Seven Ages of Starlight

Each night, after the sun sets, sit back,

look up and you can witness an epic drama playing above our heads.

One involving a cast of billions.

The stars.

Every one with its own story to tell.

There are old Red Giants,

so puffed up they're coming apart at the seams.

Supernovae, the most spectacular firework displays in the universe.

Mysterious black holes,

stellar tombstones that we are only beginning to understand.

And when the sun rises again, we can see a star in the prime of its life.

Unravelling the life and times

of these stars has revealed extraordinary secrets

about the universe and our own place within it.

At the tale's end lie clues to one of the biggest mysteries in science.

This is the story of the stars.

For thousands of years, we've told stories about the sun and stars,

populating the heavens with gods and giants.

Ancient Egyptians worshipped the sun, calling it Ra.

Orion the Hunter strode the heavens.

Stars and whole constellations were characters

that moved above our head with the changing seasons.

In the 20th century,

modern astronomers discovered that, in a way, our instincts were right.

The stars in the twinkling night sky aren't all the same.

Powerful telescopes have revealed the sheer variety

of their brightnesses and colours.

And in that diversity, scientists have discovered a new story.

When we see the stars in the sky, they look all different,

but once we put them together in order of colour,

in order of brightness, this is where we get, some kind of sense

of order, and this is what makes the whole story so interesting.

Dr Francisco Diego has devoted his career to understanding the stars,

their individual natures

and the connections that can be found between them.

For example, this is Arcturus, a very bright, red star that goes here.

This is Beta Centauri, which is a very hot, blue star.

The sun is at a medium temperature.

It has to go more or less in-between.

By plotting stars according to their characteristics,

astronomers uncovered a pattern...

..one that reveals different types of star,

each with its own personality and contribution to the universe.

But the patterns are a clue to something more fundamental.

This is telling us that, as time goes on,

the stars themselves start to change and to develop, to evolve.

And then we have a pattern here,

a kind of cycle, the lifecycle of stars.

In discovering the seven ages of the stars, scientists have uncovered

the story of the universe, and, just like for us,

it all begins with birth.

One of the most gazed at patches of sky throughout history

is the one containing a cluster called the Pleiades.

But the ancient astronomers didn't know that

the Pleiades hold a secret...

..one that modern astronomers have revealed.

This cluster, the Pleiades, mentioned by Homer in the Odyssey,

they appear in the Bible and in some of the codices by the Aztecs

and the Maya.

But the interesting thing is that the Pleiades are so young

that early dinosaurs never saw them,

because at that time, the Pleiades hadn't been formed yet.

At 100 million years old, they are like baby stars,

very, very young stars.

Some of the youngest stars that we can see in the sky.

A star is being born somewhere every day.

Each time, it's one of the most magical events in the cosmos.

One that requires mighty forces of nature.

And to set the process going, just an element of chance.

The tale starts in the cold, dark clouds of dust and gas

that lurk in deep space...

..and that have filled the mind and imagination

of Professor Serena Viti throughout her career

studying the birth of stars.

These clouds are really vast.

They can be up to 300 light years across.

And many stars form there, which is why

we call sometimes these clouds stellar nurseries.

Many regions within these clouds can stay like that for ever,

for millions of years, until something happens, a trigger,

and then a star forms.

The trigger for such a monumental event doesn't have to be much.

Two clouds can bump as they pass,

or a distant cosmic event can send a shockwave,

just something to give the cloud a squeeze.

All you need is a little bit of pressure to allow the gas

to be dense enough for gravity to take over and collapse to start.

The particles of dust and gas that had been quietly floating in space

now start being pulled together.

Gravitational attraction draws them towards each other,

faster and faster.

As the collapse continues to happen, the gas and the dust

will fall into the centre and they will become denser and denser,

and the centre of the cloud will become hotter and hotter.

The laws of nature mean that when matter gets compressed, it heats up.

Over millions of years, the protostar grows,

increasing the pressure and heat in its core,

until, finally, it reaches a critical temperature.

About 15 million degrees,

and a fundamental process will start in the core of the embryonic star.

Almost in a flash, the core of the star,

like our own sun's once did, dazzlingly lights up.

A star is born.

If you look at the night sky and you look up at a twinkling star,

you think of this little pinpoint of light,

almost like a Christmas tree light.

And, actually, what it is

is this incredible cauldron of energy being released.

To witness what's going on inside these points of light,

you have to go somewhere closer to home.

To the Joint European Torus, JET, in Oxfordshire.

Where they study what happens in the heart of stars,

the hydrogen fusion that brings them to life.

What we're trying to do in JET is essentially to make

a little star on Earth.

We're trying to create the conditions necessary

to create the fusion of hydrogen, and with it, to create

copious amounts of energy, lots and lots of energy.

If you're going to attempt to create a star on Earth,

you need something able to withstand the incredible energies involved.

You need a torus,

a giant, doughnut-shaped structure

where temperatures can reach over 100 million degrees.

Inside, an incredibly powerful magnetic field holds

the hydrogen fuel.

-OK.

-Right, trigger, please.

The conditions are so extreme that each attempt at star creation

is a tense event.

..nine, eight, seven...

So what's happening now on JET is that they are powering up

the magnets, and as they power up the magnets,

it will be pushing the electric current round the loop.

If you can see that red colour beginning to be there,

that's the beginning of the plasma firing up.

First, they have to pull apart

the basic building blocks of matter, atoms.

Then hurl them together again so they fuse and create starlight.

You can see the plasma hitting the bottom,

and so the lighting up on the bottom there... Oh, now it's really

in full bloom - this is probably about 30 million degrees right now.

This is a little bit of a star, here on Earth.

APPLAUSE

And, yes, it seems like that was about 2.5 million amps

going through that plasma right there,

and I think we had a successful shot because of all the excitement.

It lasted just a brief moment, but at JET, they've managed to replicate

what happens in the biggest objects in the universe, the stars.

And they've done it because scientists

like Professor Steve Cowley understand the smallest.

At the centre of each hydrogen atom is a proton.

And around that proton is an electron going round in a sort of an orbit.

With enough heat and pressure, the orbiting electron

will be stripped away from the proton at the centre.

Do it to enough atoms, and you create a plasma,

a soup of unattached particles.

And if the conditions are intense enough,

something extraordinary happens.

A chain reaction begins.

The protons are running around

and because they're positively charged

and they repel each other at distance,

most of the time, they just glance off each other.

At high energy, they bump into each other hard enough

that, occasionally, they'll stick.

That's the fusion process.

When four hydrogen protons ultimately fuse,

they create a new element.

Hydrogen becomes helium,

and an enormous amount of energy is released.

This is what happens when a star is born,

and it's all down to mass and the most famous equation in physics.

That helium nucleus that you just made weighs less

than the four hydrogens you used to make it.

Somehow, mass has disappeared in the process.

Anybody who knows any equation from physics knows that mass and energy

are linked by Einstein's most famous equation,

his equation E equals mc squared.

So that missing mass is energy.

But because c squared is such a large number,

a tiny little bit of mass creates a phenomenal amount of energy.

The sun only needs to use an infinitesimal amount

of its colossal mass each day to generate vast megawatts of energy.

Nuclear fusion is the process that not only brings stars into being,

it's what keeps them alive.

But when a star is born and starts its life story,

scientists have discovered

that something else very important can begin.

The first person to get an inkling

of this second story of creation was Nicholas Copernicus,

the father of modern astronomy.

And accidental social revolutionary.

In 1543, he published a book that overturned

more than 1,000 years of astronomical thought.

The belief that the sun revolved around the Earth.

Well, this is exciting.

This is one of the most important books in the history of science.

You can see from the title page that it's Copernicus's

Six Books On The Revolution Of The Heavenly Spheres,

and as well as being astronomically explosive,

it was also explosive in terms of changing humankind's understanding

of its place in the universe. And we can see that, I think, quite clearly

if we look at the famous diagram here, and you can see that here

at the centre is not the Earth, as people had thought for thousands

of years, but sol, Latin for sun, and here is the Earth going around

the central sun in this revolutionary new conception of the universe.

Earth had been relegated from the centre of the universe

to just the third rock circling the sun.

The traditional story of how the cosmos was constructed

had been shaken to its foundations.

And, in the 16th century,

this had deeply subversive implications.

At this time, people very much believed that God had created

a template for the heavens

and he'd used pretty much the same template to create

the society as well, and so with this "as above so below" belief,

any change in the heavens immediately had huge cultural implications.

By 1611, Copernicanism was sufficiently known that

the poet John Donne says, "The new philosophy calls all in doubt,

" 'tis all in pieces, all coherence gone."

Our view of our relationship with the sun had completely changed.

What Copernicus didn't know,

but what scientists have now worked out, is that the sun isn't just

at the centre of our solar system, it's the creator of it.

The birth of a star leads to the birth

of any planets that surround it.

Planets are the natural consequences of star formation.

Planets are the left-over debris of the gas and the dust forming a star.

They are like the afterbirth, if you like.

As the new star is born, the orbiting remnants of the cloud

from which it formed start creating a disc,

and over millions of years in this disc,

the dust grains start to stick together.

Blank out the light of a young star

in the northern constellation of Pegasus,

and you can see white dots, which are the planets forming

in the disc of dust that surrounds the star.

Eventually, the star is encircled by its children.

This whole process explains the distinctive shape

of all solar systems, including our own.

The reason why you see all the planets

going around the sun in the same direction on the same plane

is because they are all formed from the same belt, from the same disc.

Remarkably, just using observations with the naked eye

and the power of deduction,

Copernicus had created the first accurate family portrait of a star,

surrounded by its offspring, the planets.

But birth is just the beginning.

Every morning at dawn, the sun becomes the only star

that we can see in the sky.

A star in middle age, like 90% of all the other ones.

It's only special to us because it's so close.

Once it was realised that the sun was a star, it opens up

an enormous window to our understanding of the universe,

because the sun really is the only star that we can properly see,

and by looking at the sun,

we have this magnificent laboratory so close to us,

we can actually see it, we can actually study it,

we can actually see the surface,

make models of the interior, measure a lot of things in the atmosphere.

And by studying the sun in that way, we are studying the stars.

What most of us have learnt is that the sun's reliable,

dependable, unchanging.

But its serene outward appearance that we take for granted

belies a truth about all middle-aged stars.

Beneath the surface, there's a battle raging...

..uncovered by the scientists who know it better.

The sun is in the prime of its life.

It's a middle-aged star, but it's actually very dynamic,

very full of life.

I regard the sun as a sort of personal friend of mine

and like to know what's happening on the sun each day,

and I look at the satellite pictures to find out.

It's almost as if the sun sometimes doesn't want you to know

what's happening on it, though,

because sometimes the data links are down or something

and you can't actually see it.

That's quite frustrating,

because you want to know how your friend's getting on each day.

Dr Helen Mason's intimate relationship with the sun

has turned her into one of the world's leading solar physicists.

People think of it as quiet and boring,

but it's not at all quiet and boring,

and that makes it really interesting to study.

The work of scientists like Helen has revealed

that inside the sun, there's a fight

between two of nature's fundamental forces

that's key to the star's entire life history.

The gravity that created a star is pulling it inwards,

trying to crush it.

And the nuclear fusion that brought it to life is pushing outwards,

ready to blow it apart.

It will be disaster for the star

if either of these two forces gets the upper hand.

One 17th-century scientist who studied the sun didn't know this.

But he did quickly realise

that our parent star was more turbulent than it seemed.

That man was Galileo Galilei.

He used one of the earliest telescopes to project

detailed images of the sun,

completely transforming our understanding of it.

In the process, he shocked the world.

Well, when Galileo looked at a projection of the sun,

very much in the way that I'm doing,

what he saw were these, these spots,

these black spots on the sun.

People had seen them, previously -

I think the ancient Chinese had seen them through the fog -

but the important thing was that Galileo was actually saying

that these sunspots were on the sun

rather than satellites or something going in front of the sun,

in defiance of thousands of years of Catholic thought

that everything was supposed to be perfect,

and yet here we are with blemishes and spots on it.

Galileo's controversial work led him to end his days under house arrest,

but his observations revolutionised our knowledge of the sun.

Sunspots appeared and disappeared,

and by tracking them for several days, Galileo showed they moved,

revealing that the sun rotated.

What Galileo discovered overturned centuries of belief.

The sun wasn't a god-like immaculate disc

but a body that was constantly changing.

So this meant that the sun was not sublime any more.

It was made of the same sort of stuff as the Earth,

and therefore scientific processes

that were applied to the Earth could also be applied to the sun.

This underpins our subsequent discoveries about the sun,

the other stars and all of astronomy, really.

Building on Galileo's work,

scientists have discovered that the sun's active, changeable nature is,

in fact, the characteristic that has the biggest impact on us.

The sunspots he observed are linked to solar flares.

Sudden, colossal releases of energy that can spew

over a million tonnes of material into space.

This stream of charged particles is able to scramble

satellite communications -

in extreme cases, knock out power grids.

And all caused by the turbulent nature of the sun's magnetic field.

Sometimes these magnetic fields get twisted up.

The foot points move around, and they get really twisted up,

and they get so knotted up that eventually they crack and break.

And we have solar flares, huge explosion.

Particles are shot out into space.

In fact, this little active region, I mean, it's quite big, actually,

that we've been looking at recently, has been flaring continuously

over the past few days.

But while the sun's violent outbursts can harm us,

its active nature is what allows us to live at all.

Because the sun also ejects the solar wind,

an energised stream of particles that head out into space.

And that we can see passing Earth

as it bounces off our atmosphere...

..the aurorae.

Then the solar wind flies on.

Putting on the same show at the poles of Jupiter...

..and Saturn too.

Until, finally, 100 Earth sun distances away,

it loses its momentum and forms a boundary with deep space...

..creating a protective bubble that shields our solar system

from dangerous galactic radiation and cosmic rays -

the heliosphere.

Within it, life on a planet just the right distance away can thrive.

We are beneficiaries of the energy the sun generates

as nuclear fusion fights back against gravity.

Energy isn't created or destroyed, it's transferred, so it's transferred

from the centre of the sun through the atmospheres to us,

in many forms, warmth and light, via the plants

and via the food that we eat.

'As dawn throws into shadowy relief

'the giant pillars of Stonehenge,

'the successors of the ancient Druids await

'the first rays of midsummer sun.'

I can really understand

why ancient civilisations would have worshipped it,

because it is like a god in a sense of it provides everything

that's so important, that without it, the...life would cease to exist.

Ancient man was right to worry whether the sun would rise again.

It's been burning for five billion years,

but it's now used up half its hydrogen fuel resisting gravity.

One morning, the sun will rise on a last perfect day on Earth.

For many years, we had no idea when the end would come.

But now we can predict the sun's fate...

and our own.

We've learnt it, not by studying the sun,

but by observing all the other stars in the sky.

The breakthrough came when American astronomer Henry Norris Russell

and the Dane, Ejnar Hertzsprung, tried to create a pattern

that made sense of all the stars in the night sky.

No matter what their size,

or whether they burned hotter or dimmer.

This finally revealed that stars had a lifecycle.

At the turn of the 20th century,

astronomers have already a wealth of data about the stars.

Mainly, they have measured the colours

and the real luminosities of them.

So what Hertzsprung and Russell did was to organise

the stars in order of temperatures and in order of luminosities,

and this is the birth of the Hertzsprung-Russell diagram.

On one axis, they plotted how bright the stars would be

if they were all the same distance away from us,

from the dimmest to the brightest.

On the other axis was their temperature,

as indicated by their colour,

from blue and white hot to cooler red.

What was revelatory was the pattern that emerged.

Almost all the stars fell into a central diagonal line,

known as the main sequence.

These are the middle-aged stars, ones who still have enough hydrogen

in their cores to fuse into helium and resist the force of gravity.

But on either side were two small outcrops.

By deciphering the diagram, scientists discovered

that these outlying groups predicted the future of our sun.

Now, the sun will be burning hydrogen, as the stars do

in the main sequence, until the hydrogen is exhausted in the core,

and at that point, the star starts to die.

The outer layers of the sun will expand.

The sun will move away from the main sequence to become a Red Giant star.

From the apparent disorder of the night sky,

a map had been created...

..on which you could chart a star's journey through life.

It revealed that the fate of our own star was written in the night sky.

Once its hydrogen runs out, it will head off the main sequence

and move into the next phase of its life,

as a Red Giant.

Middleweight stars, like our sun, don't age gracefully

but catastrophically.

They swell up and become some of the largest,

most bloated stars in the universe.

Stars 200 times the size of our sun.

Thousands of times brighter.

Stars that are some of the most destructive in the universe

but also the most creative,

shining a rancid red in the inky sky.

Arcturus is a Red Giant star, very easy to find.

The tail of the Plough, the tail of the Big Bear, you follow that

and you reach the star Arcturus,

so it is in a way following the Big Bear, as a bear-taker,

which is what Arcturus means.

Surprisingly, Arcturus's striking colour is not because it's hotter

but because it's cooler.

As the balance between the opposing forces of gravity

and nuclear fusion breaks down,

the size of the star changes.

Red Giants expand,

their fiery energy spreading over a larger area,

which makes their temperature drop.

They fall from blue or white hot

to red hot,

but because they are so large,

these stars are still some of the brightest in the sky.

That's Arcturus.

When we see bright stars like Arcturus in the sky,

no doubt in many, many civilisations in the past,

they have some associations with these stars

and something that happens.

"Each star has its own distinct personality

"and it creates effects according to its character.

"When Arcturus rises, it is nearly always accompanied

"by a terrible hailstorm."

THUNDER

Actually, Arcturus is an omen

of something far worse than bad weather.

A portent of a drama more intense than any Hollywood could imagine.

When our own sun eventually becomes a Red Giant,

in five billion years' time,

it will turn into a destroyer, rather than a protector, of worlds.

Dr Robin Catchpole has devoted his middle years to studying

these devouring beasts of the night.

Their story starts the day the hydrogen in their core runs out.

Most of the star's life it spends fusing hydrogen into helium,

and this, of course, provides the pressure

that resists the force of gravity.

When the hydrogen runs out in the core and we've just got pure helium,

then there's no source of energy,

so the core starts to collapse

and as it collapses, under the force of gravity, it heats up.

And the temperature becomes high enough

to start nuclear fusion reactions in the shell around the core.

So we have what we call shell hydrogen burning.

Fusion has stopped in the core.

It's still hot, but it's dead.

The star is now fundamentally different from our twinkling sun.

The light we're seeing is still being generated by nuclear fusion,

but it's happening in a ring of hydrogen

that surrounds the core instead.

This is our new source of energy and this, of course, resists the force

of gravity and, in fact,

causes the outer atmosphere of the star to expand.

The star has begun its dramatic transformation into a Red Giant.

For our own sun, the change will be awe-inspiring

as, in its final years, it turns against the planets in its care.

The first thing that happens is it expands up

as far as Mercury's orbit here and swallows Mercury.

At this stage, it's about 1,000 times more luminous

than the sun is today.

It continues to expand and, within another million years or so,

it gets as far as Venus, and that's the end of Venus.

Venus is swallowed up, and then the sun continues out towards the Earth.

If we could see it, we would see something

nearly 3,000 times brighter than the sun is today.

It would be 260 times bigger than it is today,

but it would not have that beautiful tight compactness of the sun today.

Gas would be streaming off the surface,

it would be red and turbulent, slightly transparent.

It would almost seem to be coming apart at the seams.

Our only chance of survival would be to flee long before this crisis

and go in search of another solar system to call home.

In its angry old age, the sun will show no mercy,

even to its favoured child.

And the Earth disappears into the sun,

and I'm afraid that's curtains for the Earth.

Our planet will be engulfed by a ball of fiery gases,

never to be seen again.

The star that created and nurtured us

will ultimately, in its bloated old age, destroy us.

But while Red Giants bring annihilation,

scientists have uncovered in them the beginning of another story.

A story of creation that is about us, as well as about the stars.

They discovered that in the last stages of the battle

between gravity and nuclear fusion, Red Giants generate

two of the most abundant building blocks of the universe.

And these vital elements are being built in the heart of the Red Giant.

About half a million years after the poor old Earth has disappeared

into the sun, we get the temperature rising to the point

where we can suddenly start helium fusion,

and this is the next phase of the life of the star,

is a stage where helium is being fused in the core

to produce carbon and oxygen.

Stars, scientists discovered, aren't just twinkling points of light.

They're alchemists, creating the materials the cosmos is made of.

Most of the carbon in your body comes

from the discarded envelope of a Red Giant.

As the war between gravity and nuclear fusion

reaches its conclusion, the vast outer layers of the star detach

from the hot core, recycling carbon and oxygen into the universe.

What's left after this remarkable process is a remnant.

The star is ready to enter the next, enigmatic, phase of its life.

White Dwarfs baffled astronomers for decades.

The first problem was finding them.

It turned out they'd been hidden in plain sight.

We just needed a bigger telescope to see them.

The winter sky in the northern hemisphere brings

a set of fantastic constellations like this one, Canis Major.

Canis Major contains the brightest star in the night sky.

It's called Sirius.

A lovely star, also known as the Dog Star.

And it was discovered in the 19th century,

when the telescopes were really, really high quality

that Sirius has a companion, a very faint companion

that is lost in the glare of the very bright star.

This tiny companion to the bright Dog Star was dubbed the Pup,

and by 1922, this new type of star had an official classification.

It was called a White Dwarf.

But naming it was the least of scientists' problems.

When they compared its size to its mass,

something extraordinary emerged.

It was denser than anything on Earth,

denser than anything previously imagined.

They were a type of star that shouldn't exist.

The burnt-out remains of one whose fusion has stopped.

Their fuel is exhausted, so how can they still shine?

Mysteries that have long intrigued Professor John Ellis.

Throughout their lives, stars make their energy

by fusing together light nuclei to make heavier ones.

They start off with hydrogen and they make helium,

then they go on to fuse together helium to make carbon and oxygen.

And as time goes on, they burn up more and more of this fuel

until, eventually, it's like a car, you run out of gas.

With its helium-burning days at an end,

the White Dwarf's active life is over.

All it's left with is a dead core of carbon and oxygen.

It's not really a star at all but a cinder.

And the internal battle raging in the heart of the star,

between gravity and fusion, now has a clear victor.

Once the fusion stops,

the whole thing collapses under its own weight to form a White Dwarf.

So you've got this very small blob, which is incredibly dense.

It's going to be something like a million times denser

than it started off, so dense, in fact, that if you had a piece

the size of my mobile phone, it would weigh something like ten tonnes.

The core of the massive Red Giant collapses,

leaving the White Dwarf denser than anything

that had previously been discovered.

This raises another perplexing question.

Why doesn't gravity completely destroy them?

They were such baffling objects that one British astronomer commented,

"An appropriate response to the message from a White Dwarf

"was 'Shut up, don't talk nonsense.' "

It took a whole new revolutionary form of physics to emerge

before their secrets could be unravelled.

Quantum mechanics revealed much more about the innards of atoms,

enabling astronomers to begin to solve

the mystery of the White Dwarf.

In physics, we've got two different types of particles.

There are some particles that are very gregarious,

that like to get together, and then we've got other particles,

like the electron,

which like to be different from each other.

They're a little bit like people at a party

who are wearing the same colour dress.

They don't want to be standing next to each other,

so they're going to tend to naturally push away from each other.

That's...that's like what we physicists call pressure.

This pressure is created as the particles jostle for position.

It's a principle of quantum mechanics,

and when it was applied to stars,

the lives of dead White Dwarfs suddenly made sense.

What stopped them collapsing completely

was that gravity was resisted by the pressure generated

between the particles themselves.

In a White Dwarf, you've got a delicate balance

between the gravity which is trying to squeeze it together,

and the pressure of these electrons trying not to all have

their dresses in the same place, that are trying to push out.

And it's the balance between this gravity pulling in

and the electrons pushing out that keeps the White Dwarf the size it is.

It's also what lets a star with no fuel supply

shine for billions of years.

These White Dwarfs are very small,

so they've got a very small surface area,

which means that although they are white hot,

the light that they emit, the heat energy which they send out,

is still very limited just because of the very small size of the surface.

Now, it carries on radiating light and it gradually cools down,

it gradually gets dimmer and dimmer.

It's a little bit like a retired person sitting in an old stars' home.

It's still, you know, ticking along, but it gradually gets

sort of slower and slower, dimmer and dimmer.

There are White Dwarfs cluttering up our galaxy, all the other galaxies.

The enigma of the White Dwarf had been resolved.

Scientists had discovered how the vast majority of stars,

including our own sun, will end their days.

As White Dwarfs gently fading into the darkness of the universe.

But not all stars go so quietly.

For the most massive stars, something extraordinary happens.

They make their exit with one last spectacular hurrah.

Supernovae are the explosive, dramatic death throes

of the most massive stars in the universe.

Explosions so bright and intense

that they can briefly rival the output of ten billion suns.

They leave behind traces that paint the sky with a rainbow of colours.

Today, we know that these spectacular events

play a crucial role in creating the world around us.

Yet it took us centuries to discover it.

They're so rare that for hundreds of years, no-one saw any at all.

So the first challenge was to find them.

And that takes dedication,

perseverance and a love of the thrill of the chase.

Not just any kind of astronomer but a supernova hunter

and one with perfect timing.

You know, usually nothing much happens in astronomy.

Stars live for millions or billions of years,

so everything's the same from one night to another,

but not with a supernova.

It brightens dramatically over the course of just one night.

It happens on a human timescale.

Supernovae are so rarely seen in our own galaxy, the Milky Way,

that you need to peer much, much further to find many more.

You need to hunt for them in other galaxies.

Professor Alex Filippenko runs

one of the most successful search teams on Earth for doing just that.

In their best year, they discovered almost 100.

There's no calendar telling you

where and when to look for supernovae.

You just look kind of randomly at as many galaxies as you can,

repeatedly, and occasionally a supernova will go off in one of them.

I mean, they're rare, only two or three supernovae per galaxy

per century, so you really have to scan thousands of galaxies

in order to increase your odds of finding a few each year.

This robotic telescope automatically takes pictures

of over 1,000 galaxies a night, and it compares those new pictures

with pictures of the same galaxies it had taken previously.

If there's something new in one of the new pictures,

like a new star, that's an excellent candidate supernova.

That's the kind of thing that we want to keep studying.

The supernovae that Alex photographs

are hundreds of millions of light years away.

The only reason he can photograph them so distinctly

is because they are such colossal explosions.

And appreciating the power of a supernova's explosion

has been key to understanding the very composition of the universe.

For centuries, scientists have known

that everything we see on Earth is made up of 92 elements.

And the stars are made of the very same ones.

We can see it in their starlight.

Different elements give off different colours of light

when they're heated, when they're energised.

So if we look at a glowing cloud of gas in the sky, we can determine

what chemical elements it's made from by seeing what colours it has.

Potassium should produce a violet colour. Oh, look at that, wow!

Strontium.

Whoa, look at the strontium go!

Sodium, a bit like the light of the flames.

And, finally, I've got some copper here.

Look at the remnant of a supernova,

and you can spot the signature colours of some elements.

Modern scientists can reveal the full story

by splitting the light with a prism to create a spectrum.

And so I can see

that there's hydrogen being produced by this supernova,

and over here, the yellow/orange light

is due to glowing atoms of sodium.

It's the same sodium glow that we saw

when I sprinkled the chemical into the fire.

These ones here, in the green part, are iron,

and down here, in the violet part of the spectrum, is calcium.

The question that baffled scientists for decades, though,

was where did all the elements come from?

The breakthrough came from the mind of a doughty Yorkshireman,

Fred Hoyle.

The origin of the elements was a big question that scientists

were trying to tackle 50 years ago, and Fred Hoyle and his colleagues

thought that supernovae may be a key to unravelling the mystery.

At the time, this was a radical idea.

But Fred Hoyle was never a stranger to controversy.

'Fred Hoyle blows up stars by computer.

'This cosmic anarchist is the most controversial of theorists.'

If you think there's a mystery about why stars explode,

then you've got it all wrong.

Hoyle devoted ten years of his career

to proving his revolutionary theory on the origin of the elements.

He deduced that Red Giants are alchemists,

but he knew that they weren't hot enough to create all the elements.

He thought the ferocity of the supernova's explosion, though,

would make them the perfect furnace

and, with his colleagues, he did the calculations to prove it.

The key was the conditions created in the final stages

of a massive star's fight against gravity.

These stars are so massive and hot

that they can go through a whole series of nuclear reactions.

The ashes of one set of nuclear reactions

becomes the fuel for the next set of nuclear reactions.

The most massive stars are able to fuse heavier and heavier elements

in a series of layers,

creating the energy to resist the relentless inward pull of gravity.

There is neon and magnesium and more oxygen.

Then there's silicon and sulphur

and, finally, in the middle, a core of iron.

And that's where the fusion stops.

With fusion at an end, there's no more energy to fight back,

and gravity wins the battle.

The star is doomed.

When that ball of iron reaches a certain critical mass,

about the size of the Earth, but much, much more massive,

the electron pressure is no longer able to support it

against the inward force of gravity, so it starts to collapse.

It collapses to a ball about the size of a city

and then rebounds and that rebounds...

hits the surrounding layers,

launching a supernova explosion.

It's the speed and violence of the collapse of the star's iron core

that triggers the supernova,

an implosion that launches an explosion...

..creating enough heat and energy to forge almost all the other elements.

The supernova explosion is able to produce

some of the very rare elements heavier than iron -

the zinc, the gold, the platinum, the silver.

These things are ejected into the cosmos, having produced them

in these very special conditions of an exploded star.

The very atoms of which we are made, the oxygen that we breathe,

the calcium in our bones,

the iron in our red blood cells,

were produced billions of years ago

in stars, specifically in dying stars,

and these dying stars ejected these elements into the cosmos,

making them available for raw material for the production

of new stars, planets and, ultimately, life.

We are stardust

or rather, less romantically, nuclear waste.

In a way, the ancients were right.

The stars ARE like gods.

They are the creators of us.

To make our Earth,

several hundred generations of stars needed to come and go.

Stars born from collapsing clouds of dust and gas.

Bursting into life, to shine for millions or billions of years.

Bloating in old age to become Red Giants.

Their cores contracting into White Dwarfs.

The most massive ones exploding as supernovae,

flinging the elements they've created out into space

to form the materials for the next generation of stars.

But that's not the end of the story.

Supernovae may look like the death of a star, but for some,

there is life beyond the grave.

Understanding that took a particular breed of scientist.

They probed deep into their own imagination

and a world of calculations.

And what they found there were predictions of objects so bizarre,

so weird, that we're only beginning to understand them.

In the process, unravelling even deeper secrets about the universe.

The faintest of signals picked up from deepest space have revealed

to modern scientists exotic stellar tombstones.

Tombstones first predicted in the theoretical calculations

of the maverick Swiss astronomer Fritz Zwicky,

more than 80 years ago.

He was sure that when a supernova exploded, it left behind a kernel

so dense that a cupful would be as heavy as a mountain.

He called it a neutron star.

It seemed so preposterous that Zwicky's ideas were dismissed.

Until, that is, a new way of scouring the heavens emerged -

radio astronomy.

In 1967, the fledgling discipline picked up

a strange repetitive message from outer space.

Now, the people here say that if they got three signals

as exactly spaced as that, it would be very unusual.

If they got four, it would be phenomenal.

Well, they've had pulses as exactly spaced as that 24 hours of the day

since November.

These pulses were so exact and predictable in their pattern

that scientists even considered aliens as their source.

It turned out they were being transmitted by something

equally unlikely

and just as unfamiliar.

The most important question of all - what are they?

Well, we know that they're very small.

They're objects about the size of a planet.

We know also that they are very energetic and that the source

of energy must be far greater than a planet could really provide.

It must be something like a star compressed

into a volume the size of a planet.

Scientists worked out that the new star had to be denser

than any type previously discovered.

Could these be the neutron stars predicted by Zwicky?

Astronomers nicknamed them pulsars and immediately

set their telescopes, searching for further clues about them.

Just a year later, they found one,

in the perfect place to put Zwicky's theory to the test.

In the winter,

we have access to the beautiful part of the sky

that contains the constellation of Taurus, the Bull.

Here we have the Pleiades, or the Seven Sisters,

down here we have another cluster of the stars, which are the Hyades,

that contain the bright star Aldebaran, the angry eye of the bull.

And if we follow from Aldebaran in this direction towards

that star there, just about there, we will find the closest pulsar

to the solar system, the Crab Pulsar.

What particularly excited scientists when they discovered the Crab Pulsar

was that it was buried deep within the remains of a supernova.

In this amazing picture, we see the remnant of a supernova explosion,

but when we scan the central part of this nebula,

we find the pulsar,

which is the remnant of the core of the star that exploded.

Now that a pulsar was definitively connected to a supernova,

scientists realised that they had discovered

another of the seven ages of starlight.

It showed Swiss astronomer Zwicky was correct all along.

His theoretical equations predicted

how a supernova could leave behind such a dense remnant.

The calculations focused on a strange quality of all matter.

It's one that defies common sense but is fundamental

to the work of astrophysicists like Professor Doug Leonard.

Solidity is an illusion.

If I run up with my fist and punch a brick wall, it will hurt like heck,

but, essentially, my fist and the wall

are almost entirely empty space.

The illusion comes because we're made out of atoms,

the fundamental building blocks of matter,

and most of what an atom is is empty space.

So, if this is an atomic nucleus containing the protons and neutrons,

the electrons would be roughly where those buildings are

in the background.

Zwicky predicted the one thing violent enough to ram together

atomic particles and fill all this empty space

is the collapse of a massive star during a supernova.

A collapse that happens in a matter of seconds.

In a supernova,

the very first thing that happens is the iron core implodes,

from something about the size of the Earth down to something the size

of a small city, and in that implosion,

the densities become so high

that the protons and the electrons get squeezed together

to form neutrons.

And, essentially, all the air of the atoms gets squeezed out of it,

and what you're left with at the end is a ball of neutrons,

an incredibly dense object that we call a neutron star.

And as the neutron star formed, its magnetic field intensified.

And became billions of times stronger than our suns.

Now, as the star span,

it channelled out radio signals from its north and south poles.

Signals that swept past Earth with every rotation of the star.

This was the source of the mysterious pulses.

Some are so regular that pulsars are among

the most accurate clocks in the universe.

The discovery of neutron stars was a vindication of the power

of theoretical physics.

It set astronomers wondering if other strange bodies

that had been predicted could be lurking in space.

And there was one hypothetical object

that was even weirder than a neutron star.

The last stage of a star's life

is as much an idea of science fiction as a physical reality.

Put forward by science writer Adrian Berry in his book The Iron Sun,

the suggestion is that, in the future,

man could use black holes to transport himself instantly

around the universe, and when I say instantly, I really mean like that.

For years, most scientists dismissed black holes as fanciful conjecture.

They were apparently nonsensical structures of space and time,

spat out when Albert Einstein's equations were taken

to their extreme conclusion.

Einstein's theory of relativity does lead us into very strange

and unfamiliar paths.

Einstein himself didn't believe in black holes.

But in our search to understand them, we might have found

a clue to the biggest question of all.

The very origin of the universe.

It's like there's just a huge question mark in the sky,

where one of these things exists.

They are the most mysterious objects in space.

It's where the equations themselves break down.

Black holes are so complex, so fantastical,

that even now we know they ARE real,

they throw up more questions than answers.

How can they exist? They simply don't make sense.

A black hole represents a spot in space around which the gravity

is so intense that nothing, not even light, can get away.

It's a region bounded by something called the event horizon

within which all events are beyond the horizon of someone outside,

meaning they cannot see anything that's happening inside there,

so it's a region of space from which no information can ever escape.

Scientists think that these extraordinary monsters in space

are created by the death of the most massive stars.

Rare stars whose cores are so huge that when they collapse,

they don't turn into a pulsar.

The collapse just keeps on going.

It remains, to some extent, a theory...

..but Doug Leonard has got as close as anyone

to actually seeing it happen.

It started by getting an alert on the computer

that a supernova had gone off in a very nearby galaxy,

only 210 million light years away.

Here's a picture of the supernova indicated by the arrow,

and so what we immediately did

was trawl through the Hubble Space Telescope archives

to see if we could find a picture of that exact spot in the sky

taken before the star had actually exploded,

and, as luck would have it, someone did.

The image revealed that the supernova

was the explosion of a star dubbed LBV-1, in a distant galaxy.

Doug and his team realised that they had an unprecedented opportunity.

Because the star had been a super-massive one,

at least 50 times the mass of the sun.

It was exactly the size to test out the theoretical equations.

Could this possibly be the birth of a black hole?

Two years we waited for all of the fireworks

and embers of the supernova to disappear and go away,

so that we could get a third picture long after the supernova was gone

to see if that star in fact had disappeared, and in fact it had.

It was now gone.

It was an extremely luminous star, it blew up and now it was gone.

The evidence suggested that billions of tonnes of matter

from a massive star had shrunk to nothing.

So what we're left with here is this mind-boggling idea

of mass contained in zero volume, and that just makes your head spin,

but that's what we call a black hole.

It's these very qualities that make some scientists think

understanding black holes could hold the key not to death

but to the birth of the very first stars.

It's really an awe-inspiring story,

much more so than the classical creation myths

that make it seem so easy.

Scientists have discovered that there's one other place

you can find a point of infinite density and zero volume.

That's at the instant the universe began,

a moment studied by astronomer Dr Alan Dressler.

Today, it's scientific orthodoxy, but it wasn't always that way.

The idea that the universe had a creation event

from a scientific perspective was a revolutionary idea.

Every bit as remarkable a revolution as the idea

that the sun and not the Earth was the centre of the solar system.

Scientists call it the Big Bang,

and it was predicted by the very same equations

that discovered black holes.

There's the Big Bang theory according to which...

The universe began with a gigantic fireball on creation day,

some 10,000 million years ago.

It was here, at the beginning of the universe,

that scientists found the answer

to the ultimate question about the lives of stars.

Where did the hydrogen to make the very first ones come from?

From this very early instant came a primordial soup of energy and matter

that had to cool before it could become the elements of hydrogen

and helium that made everything else in the universe we know today.

Every hydrogen atom that fuelled every star

was made in those first few minutes of the Big Bang.

The extraordinary thing about the lifecycle of the stars

is that it's revealed the origin of the universe,

the elements, even of us.

But that isn't quite the end of the star story.

Astronomers have discovered one other tantalising fact

as they've looked out into the dark sky.

In nebulae, formed from the remnants of stars

and where the next generation are born,

they've discovered the earliest stirrings of life.

Even for NASA, nebulae are too far away to visit...

..so they've built one of their own here on Earth.

40 years ago, scientists peered into the clouds of dust and gas

created from the remains of stars and, to their surprise,

found not just elements but organic molecules.

I think it really is a shift in people's thinking about this.

50, 60 years ago, people didn't think

space had any of this kind of molecular complexity.

Now we know it does.

Many of these molecules are organic molecules.

Many of them may be complex, and, in fact, some of them

are likely to be the kinds of molecules you like to have around

if you want to have life get started.

Dr Scott Sandford is at the cutting edge of research at NASA

where they're trying to answer an extraordinary question about stars.

Just how many steps towards life can be made in the nebulae

that are the stellar nurseries and graveyards of outer space?

What we have right now is a nice little simulation

of an interstellar dense molecular cloud,

so this is a star formation region in a jar, basically.

And now we just need to let it cook for 24 hours

and then we'll be ready to pull the sample out and see what we made.

When Scott and other scientists have analysed their results,

what they've found is that as the nebulae create stars,

they make the building blocks of living things on Earth.

There's just a whole host of compounds we make.

We find that many of these compounds are very interesting,

because they play roles in life on Earth,

and so it's clear we're making many of the building blocks of life

by these very processes that happen in space.

These molecules might hold the secret

to how life began on our planet.

If they were part of the process, they'd have to firstly get to Earth,

and scientists have found a delivery system.

This is part of a meteorite that crashed from outer space

to Earth in Australia.

In it were found many of the organic compounds

vital to life on our planet.

The amino acids in this meteorite predate the arrival

of this meteorite to the Earth, so in fact these amino acids

had to have been made in space in some environment,

and so amino acids do exist out there in space

and they do get delivered to plants.

Perhaps life didn't have to start from scratch here on Earth.

Could the building blocks have been scattered from space?

We don't know if the origin of life on the Earth

owes its existence to these kinds of materials being delivered from space,

because we don't understand how life got started.

However, the analogy is if you're trying to build a Lego castle,

it's probably a lot easier if Legos fall out of the sky on you

than if you have to build Lego blocks from scratch

and then make your Lego castle.

And if those Lego pieces were available to Earth,

they could be available to planets orbiting other stars.

Well, given that we know that just about anywhere you make stars,

you're going to make these Lego blocks,

and the fact that there are a huge number of environments

where these Lego blocks will be delivered,

I personally would be quite surprised

if there isn't other life out there.

We may never know for sure whether there is life elsewhere.

But we do know a lot about where we came from.

And that's because we've learnt so much about things here on Earth

from looking far out into space.

The discovery that stars are not eternal,

that they actually have their birth, their lives and they eventually die,

is one of the greatest achievements of modern science.

And even more amazing,

that we have achieved that from this little vantage point

in the corner of a galaxy, the Milky Way.

Imagine that we live in a completely clouded planet, say like Venus,

that nobody ever has seen the stars, the movements of the sky,

I wonder, our culture, our science would have been completely different.

Our lives would be completely different.

So how lucky we are to be here on this planet

with this beautiful transparent atmosphere that allows us

to admire the majestic display of the starry night.

By looking at the stars, generations of imaginative scientists

have stretched the boundaries of knowledge,

discovering truths stranger than fiction...

and, through the stars, uncovered the story of the universe.

But like all good tales, it will eventually come to an end.

About 100 trillion years from now,

the raw materials for new stars will run out.

The last will play out their lives and their remnants gradually fade,

until, finally, the one remaining cinder goes cold

and light will be extinguished from the universe.

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