All language subtitles for The Sky at Night BBC 2018 Guides Galaxies 1080p HDTV_Subtitles01.ENG

The Milky Way.

Our home galaxy.

A glorious ribbon of stars...

..smeared across the night sky.

Almost unimaginably vast.

Home to hundreds of billions
of stars.

And yet just one of
100 billion galaxies

that make up the observable
universe.

Galaxies are the building blocks of
the cosmos.

Within them, stars and planets
form...

..and die.

The Sky At Night has covered every
major space science discovery in

more than 60 years of
occasionally quirky broadcasting.

Line your finger up with my nose.

It's absolutely tremendous.

I'm going to do just a little
mathematics.

Please don't be frightened.

Now we're going to use that archive
to tell the story of galaxies,

from their earliest origins to their
very end.

And in the process, answer some of
the biggest questions

in the universe.

Good evening. Good evening.

Welcome. Welcome...

..to The Sky At Night
Guide To The Galaxies.

Good evening. I want to begin by
showing you

a rather splendid picture.

This is a galaxy, a system made up
of thousands of millions of stars.

And we must remember that we live in
a galaxy.

Our sun is a very humble member of
the Milky Way system.

We take for granted today that the
Earth and the solar system lie

within a huge galaxy.

And that that galaxy is just one of
billions of galaxies.

But that knowledge was hard-won.

Just 100 years ago,

we didn't know for certain that
galaxies even existed.

Astronomers believed the Milky Way
was the entire universe

and that nothing lay beyond.

MISSION CONTROL: And we have
liftoff!

The quest to discover the galaxies
has been the story of how we found

our place in the cosmos.

It's also the story of some of the
most extraordinary technology

ever developed.

And it's led to some
profound questions.

Why do galaxies exist?

What are the forces that shape them?

And what will be their ultimate
fate?

The discovery of the galaxies begins
around the start of

the 20th century, when astronomers
began to look more closely at

patches of cloudy light in the night
sky that they called nebulae.

Telescopes had revealed hundreds
of them.

But one in particular became the
focus of attention -

the Andromeda nebula.

In 1978, The Sky At Night looked
back at how Andromeda revealed

the true nature of the galaxies.

I'm delighted welcome to The Sky At
Night for the first time, but I'm

sure not the last, Heather Couper,

who is a lecturer in astronomy at
the Caird Planetarium in Greenwich.

Without the powerful telescopes that
we have today,

Andromeda appeared to astronomers as
nothing more than an irregular

smudge. They couldn't figure out if
it was just a nearby cloud of gas

and dust.

Or something more significant.

Well, the mystery was solved by
Edwin Hubble in 1923.

He was using the then-new 100 inch
telescope on Mount Wilson

and he was able to take pictures of
Andromeda and he confirmed that the

Andromeda galaxy was actually made
up of stars and not gas.

What was very exciting is that Edwin
Hubble also noticed that there were

some stars in the Andromeda galaxy
which varied in their brightness and

these were the so-called Cepheid
variable stars.

Now, a variable star is exactly what
its name implies.

It's a star which brightens and
fades absolutely regularly, and the

longer the period, the greater the
luminosity.

And you can certainly see the
importance of that,

because you can find out the
luminosity of a Cepheid merely by

watching it. And once you know how
bright it is,

then you can find out its distance.

What Hubble did was to find Cepheids
in the Andromeda spiral,

study their periods, and find out
how far away they were.

And, straightaway, he realised that
they were so remote that they could

not possibly be members of our own
star system or galaxy,

and therefore the Andromeda spiral
itself must be an external system.

And I think, you know, that was
probably one of the most important

astronomical discoveries ever made.
In fact, it opened up a whole new

field of astronomy, the study of
extragalactic astronomy.

Andromeda wasn't a local gas cloud,

but a galaxy like the Milky Way -

a huge system of stars rotating
about its centre.

Once astronomers knew this, they
realised that countless other
nebulae

must be separate star systems
as well.

Most of those splodges you can see
on screen are not stars

but whole galaxies.

Now the Milky Way Was just one of
hundreds of galaxies...

..each containing billions of stars.

The universe had just got
much, much bigger.

But galaxies themselves remained
mysterious.

We still didn't know what types of
galaxies were out there,

how they developed, or even why they
existed at all.

As we set out to understand more
about galaxies,

it was inevitable that we began with
our own home, the Milky Way.

Our first challenge was simply to
define it.

Its size, its shape, and its age.

One of the first attempts to
describe the Milky Way came when

astronomer William Herschel produced
a map that I got to see in 2017.

This is the first map of the Milky
Way made by William Herschel back in

1785, in a paper he called On The
Construction Of The Heavens.

It's a wonderful thing, but it's
not accurate.

It shows the sun at the centre,
which is wrong,

and there are no spiral arms.

Over the next 200 years, astronomers
gradually expanded their knowledge

of the Milky Way, and by the 1980s,
the basic facts were

well-established.

There are something like 100,000
million stars in our star system

or galaxy. And the galaxy is a
flattened system and the sun,

with its system of planets, lies

well away from the centre or
nucleus.

In fact, over 30,000 light
years away.

We've also found out that our galaxy
is a spiral form

and that's by no means surprising,
because there are plenty of other

spiral galaxies.

But there were still huge gaps in
our knowledge.

We had only a rough guess as to how
many stars there were in the

Milky Way, and although we knew it
was a spiral galaxy,

we didn't know how many spiral arms
there were.

Luckily, there were new telescopes
on the way.

Over the years, The Sky At Night has
followed the development of many new

telescopes.

This is the MMT,

or multiple mirror telescope, on the
very summit of Mount Hopkins in the

Santa Rita Mountains of Arizona.

I am standing on top of one of the
most remarkable telescopes in the

world. La Palma is now becoming
famous in another direction.

It's here that one of the world's
great observatories is being set up.

Just look at it. It's short and
squat, with a maze of

wires and girders. It's the first of
a whole new generation

of telescopes.

Bigger and better telescopes were
built above the clouds

to lessen the distortion caused by
the earth's atmosphere.

But still, astronomers wanted more.

To really see the Milky Way, we
needed a telescope

above the atmosphere.

MISSION CONTROL: And lift off of the
space shuttle Discovery with the

Hubble Space Telescope.

The Hubble Space Telescope was
launched in 1990.

But unfortunately, when the first
images came back from Hubble,

they were a disappointment.

It turned out there was a small but
crucial flaw in the optics.

In 2015, The Sky At Night looked
back at Nasa's attempt

to solve this problem.

Repairing Hubble would require one
of the most audacious space shuttle

missions ever conceived.

Five long spacewalks would be
conducted over five consecutive

days. In terms of complexity and
ambition, nothing like this had ever

been attempted.

To pull it off, the Nasa astronauts
spent over a year undertaking

the most immersive training
programme to date.

Telescope mock-ups were submerged in
a neutral buoyancy tank.

Logging more than 200 hours
underwater,

the astronauts repeated the hundreds
of complicated manoeuvres required

to fix the telescope until they
became instinctive.

On the 2nd of December, 1993, the
mission to repair Hubble launched.

MISSION CONTROL: And we have
liftoff!

ASTRONAUT: Houston, let's go fix
this thing.

Spacewalks are never routine and
Hubble rescue mission was going to

be harder than most.

Back on the ground, the team at
Mission Control held their breath.

Carefully, astronaut Kathy Thornton
manoeuvred the new optical device

into place.

It's a very big box going in a very
small hole with about an inch

of clearance going in.

And then obviously the clearances
got tighter.

Thankfully, it was a perfect fit.

RADIO: Good work, guys.

But now, scientists on the ground
had to wait for the first images.

On New Year's Eve, 1993,
they came in.

CHEERING AND APPLAUSE

EXCITED GASPS
We did it!

What was a blur was now
crystal clear.

And our view of the cosmos was
changed forever.

Hubble marked the beginning of a new
era in the exploration of the

Milky Way.

Over the next few years,

astronomers built up a spectacular
catalogue of extraordinary objects.

It contains clusters, nebulae,
colour stars, exploding stars

and even supernovae.

I've been very impressed
particularly by Hubble's pictures of

Eta Carinae.

Eta Carinae is a very, very massive
star, highly unstable,

and could one day, I don't know
when, go off as a supernova.

Look at this picture of the
Helix nebula,

which is simply an old star which
has thrown off its outer layers and

this material looks rather like a
shower of tadpoles, doesn't it?

It just shows the great power of the
Hubble telescope.

The Pillars of Creation.

That, to me, was... It symbolised
the first of the Hubble images that

started to come through

that just gave this completely new

vision of the cosmos and it was just

completely fascinating to think that

this was what you could do
with Hubble.

Well, it's the most amazing
telescope ever built and is now a

complete and utter success.

Hubble's images took us on a virtual
tour of the galaxy.

Our nearest stellar nursery, the
constellation of Orion.

Where you can see new stars forming
from collapsing clouds of

gas and dust.

It's just one image of one object,
but it reminds me how studying the

galaxies isn't just about abstract,
distant theory.

Each of us is a product of the
processes that shape the galaxies.

The sun, the Earth, and everything
around us

came from just such a place as this.

By the start of this century, we'd
catalogued an enormous variety of

strange and wonderful objects within
the Milky Way.

But we still didn't have a really
accurate map of the galaxy.

To get one, we'd need yet
another new instrument.

Enter Gaia, one of the European
Space Agency's latest telescopes.

In 2015, The Sky At Night visited
the laboratory where

its camera was made to learn a bit
more about what makes Gaia

so special.

Gaia is a wide field telescope
designed to scan the entire sky.

Instead of staring intently at
individual objects, it's designed to

give us a broad perspective.

It's armed with two telescopes that
focus light onto a sensor,

bristling with a billion
photosensitive pixels.

What we have here is a duplicate of
one of the imaging sensors that

makes up the Gaia array.

In the full array, we actually have
106 of these, so it's pretty

impressive. Each one of these
detectors is quite similar to the

sort of thing you'll find in a
consumer digital camera -

but with one difference.

The pixels in this are much, much
bigger.

The engineers have worked out that
with bigger pixels, you can gather a

lot more light. So much so that this
sensor is capable of capturing more

than 90% of the light that lands
upon it, whereas my camera at home

would be lucky to get 20%.

Gaia is expected to detect and
measure hundreds of stars

every second.

And it will revisit the same patches
of sky 70 times,

which will allow it to do something
extraordinary.

Not only will we have the most
accurate map of our little corner of

the galaxy, but also, for the
first time, we'll record how

the stars are moving and that is a
really powerful tool.

Gaia took decades of planning
before it finally

launched at the end of 2013.

But it was worth the wait.
When the data came down,

scientists were able to use it to
construct a map like no other.

A three-dimensional map of
the galaxy.

And in 2017, I got to see it
for myself.

So, Chris, welcome to our new map of
our Milky Way.

It's beautiful. You can see
immediately the sort of big picture

structure. The first thing to notice
is this is a map of the entire sky.

But this stripe across the middle,
this bright stripe,

that's the galaxy. That's the
Milky Way Galaxy, that's right.

What we need to do is zoom in.

This is the top layer map

and now we're zooming in.

These are the inner parts of the
Milky Way here,

these big dust clouds and dust
lanes.

These are dust lanes and you start
to see that all the white light is

now breaking up into stars,

into individual stars.

As you get further in, as we zoom
in, we see more and more stars

individually. Absolutely. Look at
how they suddenly appear as you go

that little bit deeper into space
and what you thought was just white

light is actually starlight.

And how many stars are
there in the galaxy?

There's maybe 100 billion,
200 billion, 300 billion.

What we know is that there's maybe
twice as many as we previously

thought. How has Gaia doubled the
number? That seems surprising to me.

Where have they been hiding?

It's just because of the image
quality of Gaia.

Because it can see sharply?

Exactly. Gaia can tell the
difference between two or three

stars that are very close together
but, nevertheless, are separate

stars, whereas previously from our
images from the ground,

blurry sort of things, this stuff
here would have all been merged into

what we thought was one star.

And so we've been looking at the map
as it appears on the sky, but one of

the exciting things about Gaia is
that we have three-dimensional

information as well. We can go
beyond this sort of two-dimensional

picture. Exactly, and that's the
unique feature of Gaia.

Gaia measures distances as well as
all these other things.

And once we get into measuring
distances, then we can measure the

three-dimensional structure of our
Milky Way.

So far, we've only just had a taster
of that.

So, Gaia has just released two
million accurate distances, and so

here is the beginnings of a picture
in three dimensions.

This is our sun and we're about to
go and see the Hyades and Pleiades

cluster. And so here we are, for the
first time ever,

seeing a star cluster in three
dimensions.

So we can tell the difference
between the front side

and the back side. And so we have
already discovered not only how deep

the Hyades cluster itself is, but
actually it's about twice as big.

It's amazing. What other features
should we look for in this 3D map?

So this dramatic Hyades example,
one very nearby cluster,

is just a taster of what's going on.

We've got good distances now for
just two million stars.

We're going to have one billion
at least.

That will give us a
three-dimensional map of half of our

Milky Way. But even more
interestingly,

because Gaia's continuing to
observe over time,

it's telling us how everything's
moving and that combination of where

things are and how they're moving
allows us to determine how the

galaxy works as a machine, how the
Milky Way actually functions,

and how it's evolving.

Gaia's ability to map the Milky Way
over time is vital,

because our galaxy isn't static.

The positions of the stars are
constantly changing,

but on timescales that we can't
normally see.

In 2015, I came here, to the Peter
Harrison Planetarium at the

Royal Observatory Greenwich, for a
spot of time travel.

Since humans evolved, they've been
looking up at the night sky -

but not the night sky that we see.

This is what the sky would have
looked like when the first humans

emerged in Africa 200,000 years ago.

And it's an uncanny sight. Some of
the familiar constellations

are there, but others are distorted
beyond all recognition.

And if we go back further,

let's say to a million years before
the present, then things change

completely. The familiar
constellations are gone,

revealed to be nothing more
than temporary,

chance alignments of random stars.

The view we have of the Milky Way
today will be completely alien to

our descendants. It will change
dramatically over the course of the

next million years, becoming
completely unrecognisable as the

stars travel on their own paths,
going their separate ways.

Gaia is helping us map the past,

present and future of our home
galaxy in unprecedented detail.

But it does even more.

It's also revealing how the
Milky Way is structured.

I went to meet Gaia scientist
Nicholas Walton

to see the most accurate picture yet
of the whole galaxy.

This is what our galaxy looks like,
top down.

We thought we knew what our galaxy
was like, but actually,

because we're inside this very
complicated structure,

it's very difficult. So this is our
best guess as to what the galaxy

might look like. The sun would be, I
think here, something like that.

But interesting things happen near
the centre.

Yes. Towards the centre, you can see
that the spirals of our galaxy come

together and if you go in, we
believe that this is a bar.

This sort of straight structure?

The straight structure across here
and you'll notice that the spiral

arms actually start at the ends
of the bars.

At the moment, we think that we've
got a four spiral arm structure.

A few years back, we thought we only
had three.

It's changing all the time. It's
amazing. It's amazing how the

picture changes based on new
information and new observations.

This is an evolving picture.

It's humbling to think that after a
century or more of observations,

we could still be wrong about
something as basic as the number

of spiral arms in our own galaxy.

What's more, Gaia is only just
getting going.

And it's now released a second
batch of data,

almost doubling the number of star
positions and showing them in colour

for the first time.

Yet there's still one final mystery
about the galaxy that needs

explaining and it lies at the
very centre.

Exploring what sits at the heart of
the Milky Way ended up revealing

one of the defining features of
all galaxies.

The problem is it's almost
impossible to see into the centre of

the Milky Way and that meant that
for many years,

working out what was there required
a fair amount

of scientific guesswork.

When we look towards the centre of
the galaxy, we can't see it

because there's too much gas and,
more importantly, dust in the way.

And it's rather ironical, isn't it,

that we know more about the nuclei
of some other galaxies than we do of

our own? I think it's a real case
of not being able to see the wood

for the trees, actually.
Nevertheless, let's now go on a

journey to the centre of our galaxy
and see what we can find. OK.

So let's speculate, shall we?

What exactly is it.

That's the $64,000 question.

More than two decades earlier,

astronomers had detected something
lurking at the centre.

A bright, mysterious source of radio
waves that we now call

Sagittarius A-star.

But in fact, scientists
suspected it wasn't a star at all.

If astronomers are right, and there
is no direct evidence, then this

extremely hot,

superheated gas is spiralling into a
massive black hole, which forms the

centre of our galaxy. Yes. You know,
I've a nagging doubt in my mind that

some astronomers tend to introduce
black holes every time they can't

think of anything better.

Despite Patrick's scepticism,

evidence that black holes existed
was mounting.

But understanding the role they play
in the galaxy's evolution would take

decades more work.

I think it's a good time to say
something about black holes

in general. So what do we know
about them? In theory, a good deal.

Black holes were thought to form
when massive stars collapsed

in on themselves.

Now, let's go to our old, collapsed
star. As it goes on getting smaller

and smaller and denser and
denser, light cannot get away,

and if light can't do so, then
certainly nothing else can, because

light is the fastest thing in the
universe.

So our old, collapsed, very massive
star is now surrounded by a kind of

forbidden area, into which anything
can go, but nothing,

absolutely nothing, can come out.

And that's what we call
a black hole.

Clearly, we can't see black holes.

We can only detect them by their
effects upon things that we can see.

For 25 years, astronomers searched
the heart of the Milky Way for the

telltale signs of a black hole,
using near-infrared instruments to

track the motions of stars
around the very centre.

It became clear that they had to be
orbiting around something small but

incredibly dense...

..with a very strong
gravitational pull.

It had to be a black hole, and not
an ordinary one -

one that forms from the death of a
massive star.

It was a supermassive black hole.

We think that this kind of black
hole forms early in the life of a

galaxy, even before there are
any stars.

And it wasn't the only one out
there.

We now think that there are
supermassive black holes at the

centre of almost every galaxy.

Black holes seed the formation of
galaxies and they control how they

evolve. What had once seemed like
science-fiction was in fact a

fundamental part of being a galaxy.

As for how the Milky Way's black
hole might affect us here on Earth,

Patrick was on hand with some
reassurance.

I think there's one point we ought
to stress.

There's no danger at all of the sun
and the Earth and you and me being

gobbled up by a black hole.
Absolutely none.

Because black holes only suck up
things in their immediate vicinity.

Well, we told you it was like
science-fiction.

It feels like we're living in a
golden age of discovery when it

comes to the Milky Way.

We may not have unlocked all of its
secrets...

..but we know that it is 13 and a
half billion years old...

..it contains perhaps as many as 400
billion stars...

..arranged in four magnificent
spiral arms that stretch

100,000 light-years across.

And it takes our solar system
roughly 230 million years to make

one complete rotation about the
galaxy's centre.

But however much we learn,

the Milky Way can only tell us so
much about galaxies.

It's a case study of one.

To really understand galaxies, how
they form, how they evolve,

and what will become of them in the
future,

we need to cast our net wider.

The Milky Way is only one amongst
billions of galaxies and they are

staggeringly varied.

In 1996, Patrick introduced some of
the common types.

Galaxies do come in all kinds.

Some, for example, are spiral.

There's a spiral galaxy. And of
course, our Milky Way system in

which we live is itself a spiral.

But then there are spirals of
other kinds.

There are these strange barred
spirals,

where the arms appear to come from

the ends of a bar going through
the nucleus.

They're very strange indeed.
Others are elliptical.

Some have rather bizarre shapes,
what some call the

sombrero hat galaxy.

Now, it was Edwin Hubble, after whom
the Hubble telescope's named,

who first worked out a really good
idea of the

classification of galaxies.

And he produced what's always known

as the tuning fork diagram, for

obvious reasons. There we have the

ellipticals, normal spirals and

barred spirals.

But these were just the basic
shapes.

To understand the full variety of
galaxies out there, we needed to

sift through and to categorise them.

The problem? There are just so many.
And so astronomers went in search of

help from the public.

In the old days, astronomers thought
themselves lucky if they looked at

30 galaxies in their career.

These days, we have literally
hundreds of thousands of the things.

No astronomer could possibly sort
through them all, so we need to find

new ways of looking at our data.

We still want to care about each and
every galaxy individually.

They've all got their own stories.

The only way to do this is to look
at them and

amateurs have always done astronomy.

They've discovered comets,
supernovae, monitored the planets,

explored the sky.

What we did was we created a
website which showed people some of

these images and we were just asking
some questions -

is it a spiral galaxy, with spiral
arms and a disc?

Or is it an elliptical, a big ball
of stars?

Within 24 hours of launching, we
were doing 70,000 classifications an

hour. People told us about the weird
things they'd find.

The Penguin galaxy, for example,
which is rather good. Yes.

There's lots that look like flowers.

There's a whole thread of galaxy
images that look like roses.

Or the alphabet. So we can write

The Sky At Night in galaxies,

if we want to.

We know that galaxies come in a
whole menagerie of different forms.

But what was actually creating all
these distinct shapes?

The quest for an answer led to some
of the most complex and dramatic

events in the cosmos.

And at the heart of the story is a
potent force

that shapes all galaxies -

gravity.

In 2014, The Sky At Night went
galaxy hunting on a quest to

understand how gravity
creates different types of galaxy.

One particular zone we've been
looking out for tonight,

just above the constellation of
Virgo, is an area where,

many a time, you wouldn't see too
much detail in there, but get to a

dark sky area like the Brecon
Beacons and place a scope on this

particular zone, and it comes
alive wonderfully.

And you can see exactly why this
area is called

the realm of the galaxies.

There are dozens, if not hundreds,
of galaxies to look at with a small

telescope in this area of the sky,
and a bigger telescope will

obviously show you more, in
something that's called

the Virgo cluster. This image here
shows half a dozen bright galaxies

but then dozens more fainter ones.
All different shapes and sizes.

You can see elliptical galaxies that
look like spheres, so we've got a

close-up of a galaxy here.

And it looks like a round,
spherical blob.

There's not a lot of structure
there. That's the combined light of

billions of stars all
glowing together,

so it's quite a humbling thought.

So if we take a look at the very
familiar spiral galaxy,

what would be the forces that would
cause a galaxy to form rather like

this? Galaxies form from roughly
spherical-ish blobs, groups,

clumps of gas and dust.

And they collapse under gravity.

If there's a preferred direction of
rotation to that gas and dust, then

that will settle into a disc and
that's what happens with spiral

galaxies like this. The gas and the
dust collects into this disc and

then new stars form and the patterns
we see today because of this

axis of rotation.

Astronomers knew how spirals form,
but just as interesting was

discovering what was going on inside
the spirals.

To find out, we need to look through
a range of different telescopes.

Take M51, the glorious
Whirlpool Galaxy.

Switch to a radio telescope...

..and we get a very different view.

This remarkable image reveals the
distribution of hydrogen throughout

the galaxy.

Hydrogen is the raw material from
which stars are made and this image

shows that it stretches far beyond
the main disc.

But the radio image can't show us
where stars are actually forming.

For that, we need to switch to
another wavelength...

..the infrared.

What we can see in this image is
light from more stars than we'd

otherwise see in the visible. By
using the infrared, we're able to

peer through the dust that would
otherwise obscure our view and we

can go further than that and
the colour here represents the

different wavelengths of infrared
light.

The whirlpool itself has this
brilliant red glow.

That's light from the dust and
gas, the fuel of star formation,

which you can see is spread
throughout the disc.

But if you look along the spiral
arms themselves,

and only on the spiral arms, you see
these bright knots,

these bright blobs that are shining
very brightly.

They're places where thousands of
stars are being born.

So what this tells us is that it's
not enough to have the raw materials

for star formation. There's dust and
there's gas throughout the disc,

but it's only when it gets twisted
up into these spiral arms that it

can become dense enough to
form stars.

The spiral arms are where the
action is.

Spiral galaxies make up almost half
of the star systems we see in the

local universe.

The spiral arms are areas of
vigorous star formation generated as

the galaxies rotate.

But what about elliptical galaxies?

It turned out that the forces that
create these are even more dramatic.

It's a process we can investigate
close to home in the interaction

between the Milky Way and our
nearest neighbour,

the Andromeda Galaxy.

The Sky At Night explored the key
sequence of events in 2007.

The great Andromeda Galaxy,

more than two million light-years
away.

But it won't always be that way.

It's coming towards us and it's
going to collide with our Milky Way.

Not for at least a thousand
million years, but collide it will.

It is going to be quite a sight.

I'm joined now by two eminent
experts,

professors Carlos Frenk and
Derek Ward-Thompson.

Derek, Carlos, welcome back to the
programme.

Good see you again. Hello, Patrick.

First of all, why is it coming
towards us?

Well, in early times, thousands of
millions of years ago,

Andromeda and the Milky Way were
closer together and they started

expanding away from each other as
the universe expands.

However, the gravitational
attraction between these two

galaxies is so strong, the galaxies
have turned back and are heading

towards each other. So the very same
force that caused Newton's apple to

fall to the ground, that's bringing
Andromeda towards us.

When will this happen?

That's an interesting question
because we're not absolutely sure.

We know it's coming towards us at
about 100km a second.

But what we don't know is exactly
its sideways motion.

So the two galaxies will approach,
but then will come in

to an orbit together and as they
orbit around, they lose energy and

eventually, they will spiral in.

And how long the spiralling
in process will take depends...

That's the major uncertainty.

That's right. That's right. Yes.

Well, at the present moment, our
Milky Way Galaxy is a lovely spiral.

And so is Andromeda. But when they
collide, that will change.

That will change. These beautiful
discs will be destroyed.

The stars will be flung out and then
they will collect back together into

a shape resembling a rugby ball.

What we call an elliptical galaxy.

During the collision,

gas would have been heated up and
ejected out of the galaxy.

Once the train wreck is over, this
gas will cool down and settle again

into a new disc. So what started
life as a spiral disc, then went

through a phase of elliptical
galaxy, then might regenerate

itself and grow a new disc. This has
been going on for thousands of

millions of years and the galaxy can
experience all sorts of morphologies

throughout its life history. These
collisions are the answer to one of

the great riddles in astronomy.

Namely, the process by which
galaxies transform their identities.

The collusion of the Milky Way and
Andromeda will eventually create a

new, giant galaxy.

But it will be at least another four
billion years

before the merger is complete.

It turns out that this intergalactic
collision will not be a new

experience for the Milky Way.

Our galaxy is actually the product
of many previous galactic mergers.

The main disc of the galaxy is
surrounded by a halo of stars and

clusters, which live both above and
below the disc like this.

But what's new is that thanks to
Gaia, we can work out how this whole

system is moving.

Now, the disc is rotating. We've
known that for a long while.

So it's spinning in this sort of
direction.

But what Gaia has found is that some
of these clusters are moving through

the disc and they're doing so in
the opposite direction to the

rotation of the disc itself and
so what that means is that these

clusters are the remnants of
galaxies

which our Milky Way has
accreted.

Our galaxy is growing through
galactic cannibalism.

The Milky Way is not the only galaxy
devouring a neighbour.

It turns out that if you know where
to look, the night sky is littered

with similar scenes of
cosmic carnage.

From Earth, we see these events at a
single moment in time. They take

millions of years to happen, but we
see only a single snapshot.

These snapshots capture collisions
at every stage of the action.

What have you got for us?
Nick?

Well, we're looking at an image of
M51 at the moment,

and the interacting galaxy NGC 5195.

What do you reckon, Carlos?

I reckon it's just about to have
its dinner.

The big one. Snacking on the small
object and it's about to

gobble it up.

Another good example of colliding
galaxies is in Centaurus A.

You can see, it's quite dramatic in
the background -

you can see a massive elliptical
galaxy - but in the foreground,

this incredible dark dust lane is
actually the remnants of a spiral

galaxy, which is being torn apart.
And it's actually thought the new

images show a third galaxy having
been shredded to some degree in the

past. When I started my PhD in the
1980s,

this galaxy was a big mystery.

Now we know that galaxies fall into
each other all the time.

Where shall we go next?
We can look at

another popular target.

One of my favourites. Oh, wow,
yeah. The antennae.

NGC 4038 in the constellation
of Corvus.

You can see two distinct galaxies in
there, but they're very, very

heavily disturbed.

It's almost as if they're dancing
around each other.

These ones have been caught in
the act.

They are in the process of merging.

It's one of the spectacular events

that happens to galaxies of

comparable size caught in the act
of merging.

As Derek says, they engage in some
kind of cosmic dance.

And eventually they'll fuse together
and produce one object after

destroying each other in the
process.

But this is a beautiful image.

That's probably the most famous
colliding galaxy in the sky.

Let's go to some of the slightly
more obscure but interesting ones.

Wow, look at this. This is the...
The mice. The mice. The mice.

This is a Hubble Space Telescope
image, I think.

These two could well be orbiting
around each other, having already

interacted once and torn out these
streams of stars and gas from each

other's galaxies and triggered new
stars all the way along.

You can see the tails are very blue,

again showing us the characteristic
signature of young stars.

This is very nice, NGC 6770.

A nice pair of spirals.

I think that's right. The two
galaxies are not really heading

towards each other, but they have a

motion sideways and so they orbit
and then the orbits don't last very

long. We should add, of course, that
to a cosmologist,

anything less than a billion
years is fast.

LAUGHTER

OK, well, I think there's one more
image so let's have a quick look at

that. This is Stefan's Quintet.

A cluster of galaxies where two,
certainly in the middle,

are in the process of merging,

interacting at the moment and
dragging out tidal tails and all

sorts of other interesting features.
There is some sort of promiscuity

going on in here. I can see five or
six of these objects engaging in

some sort of indecent activity here!

Gravity shapes the way that galaxies
form and interact.

But what about the interactions of
galaxies on an even bigger scale?

Galaxies are the units of the
universe.

And we want to find out about the
distribution.

Surprisingly, when we look at the
arrangement of galaxies in space,

we start to see larger structures
appearing.

At the smaller end of the scale,

galaxies collect together under the
influence of gravity in groups.

For example, the Milky Way is part
of what's called the local group...

..made up of over 50 galaxies.

I know you've been making a close
study of the local group,

so, first of all, what does
it contain?

Well, as far as groups of galaxies
go, Patrick,

our own local group isn't terribly
spectacular.

It contains a couple of giant
galaxies,

like our own galaxy and the galaxy
in Andromeda,

a few medium-sized galaxies, like
the Magellanic Clouds and the galaxy

M33 in Triangulum

and a whole hotchpotch of small
galaxies, tiny dwarf galaxies, which

are scattered all around the
local group.

And here am I, standing inside the
local group and you have to imagine

I've been blown up by something like
10,000 million million million

times. And that gets me on the
right scale.

The local group is about five
million light years across

and has a sort of flattened
distribution,

as you can see around here.

We believe also that our local group
is part of a larger association

known as the Supercluster, centred
upon the group of galaxies in the

constellation of Virgo more than 50
million light years away.

Galaxy superclusters are simply
colossal structures.

But even they are not the biggest
things in the universe.

Superclusters themselves are
arranged into dense threads that we

call filaments

with huge voids between them.

These filaments form a vast,

honeycomb structure assembled from
all the galaxies in the observable

universe. For now, we know of
nothing bigger.

We now know a lot about how galaxies
develop, but there remain profound

questions to be answered.

Where do galaxies come from in the
first place?

Why do they exist at all?

What will eventually happen to them?

To find out how galaxies came into
existence, we need to travel back in

time, right back to the earliest
years of the universe.

In 1995, the Hubble space telescope
captured a photograph that took us

further back in time than had ever
been possible before.

Let's begin by showing you this.

This is a recent picture sent back
by the Hubble Space Telescope.

It shows star systems, or galaxies,

some of them so far away
that their light,

travelling at 186,000 miles per
second, takes thousands of millions

of years to reach us.

The photograph in question became
known as the Hubble Deep Field, and

for my money, it's one of the most
important scientific images

ever taken. This one image gave us a
way of seeing galaxies just a few

hundred million years after the
Big Bang.

When it was first released, I
remember staring at this image,

almost unable to believe that each
of these objects is not a star, but

an impossibly distant galaxy.

In 2015, I spoke to Sarah Kendrew to
find out the story behind the image.

Sarah, tell us how the Hubble Deep
Field came about?

The Hubble Deep Field scientists had
basically proposed to look at an

empty patch of sky, so that's quite
an unusual thing to want to do.

That seems crazy. Right,

because they had no idea what
they were going to find.

So it's quite hard to justify but
ended up being incredibly successful

and became the iconic image for the
Hubble Space Telescope.

It really is a stunning image.

We can look at it here.

What exactly are we seeing in the
deep field?

Every single point of light in this
image is an individual galaxy.

But of course, this isn't the local
universe, so we're seeing these

galaxies as they were many billions
of years ago as well.

That's correct. We're also probing
down to the first billion years of

the universe's existence and that
really was very new.

It really gave us for the first
time this quite large sample of

galaxies from the very early
universe and we can take a closer

look at them. The Ultra Deep Field,
which came a bit later,

was sort of the improved version.

But these galaxies look different
from the ones we'd see today.

That's right and that's one of the
main outcomes of the Hubble Deep

Field. The galaxies in the early
universe, you know,

have quite a different appearance.

Whereas the galaxies we see in the
universe today tend to be quite

ordered, they have a nice spiral
shape or they're blobby elliptical

galaxies, these are much more
irregularly shaped, so you can see,

for example, some galaxies that are
merging together.

The universe was much smaller in
those early days, so small galaxies

were more densely packed in, so they
would collide to then sort of form

bigger structures.

It confirms the theory that it takes
billions of years of mergers and

collisions to form the large,
regular galaxies that we see today.

And it also showed that even early
in the universe's history,

galaxies were already forming.

But it can't tell us why there are
galaxies at all.

To answer that question, we need to
look at another,

very different image, one that takes
us even further back in time.

This image shows the leftover glow
from the Big Bang,

the oldest light left in
the universe,

known as the cosmic microwave
background.

It's a picture of what the cosmos
was like when it was only

300,000 years old,

when there were no stars and
definitely no galaxies.

And so how did the universe go from
this state to one where stars were

organised into galaxies?

In 2014, I spoke to Sarah Bridle to
find out.

We're going to talk about the early
universe, which was a very different

place to the one we see around us
today. So what was it like?

Well, so, early in the universe, the
universe was much denser, so

basically today, we've got a vacuum
in space, but if we go back in time

to the early universe, the universe
was much smaller.

Everything was much closer together.

Some patches were clumpier than
others,

so there was more stuff in one place
and less stuff somewhere else.

We can see these red patches and
blue patches.

So where the red patches are, that's
where the universe was hotter and

denser, where it was really clumped
together.

But the blue patches here are where
it was cooler and more spread out.

So in fact, those hot patches where
there's lots of stuff,

that would have then gone on to form
the first stars and galaxies

that we can see today.

The cosmic microwave background
shows the first

tiny density fluctuations.

The seeds of what will eventually
become the galaxies.

But there was a problem with a
physics.

There wasn't enough matter in the
early universe to account for the

clumps that we see. There has to
have been something else there

giving an extra gravitational pull.

We're still not sure what this
something else was, but we give it

the name dark matter, and it's
thought to be crucial in forming

these first clumps,
the seeds of galaxies.

Unless we can understand and detect
dark matter,

we can't explain how the stars and
the galaxies came into being.

But the problem with dark matter is
that it doesn't normally interact

with anything except through
gravity.

That makes it almost impossible
to detect.

Despite looking for more than half
a century,

we still haven't found anything.

Our best hope is to look for the
rare instances where a dark matter

particle collides with a particle of
ordinary matter,

creating a very faint signal.

The search for dark matter is one of
the great quests in science and it

has led to the creation of some
suitably impressive

research facilities.

I went to see the latest
state-of-the-art equipment that's

searching for dark matter
early in 2018.

This is Gran Sasso in the heart of
the Italian Apennines.

But we're not here to admire the
view -

we're heading underground in search
of dark matter.

This is amazing. We're driving down
a secret tunnel underneath a

mountain, just like in a
James Bond film.

But this, this is where physics
gets done.

I met up with Ranny Budnik, who
showed me the experiment

that he's working on.

This is the xenon experiment.

This is amazing.
This place is enormous.

The lab is underground to escape
from cosmic rays -

high-energy particles that rain down
on us from space all of the time.

Down here, there are one million
times fewer than at the surface -

vital if scientists are going to
have any chance of spotting the

subtle signals of dark matter.

That's the job of Xenon1T,

a detector filled with three and a
half tonnes of liquid Xenon.

The theory is that just
occasionally, a particle of dark

matter will collide with a xenon
nucleus.

And that can be detected.

What we're looking for is basically
kind of a billiard ball interaction.

Our xenon nucleus, basically, is
being kicked.

This particle get some energy and
then deposits the energy inside our

detector. OK. So you're looking for
these direct hits.

Exactly. These rare cases where the
dark matter particle happens to hit,

directly, a xenon nucleus. Yes.
That's the cheerful possibility.

You could press the button and see
nothing.

Exactly. That's actually what
usually happens. Yeah, yeah.

Or what happened all the time so
far.

Hundreds of scientists have spent a
decade looking for dark matter at

Gran Sasso - and they're still
looking.

So for now, our best understanding
of how dark matter helped create the

early stars and galaxies comes from
computer models.

In 2015, Jim Al-Khalili went to meet
Andrew Pontzen

to see the latest model in action.

We think dark matter had a crucial
role making the universe like it is

today. I can actually show you here
a simulation of the way that dark

matter behaves. Because it's in a
computer,

we can paint the dark matter any
colour we like.

We can make it visible.

So I'm going to paint it green and
show you what happens over the first

billion and a half years or so.

So I can hit go...

..then we've just seen the Big Bang.

The whole universe, or at least this

chunk of the universe we're looking

at, is expanding and as it does so,

you're seeing what happens to the

dark matter over the first

billion and a half years or so.

It started out quite evenly spread

out, coming towards us,

but over time, it forms into clumps.

So, of course, this is the dark
matter,

which we wouldn't be able to see.

How does that relate to the visible
universe?

Yeah, we can switch views and show
what the computer thinks the visible

universe would look like at
this time.

Every dot of light you see here is a

forming mini galaxy.

It's got maybe few million stars in

it, but the key thing is that they

wouldn't be there unless the dark

matter is there in the first place.

It's that extra gravitational pull
that all that dark matter is

providing that pulls the gas in and
allows it to sit there and start

forming stars, start forming the

galaxies we know today.

They're coming together not because
of their own gravity, although that

must be important, but because of
the gravity of the dark matter,

which is much more dominant.

Yeah, that's what we think the key
role of dark matter is,

to pull all this stuff together and

actually clump it into something

that can form these galaxies.

How does that evolve, what does it
look like today?

Well, we can use these computer
models to work out precisely that.

If I switch to another view -

here we've zoomed in on one of

those single points of light that

you saw and this is going to turn

into something like our Milky Way

galaxy today. So if I restart time,

then what you see happening is all

these different little mini
galaxies

start merging together and, once
again, we think dark matter is

playing a key role in this.

It's the gravitational pull

associated with the dark matter that

actually pulls all these different

bits and pieces together

and starts assembling the present

day Milky Way, which just grows and

grows.

We think we understand how the
galaxies appear after the Big Bang,

but there's one big question left to
consider -

perhaps the most important of all.

What about the future? Will the
universe end?

What's going to happen to it?

It's now 70 years since Edwin
Hubble discovered that the universe

is expanding. All the groups of
galaxies are racing away from all

the other groups and the expansion
is universal.

Understanding this expansion of
galaxies away from each other is the

key to their ultimate fate.

The question is - will the expansion
continue or will it eventually stop?

In a universe with just normal
matter and dark matter,

gravity should be able to slow down
the cosmic expansion.

There's just one problem with
this theory...

..that's not what we see happening.

Gravity isn't slowing the
galaxies down.

On the contrary, something else is
speeding them up.

And the question is - what's causing
this acceleration?

Jim Al-Khalili went on a quest for
the answer.

Well, the truth is we don't know.

But at least we've given it a name -

dark energy.

A weird, new force that pushes the
universe faster and faster.

The fate of the galaxies, and indeed
the entire universe, comes down to a

straight fight between gravity...

..and dark energy.

Without dark energy, gravity is the
most significant force dictating the

fate of the universe.

If gravity is the dominant force,

then it means that one day the
universe might stop expanding and

start contracting.

Eventually it'll collapse
together...

..in what's known as the Big Crunch.

But if dark energy turns out to
dominate,

then the end of the universe could
be much lonelier.

As the universe spreads out, the
influence of gravity becomes weaker,

until everything is too far apart
for it to have any effect.

Then dark energy will be the only
player in town.

As dark energy keeps pushing the
universe apart,

eventually all galaxies will move so
far away, they'll become invisible

to each other. The distances between
them will become so great that light

from one would never reach
the others.

And the universe would disappear
into darkness forever.

So, what's it going to be?

The truth is we don't really know
what dark energy is, and so we don't

know how the story of the galaxies
will end.

We've seen back in time to their
earliest origins...

..we discovered how they grow
and change...

..and we've seen the most
astonishing events and structures

inside them.

It's quite an achievement.

Not bad, considering that we've
worked all of it out from here,

on a tiny observation post in an
obscure corner

of the Milky Way galaxy.

We may not yet know the whole story
of the galaxies, but even without an

ending, it's already clear what an
epic story it is.