with the Big Bang.

[loud explosion]

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 outer 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 surface is 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.

Recognizing 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.

And 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.

(Richards) It's midwinter.

It's about 11:00 in the morning,

and it's still not light completely.

There's a strong wind coming in off the Atlantic,

and it's cold, and it's wet.

And that's pretty much typical

of this time of the year up here.

(narrator) Yet despite the cold, in the Stone Age,

5,000 years ago, a civilization 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 the finest examples of Stone Age architecture.

While entering Maeshowe, I have to crouch right down

and then confronted with a passage

which seems to actually go on and on and on.

Slightly feeling the impression of going uphill, upslope.

I'm 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, when it was first entered

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.

(narrator) 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.

(Richards) Well, 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, the 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.

(narrator) 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.

(Mason) Most people think of the Sun as quite a boring,

constant sort of thing. (Mason) Most people think of the Sun as quite a boring,

constant sort of thing.

But in fact, it's not at all.

It's changing all the time, and if you look,

you can see those changes in a matter of minutes or hours,

and it's far from static and boring.

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

(narrator) 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 vaporize 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.

(Alexander) It's easy to appreciate the power of the Sun on a nice

hot summer's day, like here on the Texas Gulf Coast.

And you can feel the power of the Sun on your skin.

Sunscreen's on.

But man, the Sun is just, you know, the actual physics

of what's going on inside the Sun,

the power that the Sun, the energy that the Sun

is releasing, is almost beyond comprehension.

(narrator) But it is only almost beyond comprehension.

And you can measure its power output

with some simple apparatus.

(Alexander) One of the earliest experiments to try and measure

the actual power of the Sun was by astronomer William Herschel

in the 19th century,

when he had the brilliant idea of just watching

a piece of ice melt to see

how long it would take, and therefore,

from the properties of the ice,

work out how much sunlight was coming to the ground.

[beeping noise] work out how much sunlight was coming to the ground.

(narrator) As a demonstration of the Sun's power,

it doesn't look that impressive,

but Herschel realized that he could

use the time it takes to melt one bit of ice

to calculate the Sun's total power output.

(Alexander) So here we see the ice is almost completely melted.

Roughly 29 minutes, almost half an hour.

But Herschel was able

to use this experiment and the time that it took

to melt the ice to work out

some basic properties of the Sun.

(narrator) 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 kilometers.

A shell 1/2 a centimeter thick and 300 million kilometers

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 Herschel's time,

it was a mystery.

(Alexander) One of the issues was, of course,

what powered the Sun?

And some very clever people actually considered the fact

that the Sun might be powered by burning coal.

I mean, it seems ludicrous, but why not coal?

That was an important source of energy on the Earth

at that part of the 19th century.

(narrator) 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.

(Hathaway) You can see the strata in the lines

in the rock in back of you that represent hundreds

of million years of geological history.

The top of Sacramento Peak up beyond this,

we find fossils that are 300 million years old.

Below Sacramento Peak, we've got more than a kilometer

of these 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 that the Sun was only 10,000 years old.

If the geologists were right,

then astronomers had to find some other source

for the Sun producing its energy.

(narrator) 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.

(Hathaway) When you take the light from the Sun

and pass it through a prism-- spread it out into the colors--

as you look through it,

you notice that it isn't uniform,

that 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.

(narrator) Each chemical element absorbs light

at specific frequencies,

removing a strip from