Original subtitles

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.