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  • The story of the Sun starts 13 billion years ago

  • 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 the spectrum.