The Sun

It is dawn and the sun is rising, as it has every day for the last five billion years.

For millennia it has been a constant golden disk shining its unchanging light onto the Earth.

But look through the glare and the true face of the sun is revealed.

Not constant, but constantly changing.

Turbulent and violent.

Its worst tantrums can wreak havoc on the Earth.

To understand the sun is to understand the forces that drive the universe.

If we can control those forces, we can unlock the power of the stars.

The power of our sun.

The story of the sun starts 13 billion years ago with the big bang.

In an instant, the universe was born, and since then it's been expanding at the speed of light.

Within the universe there are 100 billion galaxies.

Our galaxy is but one of them.

In it, there are 100 billion stars.

And towards the edge of one of the spiral arms is an almost insignificant dot.

A medium-sized, not very bright, undistinguished star.

Up close, it's a different story.

On the planets closest to the sun, Mercury and Venus, the heat is intense, their surfaces scorched.

Further out through the solar system, the sun's rays weaken,

until they are powerless against the chill of space.

The outer planets are frozen.

But in the middle lies the Goldilocks planet.

Not too hot, and not too cold.

In fact it's just right, and life has flourished in the warm glow.

All life on Earth owes its existence to the sun.

It powers every natural system and sustains every plant and animal.

Without the sun, the planet would be a barren, lifeless ball of rock.

Recognising that power, humans have always worshipped the sun.

But we have also always striven to understand it.

These monuments are more than just temples.

They are calendars and observatories.

Tools for studying the sun.

Some of them are still operational.

This is Orkney.

To live here is to know the importance of the sun.

In the summer, the days are long and full of light.

In December it's a different story.

It's mid-winter.

It's about 11 in the morning and it's still not light completely.

There's a strong wind coming off the Atlantic and it's cold and it's wet.

That's pretty much typical of this time of the year up here.

Yet, despite the cold, in the Stone Age, 5000 years ago, a civilisation thrived here.

The island is covered in the remains of their society.

The ruins are full of mystery.

We know little about the people who lived here,

but they did leave evidence of the important role the sun played in their lives.

Maeshowe, 1,000 years older than the Pyramids,

is one of finest examples of Stone Age architecture.

Entering Maeshowe,

you have to crouch right down and are confronted with a passage

that seems to go on and on and on.

Slightly feel an impression of going uphill, up a slope.

Coming through clearly another doorway,

suddenly...

the whole thing opens out into the most amazing chamber.

This alone is probably the highest and largest enclosed space

that Neolithic Orcadians would have experienced.

When it was excavated, back in the 19th century,

the clay floor was littered with broken pieces of human skull.

This is a place of the dead.

This is a house of the dead.

Most of the time the occupants of the tomb were left in complete darkness.

Then, at sunset on the Winter Solstice,

the shortest day of the year, something amazing happens.

The light of the setting sun shines straight up the entrance tunnel

and illuminates the interior.

The significance is that it's marking the shortest time of the year, with the least light,

and from that point on, slowly and gradually,

light is going to increase, the days are going to grow longer.

So what's happening here is that the dead, the ancestors,

are being awoken on that shortest day.

The Winter Solstice events at Maeshowe demonstrate

an intimate and precise knowledge of the sun's movements through the sky.

It was the first step on our journey

to understand the sun and its many effects on us.

To complete that journey, we've had to travel

to the furthest depths of space

and to the heart of the smallest atom.

And with every closer look, the sun has always surprised us.

To our ancestors, its power was its reliability.

Always on time.

Never changing.

But the reality is proving to be very different.

Most people think of the sun as quite a boring, constant sort of thing, but it's not at all.

It's changing all the time.

If you look, you can changes in a matter of minutes or hours.

It's far from static and boring.

It's changing and it's got a life of its own.

Modern solar observatories magnify and filter the sun's light

to get past the constant glare and give a clear view of the surface.

This is the actual face of the sun.

It is turbulent and boiling.

Never the same from one second to the next,

the surface bubbles like a giant bowl of porridge.

Each bubble is 1,000 miles across.

The heat and light brought to the surface raises its temperature to 6,000 degrees centigrade -

enough to vaporise solid rock.

And the sun is huge.

You could fit the Earth inside it a million times over.

Periodically, huge explosions rip through the surface,

releasing the energy of a billion atomic bombs in seconds.

All this is on the surface.

To understand the sun, we must know what is going on deep inside.

That is where the power is generated.

So, for centuries, scientists have been devising ways

to probe the heart of the sun.

Some of them have been complex and some of them very simple.

And the first step is to figure out just how powerful the sun is.

It's easy to appreciate the power of the sun

on a nice hot summer's day on the Texas Gulf Coast.

You feel the power of the sun on your skin, sunscreen's on.

But, man, the sun is just...

the actual physics of what's going on inside the sun,

the power of the sun, the energy it's releasing,

is almost beyond comprehension.

But it is only almost beyond comprehension.

And you can measure its power output with some simple apparatus.

One of the earliest experiments to measure the power of the sun

was by astronomer William Herschel in the 19th century,

where he had the brilliant idea of watching ice melt to see how long it would take.

Therefore, from the properties of the ice,

he worked out how much sunlight was coming to the ground.

As a demonstration of the sun's power,

it doesn't look that impressive.

But Hershel realized that he could use the time it takes to melt one bit of ice

to calculate the sun's total power output.

So here we see the ice is almost completely melted

in roughly 29 minutes - almost half an hour.

Herschel was able to use this experiment and the time that it took to melt the ice

to work out basic properties of the sun.

Here's how Herschel's thinking worked.

In the time it takes to melt a slab of ice on Earth,

the sun is radiating heat in all directions -

enough to melt a complete shell of ice around it,

a diameter of 300 million kilometres.

A shell half a centimetre thick

and 300 million kilometres across contains a lot of ice -

enough to make an ice cube bigger than the Earth.

To melt that much ice in just 30 minutes

would take an energy input of a billion billion billion watts.

It's a rough but surprisingly accurate experiment.

Modern satellite readings confirm the figures to within a few percent.

It's an almost unimaginable amount of energy.

If we could harness the sun's power output for a single second,

it would satisfy the world's energy demands for the next million years.

But it's one thing to know how much power the sun is producing.

It's something else to know how it's doing it.

Until the middle of the 20th century,

no-one had any idea what made the sun work.

For scientists in Hershel's time, it was a mystery.

One of the issues was what powered the sun.

And some very clever people actually considered the fact

that the sun might be powered by burning coal.

It seems ludicrous, but why not coal?

That was an important source of energy on the Earth

in that part of the 19th century.

If the sun was made entirely of coal,

there would be one unfortunate consequence.

It would burn itself out in just a few thousand years.

Today that sounds ridiculous,

but 200 years ago, it didn't seem so unlikely.

It was widely believed that the Earth

was only a few thousand years old.

But in the mid 19th century, a new science was emerging

that was painting a very different picture of the age of the Earth.

By looking at the deepest layers of rocks,

geologists were discovering that the Earth was much older

than anyone had previously imagined.

If that was true,

then the sun also had to have been burning for much, much longer.

You can see the strata and the lines in the rock

that represent hundreds of millions of years of geological history.

The top of Sacramento Peak, up beyond this,

we find fossils that are 300 million years old.

Below Sacramento Peak you've got more than a kilometre of strata

that are far older than that.

From the age of these strata, geologists knew that the Earth

was at least a billion years old.

At the same time,

astronomers thought the sun was only 10,000 years old.

If geologists were right, astronomers had to find another source for the sun producing its energy.

The search for the source of energy

that could power the sun for billions of years

lasted for nearly a century.

Eventually, scientists would find the answer

in the forces that hold atoms together,

and in the nature of matter itself.

But first, you have to know what the sun is made of.

To find that out, you need to take a very close look at sunlight.

When you take the light from the sun and pass it through a prism,

spread it out into the colours,

you notice it isn't uniform - there are places that are darker.

Each of those dark lines is due to a specific chemical element.

Each element has its own series of lines that are specific to it.

Each chemical element absorbs light at specific frequencies,

removing a strip from the spectrum.

As the light passes through the sun,

all the elements leave their mark.

So, when the light arrives at the Earth,

it contains the complete chemical formula of the sun.

If we take the spectrum and spread it out to fine details -

and what we've done here is stacked up pieces of the spectrum on top of each other -

you see all of these dark areas.

The key to figuring out how much of these elements are there

is the breadth and the depth of the line -

how dark is the line and how broad is it?

The composition of the sun lies in this barcode.

All the thin lines are caused by tiny amounts of complex elements -

metals like iron and magnesium.

But there are three very deep, broad lines,

and they are all caused by an enormous amount of a single element.

There's some helium and traces of heavier elements.

But over 90% of the sun is hydrogen.

It's the simplest and most common element in the universe.

The secrets of the sun's power must lie in this gas.

Look carefully into the night sky,

and you'll see clouds of hydrogen floating in interstellar space.

These are nebulae, and they can be hundreds of light years across.

They are some of the brightest areas in the sky,

lit up by the intense light of newly-formed massive stars.

In them, we can see stars being born.

It's to areas like this that astronomers turn their telescopes

when they want to study how our sun was formed.

There she is. OK.

So let's re-centre on her.

By studying different nebulae,

it's possible to piece together the stages in which stars are made.

And it all starts in a cold, dark cloud,

floating around waiting for something to happen.

Cold clouds like this are actually quite stable.

They will sit there for a very long time,

for thousands or millions of years,

before they will actually do anything.

What you need to get the star formation process going is to kick it with something.

That can be an impact on the cloud from one side by a supernova blast wave -

a massive star nearby has gone phoom at the end of its life,

and that sends out in all directions very energetic compression waves

that hit the gas and compress it.

The shock waves knock the cloud out of balance,

causing localised clumps of hydrogen to form.

These are the seeds from which stars grow.

The increased gravity of the seeds sucks in more and more hydrogen

in a runaway process that lasts for a million years.

As more hydrogen is squeezed into the clumps, the temperature rises.

They are not yet producing light,

but they are well on their way to becoming stars.

As they grow bigger and bigger, they start to spin

and throw out a disk of debris that coalesces to form a solar system.

This kind of process is exactly what would have formed our solar system.

When we look at our solar system,

all of the planets rotate around the sun in the same direction,

and they all are in the same plane - the same flat sheet around our sun -

and this is exactly a consequence of the fact

that the early solar system formed out of a broad disc.

With the solar system in place,

all that is left is for the young star to light up.

When it happens, it is sudden and irreversible.

Ultimately, the process starts.

And because it liberates so much energy with that first fusion,

then the process takes off.

It lights up a large area

and it starts to shine on its own within a matter of minutes.

It's a very quick process.

In that first burst of light,

the star has begun its lifelong activity

as a factory for making other chemical elements.

Every atom in everything around us was made in the heart of a star,

and all were made from the same starting ingredient.

The simplest element that we have is hydrogen,

and it's the building block for all the other elements that we have.

In the heart of the sun, hydrogen nuclei - protons -

are stuck together to make helium.

It sounds straightforward,

but it can only happen in the most extreme conditions.

In order for these protons to come together,

because they're both positively charged -

they've actually go to be pushed together.

In order to do this, you need very high temperatures -

so they're moving very fast, and you also need very high pressures.

The only part of the sun that is hot and dense enough is the core -

an area that contains over half the star's mass

in less than 2% of its volume.

Here, at 15 million degrees,

the protons are bashed together so hard that they fuse.

A helium nucleus is a tiny bit lighter

than the combined mass of the four protons it is made from.

And, as Einstein tells us,

it is that tiny bit of lost mass that provides the power.

Energy is equal to mass times the speed of light,

times the speed of light.

Now, the speed of light is a very, very big number.

So, if we take a small amount of mass, we get a huge amount of energy.

That's the energy which powers our sun.

Every second, five million tonnes of the sun is converted to pure energy.

And although it has been burning for five billion years,

it is only halfway through its supplies of hydrogen.

The light produced in the core must travel over half a million kilometres to the surface.

And it does so very slowly.

The heart of the sun is so dense,

the speed of light is less than 1mm a second.

It can take 200,000 years

for the light to travel from the core to the surface.

It takes just another eight minutes to get to the Earth.

This is what the power of nuclear fusion looks like

from 150 million kilometres away.

This is what it looks like close up.

The H-bomb was man's first attempt at recreating the sun on Earth -

a balloon full of hydrogen squeezed until it released its energy.

In contrast to its destructive power,

it's long been realised that controlled nuclear fusion

could solve the world's energy problems.

It has been one of the holy grails of science for half a century.

This kind of power, the H-bomb, is a manmade version of this - the sun.

In 1958, Britain announced that she could produce this power in a laboratory,

in a machine called Zeta.

There is a prospect of unlimited energy from controlled thermo-nuclear fusion.

Unfortunately, it wasn't that easy.

But now, nearly 50 years later,

in the same laboratories in Oxfordshire,

scientists are finally managing to create their own star on Earth.

-Ready when you are.

-OK, ready.

Requiring shot 14658.

Starting shot in five seconds.

It might not seem like much...

..but slowed down by 300 times, the pictures reveal how the gas plasma

is being squeezed and heated

to create the most extreme conditions in the solar system.

Plasmas I always like to think of as being like naughty children.

They're full of energy and they want to misbehave,

and it's our job to try and control that misbehaviour.

For the particles to fuse on Earth,

the temperatures need to be raised

to ten times those found at the heart of the sun.

Bombarding the gas with a stream of fast neutrons

raises the temperature to over 100 million degrees.

Only then can the energy of nuclear fusion be released.

After years of learning to control the plasma, scientists now believe

they are within sight of harnessing the sun's power.

The aim of it is to be able to produce cheap, clean

and effectively an inexhaustible supply of electricity for future generations.

This is only a small experimental reactor.

It can only run for a few seconds

and sucks up more energy than it creates.

But the next generation of bigger reactors is already being built.

When operational, they will produce ten times more energy than they use.

They will be stars on Earth -

power stations that won't deplete natural resources,

or produce dangerous waste products.

It sounds great, but recreating the sun isn't easy

and it may be another 50 years

before the fusion power station becomes reality.

Until then, we'll just have to make do with the real thing.

But that's not so bad.

Just seeing sunlight is enough to cheer us up.

Many people think it's the warmth of the sun

that is associated with us feeling good.

Of course that's true, but research has shown

it's not really the warmth - it's the light that's important.

Sunlight controls our daily cycle,

making sure we wake up in the morning and go to sleep at night.

When there's not enough light those patterns get disturbed,

with miserable effect.

It's a clinical fact that depression is more common in winter

because of the lack of sunlight.

They call it SAD - Seasonal Affective Disorder.

Seasonal Affective Disorder is a depressive illness.

It the starts during the autumn and early winter

and usually goes away completely during spring and early summer.

Some people feel quite miserable, depressed and gloomy

during the winter months and some people - a small proportion -

will go on to develop clinically significant depression,

which requires treatment.

This is Rattenburg, a fairytale Austrian village

cursed by a lack of sunlight.

Due to a quirk of geology,

it gets no sunlight at all between November and February.

During those winter months, the sun never rises high enough

to clear the brow of Rat Mountain.

And the town lies in permanent shadow,

while it's neighbour across the river basks in the sunshine.

In winter, of course, it's very cold.

It's shadow and, as you can imagine, it's cold.

We are freezing and if you want to have some sun, you have to move to the next village.

It makes you happier to sit in the sunlight

and not to freeze in the shadow.

Frozen and in the dark, the residents have been forced

to take desperate measures to bring some light into their lives.

Helmar Zangirl is a lighting expert who specialises

in bringing natural light into some of the world's biggest buildings.

He may be the salvation of Rattenburg.

Evolutionary speaking,

man has adapted to natural light,

and has adapted to the sun, and if you deprive man of the sun,

clearly it's a different quality of life.

How do you feel if you sit for one week in a place with fog?

And how do you feel if you sit for one day in a place where the sun shines?

I personally feel so much happier in sunshine, I can tell you that.

The solution for Rattenburg is simple -

steal some of the sunlight from the other side of the valley.

Eventually, a huge system of mirrors will reflect the light of the sun

to a second set of mirrors on the castle above the town,

and then reflect it down into the streets

to brighten the lives of the citizens of Rattenburg.

The main effect for the people in town will be

that part of the facades of buildings

and parts of the street, at least of the main street,

will be brightly illuminated,

and will clearly be recognised as sunlight.

It sounds crazy, but it's true.

People will go to extraordinary lengths for a bit of sunlight.

But the sun is much more than a giant light bulb.

There are other forces at work in the sun.

Forces that change over minutes and over years.

Forces that tear the surface apart.

We are only now beginning to understand these forces

and the effects they have on the Earth.

But we've known about them for hundreds of years...

because the sun gets spots.

Sun spots are dark regions on the surface of the sun,

typically about the size of the Earth, in terms of area.

Here's a live shot of one today.

Unfortunately, we've got a very windy day with high thin clouds, so the picture's not very sharp.

With very high resolution images, we can see detailed structure.

The inner part of the sun spot, the dark umbra,

is dark only in comparison to the rest of the Sun.

It's actually bright enough to blind you if you looked at it alone.

The spots are not static.

These video images show the edges crawling,

almost as if it was alive.

The movements of sun spots have been studied for 400 years,

ever since Galileo trained his telescope on the sun

and made the first crucial discovery about its behaviour.

Surprised to see black dots creeping over the surface,

he kept track of them over a number of days,

and found that they were all moving in the same direction.

To Galileo, the meaning was clear -

the sun was rotating,

and was turning faster at the equator than at the poles.

It was a discovery that was to prove critical in our understanding of how the sun works.

Ever since Galileo,

records have been kept of the coming and goings of sun spots

and variations soon became clear.

Sometimes the sun is covered in hundreds of spots.

Other times, there are none at all.

And after a while, a pattern emerged.

If you observe sun spots over several years,

they come and go with about an 11-year cycle.

From a minimum, where there can be no spots at all on the sun for weeks or months at a time,

to a maximum where you can have 100 spots visible on the surface of the sun,

and then a decline again, usually over about six years or so, back to minimum.

Until recently, no-one knew what was driving the solar cycle.

Or where sun spots came from.

But people had always suspected that the scars on the sun's surface

had an effect on the Earth.

But no-one could quite put their finger on what it was.

It has been correlated with all kinds of different things -

the price of wheat, the thickness of fur on animals.

I get a guy that comes to my website trying to predict the stock market

from the daily sun spot areas.

"Oh, you didn't post them today! I gotta know!"

"You making money or what?! I want some if you are!"

One effect sun spots do have on the Earth is on the climate.

But it is a subtle effect.

Its greatest impact was only noticed 300 years after the event.

It was discovered in Greenwich by the astronomer Robert Maunder,

who was studying the hundreds of years of sun spot records

held at the Royal Observatory.

In the late part of the 19th century,

he became particularly interested in sun spots.

He discovered there had been a peculiar effect in the second part of the 17th and 18th centuries

when sun spot numbers diminished to a fraction of what they are today.

He described this as the Maunder Minimum.

For 70 years, from 1645-1715,

sun spots disappeared.

It was as if the engine that drives the solar cycle had stopped.

And it correlated almost exactly

with the last period of prolonged cold

to strike the northern hemisphere.

They call it the little ice age.

What's interesting about a period like the little ice age

is that you don't see a very large dip in temperatures

but even a degree or two is enough to see some really quite dramatic effects.

Like pack ice advancing south from the North Pole,

like the Viking colonies in Greenland being wiped out by the changing climate

and the population of Iceland falling by half.

In Britain, the weather was cold enough

the Thames would freeze in winter,

and one of the classic depictions of life at the time is frost fairs,

when the population held a fair on the frozen river.

So things must have been a lot harsher than today.

When the sun spots disappeared,

something on the sun changed that cooled the Earth down.

But it wasn't that the solar output changed.

No matter how many sun spots there are,

the sun doesn't get any hotter or brighter.

So the spots must have another effect.

And to see it, we need a different way to look at the sun.

It's not just heat and light that the sun is throwing at the Earth.

As every sunbather knows, there's ultraviolet light too.

Enough UV reaches the surface to burn our skin,

but it is only a fraction of the sun's UV output.

The rest is filtered out by the atmosphere.

It means we don't get a complete picture of the sun from the Earth.

To see it in all its glory, you have to go into space.

From here, you can really see the changing sun.

In the extreme UV, the sun spots burn a brilliant white.

In the X-ray frequencies they look even more dramatic.

Huge plumes of superheated gas spout from the spots.

When you're seeing the visible, you're seeing the surface of the sun.

When you see the ultraviolet with X-rays,

you're seeing that hot atmosphere - a million degrees.

The surface is about 6,000 degrees.

So you're seeing a different part of the sun

and you're seeing a part that's constantly changing.

These phenomenal displays of solar power

were only discovered in the 1970s by the astronauts on Skylab.

No-one had seen the sun like this before.

Since then, a number of space telescopes have been deployed,

whose sole purpose is to look at the sun.

The most used is SOHO -

the Solar and Heliospheric Observatory.

It sits a million miles away from the Earth, at the Lagrangian point,

where the gravitational pull from the Earth and the sun is equal.

Fixed in space, it has an uninterrupted view of the sun and its tantrums.

It has completely transformed our understanding.

You can essentially see right from

inside the sun, right through to the coronal mass ejections

as they're leaving the sun, so you're seeing out to 30 solar radii.

SOHO has played a key role in understanding the explosive power of the sun.

By blocking out the disc, it simulates an eclipse,

revealing the outer atmosphere

and the true scale of the sun's largest eruptions.

These are solar flares and coronal mass ejections

and they erupt from the heart of sun spots.

The temperatures in a solar flare will be tens of millions of degrees,

so it's an extremely hot, very dramatic change in temperature over a short period of time.

When they erupt completely,

you can get masses which are roughly the mass of Mount Everest

being flung out into the solar system.

At solar minimum, flares are infrequent.

But every 11 years, when the cycle peaks at solar max,

the sun puts on the best firework display in the solar system.

Solar astronomers are now beginning to understand the cause of these explosions.

They are not caused by the power of fusion.

There is another force at work.

It is the force of magnetism.

The sun is covered in a complex network of magnetic fields.

A magnetic map shows a familiar patchwork on the face of the sun.

The areas of the strongest fields coincide exactly with the position of sun spots.

Here, the magnetic field strength can be amplified 10,000 times

The regions that have the strongest magnetic field

on the sun are in the sun spots.

And in the units that we use,

sun spot magnetic fields are roughly 1,000, 2,000, maybe 3,000 gauss.

But if you look at magnets like the ones I'm playing with here,

the magnetic field of these is about 1,000 - 1,500 gauss,

so these have the same magnetic field strength as a sun spot.

The key difference is that a sun spot is an awful lot bigger,

so the total energy - the amount of energy in the magnetic field on the sun - is enormous.

But the field strength in any given location

is something you can hold in your hand.

Sun spots are just the visible effect of magnetic fields so strong

that they can prevent heat and light rising from the sun's interior.

With the right viewing equipment,

you can even see the magnetic fields.

Magnetic loops arch off the surface,

like iron filings around a bar magnet,

their shapes are mapped out by plasma heated to a million degrees.

The largest are 200,000 kilometres high.

And they are packed full of unstable energy.

When you add up the total energy content in these loops,

it comes out to roughly 10 to the 21 joules of energy.

That's roughly ten times the annual energy consumption of the United States.

Of course, we can see thousands of them at any one time.

The loops are caused by the twisting of the sun's basic magnetic field.

Because the sun rotates faster at the equator than at the poles,

it drags the field lines with it,

stretching and twisting them like elastic bands.

As the solar cycle goes on,

the fields get more and more twisted

and break through the surface.

Until, at solar max,

the whole sun is covered in loops stretched to breaking point.

Solar flares are what happen when the strain gets too much

and the loops snap.

Basically, all that energy comes out of the catastrophic release

of energy that's been stored in the magnetic field.

So, like if you wind up an elastic band too much

and let it go with your fingers, that band flies across the room.

When the energy bound within sun spots is released,

billions of tonnes of plasma are fired far into space at huge speeds.

And sometimes they are aimed straight for the Earth.

The flares fly through space for two days.

When they reach us,

the Earth's own magnetic field deflects most of the blow.

But it's the impact on the magnetic field which affects on Earth.

It's called space weather

and the best of its effects are magical.

The auroras, dancing displays of celestial light,

are caused by particles from the solar storm

smashing through the magnetic field at the poles.

When they strike the upper atmosphere,

they light up the polar skies.

In the strongest storms, at the peak of the solar cycle,

the northern lights can be seen as far south as Athens and Cuba.

But the buffeting of the magnetic field has other unseen effects.

Migratory animals that navigate using the magnetic field can lose their bearings.

Racing pigeons don't come home.

And whale strandings have been seen to increase with solar activity.

But most worryingly for us is the effect that the disrupted magnetic field can have on electronics.

The strongest storms can damage or destroy satellites

with devastating effects.

We're more sensitive to the sun than we probably realise

because when the sun releases this magnetic energy,

in the form of solar flares and coronal mass ejections,

maybe you don't notice it at first,

but a crackle on your cell phone, or your cell phone going out,

may actually be caused by enhanced activity on the sun.

Mobile telephones, television,

aeroplane navigation, even weapons guidance systems,

all rely on satellite communication

and all can be disturbed by space weather.

The more we are reliant on these systems,

the more we will feel the effects of the sun's tantrums.

But we still don't understand all of the effects of space weather.

No-one can explain the effect on the climate

or why the disappearance of sun spots should cause an ice age.

It may not matter.

The small effect that solar variation has on the climate

has long since been drowned out by man-made global warming.

As the world warms up,

the sun may yet prove an unlikely source of cooling.

The greenhouse effect could be stopped by harnessing its heat.

DRUMS AND CHANTING

The American Indians of the south-west deserts

have understood the importance of living sustainably with their environment for hundreds of years.

We have these natural disasters, like the big old fires,

the volcanoes erupting, the big old earthquakes, the tsunami.

That's the Earth telling us that it's getting pretty tired.

It's warning us that we need to slow down.

On the Mesas of Arizona, far beyond the reach of the electricity grid, lies Old Oraibi -

the oldest occupied settlement in North America.

For 1,000 years, the Hopi Indians have lived here in harmony with their environment.

But that doesn't mean that they have to live without the conveniences of modern life.

They have just realized that they are already getting all the power they need.

Under the clear skies of the desert,

a family can be supplied with electricity from a couple of solar panels.

Being that we pray to the sun every day,

it's got our heartfelt desires. He receives it every day.

We thought the best source of energy should come from the sun.

A few miles across the desert,

the idea is finally catching on with the rest of American civilization.

But, naturally, it's on a much bigger scale.

These are Stirling dishes - the daddy of solar power systems

and they may be about to solve the energy crisis facing the cities of the American west.

People want to run their air conditioners, their computers,

their lights during the day and we run out of energy.

We have a solar furnace 90 million miles away that provides life for all of Earth.

In the past, we have been unable to harness that potential

because the efficiency, the cost of the systems, didn't make it viable.

What's changed is technology.

The technology now allows us to build large-scale, highly efficient, cost effective systems.

Each dish tracks the movement of the sun across the sky,

focusing the heat onto a single point, where it is converted to electricity.

They are twice as efficient as any other solar power system.

With one, you could power a small village.

A field of them could power a city.

Each system produces about 25 kilowatts, which is enough to power about ten homes.

However, we plan to deploy these on a large scale -

20,000 of these, which is massive - the equivalent or a coal-fired plant or maybe even a nuclear plant.

The ink is just drying on the contract to build the first commercial solar power station.

It will fill five square miles of the vast Californian desert with mirrors

and will supply electricity to the city of San Diego.

In time, as the technology and efficiency improves,

systems like these may spread around the world.

These dishes are direct descendents of the stone monuments of Stonehenge and Orkney.

Separated by 5,000 years,

they all embody our desire to understand the sun,

to be bathed in its light and to tap into its awesome power.

He's quiet, he's clean, he's powerful.

This is the way I was raised - I was raised to respect him

and to offer those prayers to him for keeping us for the length of time that he has.

He's done this for generations and generations,

and hopefully he will, you know, in the future.

The sun will probably shine down on our civilisations for as long as they exist.

If we survive for another billion years, the sun will still be there.

But, just as the sun was once born, it will one day die.

And when it does so, it will take the Earth with it.

In about five billion years' time, the sun will run out of hydrogen.

When it does, it will become a red giant.

Deprived of fuel, the core will shrink,

generating so much heat that the outer layers will balloon into the solar system.

The inner planets will be swept up.

It may even swallow the Earth.

Whether it is engulfed or not, the Earth is doomed.

The sun, burning 2,000 times hotter than it does now,

will melt and seal the outer layers of the planet.

Then, suddenly, the sun will stop burning.

As the remains of the solar system is plunged into darkness,

the sun's core will collapse

and, with its last gasp, it will blow its remaining shroud of gas into space.

It will be forever night.

The story of the sun and the Earth will have ended

and a small part of the outer arm of the Milky Way will be a little bit darker.

But around it, in the rest of the universe,

the same story will be being told

by a billion other small, insignificant stars

twinkling in the vastness of space.

Subtitles by Red Bee Media Ltd 2006

E-mail subtitling@bbc.co.uk