways, you know,
it's a lot like the Milky Way,

this beautiful spiral galaxy.

So you might think that they're,
you know, going to be

very similar galaxies
with a very similar history.

But what we've learnt through
studying Andromeda over time

is that actually,
they're not quite the same.

In fact, Andromeda
is 50% bigger than the Milky Way.

And that's not all.

The Andromeda galaxy
is actually heading towards us

at about 250,000 miles per hour.

In about four-and-a-half billion
years' time,

that faint smudge of light
we saw in the sky

will collide with
the Milky Way galaxy,

changing our galaxy forever.

The Milky
Way as we know it today

is not eternal.

And Earth will witness
the final act.

Two galaxies in a single sky,

gradually, but inevitably,
merging into one.

There is absolute
evidence that Andromeda

is going to collide with the
Milky Way one day,

because they are pulling each
other closer

and closer over time,
and one day,

they're just gonna come so close
that they will collide.

Andromeda and the Milky Way,

when they come together,
sparks fly.

It's going
to be an incredible time.

If we were able to view
this collision happening,

it would be amazing to watch
the night sky change over time.

It'd be a
really nice sight, actually.

You know, just watch it coming.

I mean, there's nothing
you can do about it

except sit back
and enjoy the view.

We'll end up smashing
these two galaxies together.

There may be a huge burst
of star formation initially,

which will sort of light up the
night sky with fireworks.

And then over time,
that will sort of

burn off all the remaining gas
we have in those two galaxies.

But unlike
in previous collisions,

this time, our galaxy is the
smaller of the two.

Andromeda and the Milky Way

pull at each other's
spiral arms...

Scattering stars...

Until no trace of the original
structures remain.

Two spiral galaxies,

merged into one colossal mass
of stars.

Watching the motion of galaxies

is like looking at a really,
really exquisite ballet

in really, really slow motion.

When that dance
is finally complete,

the structure of the Milky Way
will be forever altered.

While this collision
will extinguish the Milky Way

and Andromeda as we know them,

it will also create
a whole host of new stars,

and around those new stars,
there'll be new planets,

and maybe another generation
of people

asking the same questions
that we're asking now.

Where have they come from?

What's their place in the
galaxy?

And what's going to happen
in their future?

We will not be
able to see the beautiful galaxy

that we see right now,
but the universe will carry on.

As we look
even deeper into the future,

all of the galaxies in our
local group

will eventually merge into one
enormous entity.

Floating in isolation.

As the universe expands,

the distance between all the
galactic groups will increase

and the other galaxies will
simply disappear from view.

Knowing that we
can sort of look into the future

many billions of years
and understand

what will happen to our galaxies
is mind-blowing.

And all of
this we have determined

by looking up at the skies from
one tiny,

unremarkable outpost in the
Milky Way.

Even though we as humans
have such an insignificant role

in the grand scheme of things,

there is so much about the
vastness of space

that we can understand just
from our

unique perspective here on
Earth.

Earth is a tiny little rock

in a really indescribably vast
cosmic ocean, right?

We are just a tiny little planet
spinning in the void.

But the story of our
night sky is far from being complete.

And there is so much more
to discover.

Is there life in the universe?

And has there been life in the
universe

from the very beginning?

What is dark matter?

What is dark energy?

How does it affect our universe?

Particularly, how does it affect
our Milky Way

and even our own solar system?

We want to
know where we come from.

We want to understand
our origins and our destiny.

And also, we just love
a good story.

We love mystery.

And the story of the universe is

the greatest story of all.