All language subtitles for The Sky at Night 2015 The Billion Pixel Camera BBC 1080p HDTV_Subtitles01.ENG

This is our galaxy,
the Milky Way, in all its glory.

Or, at least, it might be...

because like every other image that
shows the whole of the Milky Way,

this is actually a computer graphic
that is based on our best guesswork.

So what DOES the Milky Way
really look like?

We're about find out.

Quatre, trois, deux, un...

Decollage.

In December 2013, the European Space
Agency launched Gaia -

a scientific instrument
that will transform

our understanding of the Milky Way.

It's the most advanced
astronomical camera ever made,

and contains the biggest array
of sensors ever to leave the Earth.

The camera is packed
with a billion pixels -

over 60 times more than
the Hubble Space Telescope cameras,

and it will allow Gaia to learn more
about our galaxy than ever before.

We've come to the company that built
this incredible sensor

to explore the story behind it.

Also, what DOES our galaxy
actually look like?

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

A few years back
we thought we only had three.

It's amazing
how the picture changes.

And physicist Jim Al-Khalili
finds out

how the mission will let us trace

the hidden matter
that shapes our galaxy.

Without dark matter, the universe as
we see it simply wouldn't exist.

Plus Pete, showing us how to see
the Milky Way for ourselves.

Seen from Earth, the Milky Way is
a glorious sight

as it stretches across
the night sky.

But we only get a very narrow
perspective on our galaxy.

And, in fact, we know
surprisingly little about it.

We estimate the Milky Way contains
at least 100 billion stars,

but so far we've only been able
to catalogue a tiny fraction -

around 200 million.

We believe
that it's a spiral galaxy,

but its exact structure remains
an open question.

As does what lies in the mysterious
region around its centre.

But Gaia is hoping
to change all this.

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,

and 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.

It will allow us to predict what the
galaxy will look like in the future,

and how it evolved.

It should also help solve a puzzle -

what is the exact shape
of our galaxy?

The problem is how do you work it out
from the inside?

To find out why this is such
an important question,

Chris is speaking to Nicholas Walton.

This is a representation of what
our galaxy looks like, top-down.

We can see the spiral arm
structures.

We can see a bar structure,

and here we see directly that
looking at it from this direction,

looking down on it,

we wouldn't know
if that was a flat bar,

we wouldn't know if it was a bulge,

because we haven't got
the idea of the 3D volume.

And so this is our best guess as
to what the galaxy might look like.

The sun would be 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.

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

but a few years back we thought
we only had three.

It's changing all the time. It's
amazing how the picture changes,

based on new information
and new observations and so forth.

If you go in, you'll see that
the star densities actually increase

and you'll see
what looks like a bulge.

But now we believe this is a bar.
A sort of straight structure?

A straight structure across there,

and you'll notice
that the spiral arms actually

start at the ends of the bars,
the north and the south bar.

In the centre, the idea is that

actually some of the stars
are forming

because of material forming
and flowing down the bar into there.

So this material has perhaps
flowed along the bar.

And actually there were
some recent results

that make it more complicated again.

Yes, indeed. We've had some recent,
very exciting survey data,

looking for very bright stars
towards the centre,

and now we find that there's
evidence of cross structure.

So there's two bars. So, all of a
sudden, it's even more complicated

and it's a real challenge to
understand how we put this together.

But this is an evolving picture.

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

because we're inside this very
complicated structure,

it's very difficult.

So when we look ahead a few years
to the results from Gaia,

it will give us a definitive picture
of what's happening.

So what does it mean?

If our galaxy has these complicated
structures at the centre,

and Gaia reveals them to us,

what will that tell us
about the formation of the galaxy?

Well, did our galaxy form

because it was the merger of two
massive galaxies, for instance,

or was it the merger
of a massive galaxy

where we actually then went on and
accumulated and accreted lots of...

gobbled up, almost,
lots of little, smaller galaxies?

These are the answers
that Gaia will enable us to get.

And if we can get them
for OUR galaxy,

we can understand other galaxies.

We can understand other galaxies,

we can understand what the universe
looked like further back

towards the Big Bang, all the way
through to our present-day universe

and get a good understanding
of how galaxies were built,

how they were formed, and how stars
were formed within those galaxies,

and that links all the way through
to how planets are formed.

The main focus of Gaia's work will
be to catalogue millions of stars,

but scientists are hoping
that it will do much more,

that it will shed some light
on one of the most mysterious

and hard-to-imagine constituents
of our galaxy -

dark matter.

Dark matter is thought
to be everywhere -

in the depths of space,
all around us.

Billions of particles passing through
your body every second.

And yet it is also
completely invisible.

Physicist Jim Al-Khalili
is investigating

what dark matter really is,

and how Gaia might help us find out
more about it.

For almost a century,

scientists have been searching
for dark matter without success.

But we know something is there.

The key is gravity.

Look up at the night sky,
and you expect gravity to explain

the distribution and movement
of all the stars.

But the mass of the visible universe
only provides

15% of the gravitational pull
needed to explain everything we see.

There's no visible evidence
of the missing stuff that provides

the other 85%
of the required gravity.

Dark matter is simply the name
that scientists have given

to whatever it is that's creating
the rest of that gravitational pull.

Without dark matter, the universe
as we see it simply wouldn't exist.

But if dark matter makes up
such a large part of our galaxy,

why haven't
we been able to find it yet?

To begin to get your head round
just why searching for dark matter

is such a challenge, you first
have to understand what it ISN'T.

You might think it would be easy
for me to show you dark matter,

but it's much more difficult
than that.

Nothing in this wood, no matter
how dark, is as dark as dark matter.

That's because all normal matter
reacts to light.

Shine a torch, and some of that
light will be reflected back.

But dark matter is different.

It's a substance that doesn't
respond to light in any way.

It neither emits light
nor reflects it.

But even that is only the beginning
of just how profoundly different

dark matter is.

You see, dark matter isn't "matter"
in the conventional sense at all,

in that it's not made up
of the same stuff as normal matter.

Most of the evidence
for the existence of dark matter

is therefore indirect.

And this is
where the difficulty lies.

The problem is, we just don't know
what dark matter is.

And that's why
it's so hard to study.

Scientists are trying
lots of different ways

to detect
and understand dark matter.

For example, there are experiments
buried deep underground

trying to capture the particles
of dark matter

using specially designed detectors.

There are particle accelerators,
like the Large Hadron Collider,

which are trying to create
the particles of dark matter.

But Gaia will be trying
a different method.

It will allow us to look up
at our galaxy

and deduce the properties
of dark matter

from what we can actually see.

It will measure, in extreme detail,
the effect dark matter has

on the visible matter
in our own galaxy.

'Cosmologist Dr Andrew Pontzen

'simulates
what this might look like.'

Of course, we think about dark
matter being out there today,

but you're interested
in the role it played

very early on
in the formation of the universe.

That's right, and we think dark
matter had a crucial role in making

the universe like it is today,
so a lot of what

I do uses computers
to try and work out or model

what dark matter would have done

throughout the history
of the universe.

So I can actually show you here
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...

And 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've seen 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?

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
a few million stars in it.

But the key thing is that
they wouldn't be there

unless the dark matter
that we were seeing just before

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 and start
forming the universe we know today.

This is the important point.
They're clumping 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, there's so much more dark
matter than normal matter

that 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 stars.

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 we saw.

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 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 the different
bits and pieces together and

starts assembling the present-day
Milky Way, which grows and grows.

'Andrew's simulation
can also predict

'where the dark matter
that surrounds our galaxy is today.'

This is the final picture
that comes out of the computer model

we've been watching.

What you've got here
is that the galaxy as we know it

is some tiny little bit
in the centre, which we can't see

cos we're only looking
at the dark matter here.

And then you see there are all these
other blobs of dark matter around.

Now, a few of these will have
some stars in as well,

but most of them are just
too small.

So we think that our Milky Way
galaxy first of all is

shrouded in dark matter, but also
has

all these extra blobs of dark matter
around it,

which are essentially
failed galaxies in their own right.

Of course, this is a simulation.

How do you test that this
idea is correct?

That's where something like Gaia
comes into the picture

because if we switch
to where we think

the stars are in our computer model,
it looks something like this.

The main galaxy is still
in the centre there,

and as those small bits of dark
matter fall into our own Milky Way,

the stars are stripped out

and they are left in these big
streams that you can see here.

So, if we could see the streams
of stars falling into the centre,

that is evidence
that there is dark matter

spread out to large distances.

Exactly, and that's just what we're
hoping Gaia will let us confirm.

So Gaia may well give us
our best evidence for dark matter

and how it behaves.

But I give you one last thought.

It might be that after we've had
time to analyse

the massive amount
of information provided by Gaia

over the next five years,

we discover something
completely different.

It might be that all our
calculations were in fact wrong,

and that dark matter doesn't exist
at all.

Now, in my view,
this isn't very likely,

but it's what makes astronomy
so exciting.

The key to the success of Gaia

are the imaging sensors that make up

the spacecraft's
billion-pixel camera.

Producing these sensors requires
some very delicate engineering.

We're back here at the e2v
laboratories,

and what you can see in front of us
is where they assemble the detectors.

It's in a clean room because
of contamination, such as dust.

This can play two roles.
If it sits on top of the detector,

it stops the light or the photons
getting in.

But if it's embedded
in the electronics,

it kills the detector dead.

Now, contamination is such an issue
that I'm not allowed in.

But we've cleaned up our cameraman,
Nick,

and he's showing us
the close-up detail.

The battle with dust is extreme.

In this clean room there is
35,000 times fewer dust particles

than in normal air.

They've got over 300 people
making sensors

for all sorts of space missions.

As well as Gaia,
this company has built sensors

for the Rosetta
comet landing mission,

planet-hunter Kepler
and the Mars Curiosity rover.

What they're actually doing
in there is cleaning

the detectors before
they are sent off for testing.

Dust isn't the only challenge
in making the sensors.

They also have to be designed
to cope

with the violent experience
of being launched into space.

'Chief engineer David Morris
simulates

'the conditions of a launch
with a vibration test.'

So, what sort of force
are you putting it under?

It goes up to 50 times
the force due to gravity on Earth

simply by this vibration
in three axes.

So it seems really odd
because you spend so much time making

these wonderful detectors,
and then you put them through hell!

They have to go through hell because
otherwise we won't be confident

they'll survive when they go
through the hell of launch.

But it is always traumatic, worrying
about whether or not

what we've designed and built
will survive

this sort of extreme violence.

This process reveals the key
to successful space engineering -

extreme precision
married with extreme robustness.

Next, how can you view the Milky Way
for yourself?

Pete's here with a few tips
and a guide to touring the galaxy.

It's a great time of year
to spot the Milky Way.

Unlike in the spring and the autumn,
the plane of the Milky Way rides

very high in the sky
during the winter and the summer.

And each view gives us a completely
different perspective

of our own galaxy.

In the summer
we look through the Milky Way

towards the galactic centre.

This bright, summer Milky Way
snakes across the sky,

revealing a dense path of stars,
punctuated by dark dust clouds.

In the winter we get a much more
subtle view of the Milky Way,

rather than the bright view
we get during the summer months.

And this is because during the
winter we are looking out through

a thinner portion of the Milky Way
to the universe beyond.

The result is remarkably different,

with a faint band stretching up
from the horizon.

However, it is no less beautiful

and is packed with some stunning
deep-sky objects.

At this time of year the best place
to see the Milky Way

is within what's known
as the Winter Triangle,

and this is made up
from three bright stars.

The Winter Triangle is formed
by drawing an imaginary line

between the stars Betelgeuse
in Orion, Sirius in Canis Major,

and Procyon in Canis Minor.

The Winter Triangle
is a very clear pattern,

and if you have a dark sky,
you can often see the Milky Way

running right the way through
the centre of it.

But if you do have trouble
making out

that subtle, faint light
from our own galaxy, then it's worth

scanning the region
with a pair of binoculars

because there are some fantastic
clusters to be seen there.

A few favourites are NGC 2244, in
the centre of the Rosette Nebula...

..and M41, located
just below the bright star, Sirius.

Of course the Milky Way isn't
the only thing on offer this month.

Jupiter is really well positioned

and it's about to offer up
a really rare event.

So, with details of this

and other highlights visible
in this month's night sky,

here's my star guide.

The open cluster M41 is located just
to the south of the Winter Triangle,

below Sirius, and should be easy
to find with binoculars.

Southeast of the triangle,
two further clusters -

M46 and M47 - can be seen.

The main constellation in this
direction is faint Monoceros,

the unicorn.

Within the mythical beast's
triangular head sits NGC 2244,

the open cluster at the heart
of the wonderful Rosette Nebula.

As the Milky Way heads north,

it passes a distinctive curve
of faint stars in Gemini.

And it's here you'll find M35

and the dimmer NGC 2158.

Finally,
look out for brilliant Jupiter,

which dominates the view
high to the south around midnight.

On the 24th of January,
between 0628 and 0653,

three dark moon shadows of
Io, Europa and Callisto

will be visible on the planet's
disc at the same time.

This doesn't occur very often

so make sure you get outside
and have a look.

Now back to Gaia,

and it will be a few years
before we get the first

new star catalogues and maps from
the data it's sent back to Earth.

But it's already giving us
exciting science

about unusual phenomena that
we might call transient signals.

Every now and then
our telescopes record

flashes in the night sky.

Many of which are the results
of stars exploding

in what we call supernovae.

These are rare events, but they have
enormous scientific value.

In these stellar deaths we can
reveal how many of our elements

were created, and also
how the universe is expanding.

What Gaia will do is help us find
more of these things as they happen,

and it might even help us discover
a new type of exploding star.

Maggie is talking
to Dr Simon Hodgkin,

who'll be releasing information
about these signals

as they are found.

Simon, can you describe to me,
what is a transient signal?

A transient signal is one that does
not last for a terribly long time.

So it's only there for a small
amount of time and it's important

to react quickly to learn what it is
before the light disappears.

So there is a whole range
of transient events.

They can be superflares
on stars like our sun,

which can last for hours to days.

But there can be supernova events
which last for weeks to months.

So, if you find a supernova, what can
it tell us? Supernovae are mystery.

We've known about them
for a long time.

We know there are lots of different
kinds of supernovae,

but the physics of what kind
of star gives rise to which

kind of supernovae is a little bit,
I would have to say, flaky.

So we're trying to find
as many nearby supernovae.

It's possible to try and understand
where they came from

and what kind of supernovae
there are.

'And they've already made
a number of discoveries.

'One of which caught the scientists
totally by surprise.'

We found something really rather
rare and really rather exciting.

'They were alerted
after Gaia recorded

'an explosion
on a star in our galaxy.'

We took an image, and this was taken
on a telescope in Italy.

So we decided to take a sequence,
a very short-duration image,

so this is... The first 20 or so
images look like this,

and this is another one,
which looks pretty much the same.

And it's this star here.

That star blew up. And essentially
it's still there, it looks good.

The very next image,
30 seconds later, the star vanished.

It's got 1,000 times fainter,
essentially.

So suddenly we've gone
from a star that's pretty bright to

a star that is not there.
We carried on looking at it,

every 30 seconds we took a new
exposure, and over the next minute,

it started to reappear
and got back to its previous level.

So what's going on there?

So our interpretation
is that we have

two degenerate white dwarf stars
orbiting each other,

and in fact these disappearances
happen periodically.

As one goes in front of the other?
As one goes in front of the other.

Every 15 minutes one star
is occulted by the other,

it's called an eclipse.
But that doesn't make sense to me

because if you've got two bright
stars, you have lots

of brightness, half the brightness
then lots of brightness again?

So why does it disappear?
That's right.

So both objects need
to be the same size,

but one is essentially bright
and the other is very dark.

So when the dark star
passes in front of the bright star,

it blocks it out.

When the dark star goes behind
the bright star,

you don't really see any change
in the brightness.

And because they are
so perfectly aligned, we can

do something you can't do with
these kind of stars normally.

It is very rare, we're very lucky
to have found the explosion

with Gaia and done this follow-up.
We can measure their masses,

we can measure how heavy they are
and we can measure their radii.

And if they are
what we think they are,

and these two stars will merge
together as they lose energy through

gravitational radiation, they could
form a type 1a supernova.

Type 1a supernovae are a unique form
of exploding star

that are important for astronomy.

Cosmologists can use them
to measure the size of the universe.

But exactly how they form
is a mystery.

So the question is,
is this a progenitor of a 1a,

and I hope in my lifetime we'll
be able to make the measurement

that will tell us whether or not
these are growing together

and could explode,
or they're growing apart

and this is not the 1a progenitor
we're looking for.

I suppose,
if you see them coming together,

then the hope is
that they will go supernova.

That is exactly right,
but it may take millions of years.

It's a fantastic discovery,

and one of the great things
about this mission

is that you could help find others
like it.

Because Gaia data
isn't just for scientists.

So the idea is, every time
we find something dramatic,

a transient event
happening in the sky,

we publish it to our website,
so we write down the coordinates

and the brightness of the object,
and all those data are public,

and anyone can go and look at them.

And if they have access
to a telescope they can go and

follow up our objects, and I would
love it if they could tell us what

they are, share their data, and help
us classify

the kind of things we're finding,

which are potentially very rare,
very exciting.

Cos I guess Gaia's going to be
inundating you with lots of these,

and you want the amateur community to
join in and help solve the problems.

That is absolutely right.
I can't keep up.

We are finding three or four
or five a day,

we do not have the resources
to follow them up, whereas the

amateur community can help

for the ones that are bright enough
for them.

And even in schools, there is
access to a network of telescopes

that schoolchildren
can have access to,

and trigger follow-up observations
of objects we have found.

So school kids can get involved
and do real science.

That would be fantastic, I would
love that to start happening.

Exciting times ahead.
It's a busy, exciting time. Yes!

Thank you so much. Thank you.

To find out more about Simon's
project, you can visit our website -

It really is a fabulous chance
for schools to get involved

in pushing back
the frontiers of knowledge.

That's it for this programme.

Next month we'll be
scanning the skies for UFOs,

to reveal how the search
for little green men

has transformed our knowledge
of the universe.

In the meantime, get outside
and get looking up.

Good night.