Nova (1974–…): Season 48, Episode 18 - The Universe: Milky Way - full transcript
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."
♪♪
♪ See me when I float
like a dove ♪
♪ The skies above are lined
with trees ♪
♪ I'm on my knees,
begging please ♪
♪ Come and take me away ♪
♪♪
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.
♪♪
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."
♪♪
♪ See me when I float
like a dove ♪
♪ The skies above are lined
with trees ♪
♪ I'm on my knees,
begging please ♪
♪ Come and take me away ♪
♪♪
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.
♪♪