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0:00:04 > 0:00:05- The sun.
0:00:07 > 0:00:11- Without it, we'd be in darkness - and nothing could live.
0:00:15 > 0:00:18- The sun is essential - to all life on earth.
0:00:23 > 0:00:28- Life relies on the amazing journey - of a small packet of light.
0:00:28 > 0:00:29- The photon.
0:00:31 > 0:00:35- The photon has been - on a very long journey...
0:00:36 > 0:00:38- ..and has struggled to reach us.
0:00:39 > 0:00:44- The photon, which brings us - light and heat, is ancient.
0:00:45 > 0:00:49- It's fantastic that the light - we see today from the sun...
0:00:49 > 0:00:52- ..was created - in the age of the dinosaurs.
0:00:54 > 0:00:59- In this programme, we follow the - photon on its incredible journey...
0:00:59 > 0:01:03- ..from the sun's core - to its escape to earth.
0:01:05 > 0:01:09- And we learn how, and why, - everything on earth relies on it.
0:01:24 > 0:01:26- The cosmos.
0:01:26 > 0:01:29- Home to billions of galaxies...
0:01:30 > 0:01:32- ..each made of billions of stars.
0:01:34 > 0:01:39- And in an unremarkable corner - of the Milky Way, our galaxy...
0:01:39 > 0:01:41- ..lies the sun, our closest star.
0:01:46 > 0:01:48- A dazzling sphere of intense light.
0:01:50 > 0:01:52- Too bright for the naked eye...
0:01:54 > 0:01:58- ..but strip away that glare, - and the sun transforms.
0:02:07 > 0:02:11- The sun is the most important thing - in our local system.
0:02:12 > 0:02:15- Think of the sun as the mother.
0:02:15 > 0:02:20- All the planets, including earth, - are like small children around her.
0:02:23 > 0:02:26- The sun drives everything - in our local system.
0:02:35 > 0:02:39- The sun generates heat and light. - It sustains life.
0:02:42 > 0:02:47- The energy we get from the sun - drives all life on earth.
0:02:51 > 0:02:55- Unbeknown to us, - the sun is our nuclear powerhouse.
0:02:58 > 0:03:00- Our entire existence...
0:03:00 > 0:03:05- ..is powered by the energy emitted - in nuclear reactions in the sun.
0:03:05 > 0:03:08- We are here - because of the light from the sun.
0:03:09 > 0:03:13- No aspect of our existence - could persist if it wasn't there.
0:03:17 > 0:03:19- Light is essential to life.
0:03:31 > 0:03:33- Light is the fastest thing - in the cosmos.
0:03:34 > 0:03:38- It travels - at 186,000 miles per second.
0:03:44 > 0:03:47- The sun is about 93 million miles - away from us.
0:03:48 > 0:03:53- Light travels at a speed - of around 186,000 miles per second.
0:03:55 > 0:04:00- Light takes around eight minutes - to reach us from the sun.
0:04:02 > 0:04:04- The final leg through space...
0:04:04 > 0:04:08- ..is just a tiny part - of the photon's journey.
0:04:15 > 0:04:18- It can take up to a million years...
0:04:18 > 0:04:21- ..for light to escape - from the sun's interior.
0:04:30 > 0:04:35- When we see light coming from - the sun, we don't realize...
0:04:36 > 0:04:38- ..how old those photons are.
0:04:38 > 0:04:41- They took centuries to travel - from the sun's interior...
0:04:42 > 0:04:45- ..before whizzing for eight minutes - to reach earth.
0:04:47 > 0:04:49- I remember first hearing that fact.
0:04:51 > 0:04:54- It's remarkable - that the light we see today...
0:04:54 > 0:04:58- ..was created when our forefathers - were monkeys in Africa.
0:05:03 > 0:05:06- The journey starts - deep inside the sun.
0:05:09 > 0:05:14- If we could open up the sun we'd see - layers of dense hydrogen gas...
0:05:15 > 0:05:18- ..hundreds of thousands - of miles deep.
0:05:19 > 0:05:22- And at its centre, - the sun creates light...
0:05:23 > 0:05:27- ..out of the most violent reaction - in the cosmos.
0:05:29 > 0:05:31- Nuclear fusion.
0:05:32 > 0:05:35- A nuclear reaction - happens in the sun...
0:05:35 > 0:05:39- ..when protons, - the nuclei of hydrogen atoms...
0:05:39 > 0:05:42- ..fuse together - to create nuclear fusion.
0:05:44 > 0:05:46- It sounds simple but it isn't.
0:05:47 > 0:05:50- These protons have equal charge.
0:05:50 > 0:05:53- When they collide, - they try to separate.
0:05:54 > 0:05:58- If a proton is positive, - and we put two positives together...
0:06:00 > 0:06:04- ..it's like trying to push - two magnet heads together.
0:06:05 > 0:06:09- You need a huge pressure and heat - to force them together.
0:06:10 > 0:06:15- Forcing protons together takes - intense heat and huge pressure.
0:06:17 > 0:06:21- This happens - due to the power of gravity.
0:06:24 > 0:06:29- The sun contains almost all - the matter in the solar system.
0:06:29 > 0:06:34- Incredible amounts of gravity - stops the sun from breaking up.
0:06:38 > 0:06:43- The gravity created by all this mass - creates huge density in the sun...
0:06:44 > 0:06:48- ..which allows nuclear fusion - to happen.
0:06:51 > 0:06:56- In this nuclear powerhouse, hydrogen - atoms constantly slam together.
0:07:01 > 0:07:04- Atoms fuse, releasing energy...
0:07:12 > 0:07:16- ..but, unexpectedly, - fusion does not happen every time.
0:07:23 > 0:07:27- Less than 1% of this mass - is converted into energy.
0:07:28 > 0:07:30- It's an inefficient process.
0:07:33 > 0:07:37- Protons smash together - but most don't react.
0:07:40 > 0:07:43- Most of the protons that collide - don't fuse.
0:07:43 > 0:07:49- Electromagnetic force - pushes them apart before they fuse.
0:07:53 > 0:07:55- Once in every billion years...
0:07:55 > 0:07:58- ..a proton - will be lucky enough to fuse...
0:07:59 > 0:08:01- ..and this energy sustains the sun.
0:08:04 > 0:08:05- That's good news for us.
0:08:09 > 0:08:13- Most protons wait - almost the lifetime of the earth...
0:08:13 > 0:08:15- ..before they have a reaction.
0:08:15 > 0:08:19- If they all reacted right away, - the sun will go poof in an instant.
0:08:20 > 0:08:23- That means the sun can keep going - for much longer...
0:08:24 > 0:08:27- ..than if all the protons fused - with every collision.
0:08:32 > 0:08:36- The sun's core is so dense, - atoms are forced together...
0:08:36 > 0:08:39- ..and the rare reaction of fusion - is achieved.
0:08:41 > 0:08:44- Millions of tons of hydrogen...
0:08:44 > 0:08:48- ..are turned into helium - in the sun's core every second.
0:08:50 > 0:08:54- This is like many Mount Snowdons - coming together every second.
0:09:05 > 0:09:08- Nuclear fusion - also happens on earth.
0:09:14 > 0:09:18- Hydrogen bombs harness the raw power - of fusion reactions.
0:09:19 > 0:09:24- A huge amount of energy is created - from a tiny amount of hydrogen.
0:09:27 > 0:09:32- On earth, the closest we've come - to re-creating a similar process...
0:09:32 > 0:09:34- ..is hydrogen bombs.
0:09:35 > 0:09:39- Huge energy can be created - from a small amount of matter.
0:09:42 > 0:09:45- The biggest nuclear bomb - ever detonated...
0:09:46 > 0:09:52- ..had ten times more power than all - the explosives used in World War II.
0:09:54 > 0:09:56- An immense amount of energy.
0:09:57 > 0:10:01- But this is nothing - compared to the sun.
0:10:04 > 0:10:09- When we talk about the sun, we're - talking about a much larger scale.
0:10:09 > 0:10:14- Around 100 billion hydrogen bombs - explode every second in the sun.
0:10:18 > 0:10:22- In the core of our sun, - hydrogen atoms collide and explode.
0:10:26 > 0:10:28- Each collision creates a photon...
0:10:29 > 0:10:32- ..a tiny but incredibly powerful - packet of light.
0:10:34 > 0:10:40- The photon delivers its energy to - earth, where it makes life possible.
0:10:40 > 0:10:45- But when it starts its journey, - it's nothing like the light we see.
0:10:46 > 0:10:49- It has massive amounts of energy...
0:10:49 > 0:10:51- ..and it can be deadly.
0:11:01 > 0:11:01- .
0:11:04 > 0:11:04- Subtitles
0:11:04 > 0:11:06- Subtitles- - Subtitles
0:11:14 > 0:11:18- The light we see - from our star, the sun, is old.
0:11:19 > 0:11:21- It's much older...
0:11:21 > 0:11:25- ..than the eight minutes it takes - to get from the sun to earth.
0:11:27 > 0:11:32- That short leap across space is - the end of a long, hard journey...
0:11:32 > 0:11:35- ..which starts deep inside our sun.
0:11:41 > 0:11:45- When we look at the sun, - we see a bright ball of light...
0:11:45 > 0:11:47- ..which is fairly calm.
0:11:47 > 0:11:52- But there's much more going on which - we can't see with the naked eye.
0:11:57 > 0:12:01- The journey starts in - the immense heat of the sun's core.
0:12:05 > 0:12:08- Crushed together - by the sun's gravity...
0:12:09 > 0:12:12- ..atoms smash into each other - and fuse...
0:12:13 > 0:12:16- ..releasing a tiny packet of energy.
0:12:17 > 0:12:19- A photon of light.
0:12:20 > 0:12:24- Far smaller than an atom, photons - travel faster than anything else...
0:12:28 > 0:12:30- ..and they never stop moving.
0:12:36 > 0:12:40- Photons don't just come from - the core of stars.
0:12:41 > 0:12:45- Any matter that moves creates light.
0:12:45 > 0:12:50- Matter at any temperature - above absolute zero moves...
0:12:50 > 0:12:52- ..and creates light.
0:12:56 > 0:13:01- It's the temperature that determines - how much light is generated.
0:13:01 > 0:13:05- Everything consists - of different elements.
0:13:05 > 0:13:11- When moving, they emit light - from the electromagnetic spectrum.
0:13:12 > 0:13:17- We humans are made out of these - elements, so we also emit light.
0:13:22 > 0:13:26- White light can be split - into the colours of the rainbow.
0:13:29 > 0:13:32- Each colour is a photon of light...
0:13:32 > 0:13:35- ..with slightly different amounts - of energy.
0:13:36 > 0:13:40- But what we can see is only - a fraction of the light spectrum.
0:13:41 > 0:13:44- We can see light - in the optical spectrum...
0:13:44 > 0:13:49- ..so we can see the sun's light and - we can see things from day to day.
0:13:49 > 0:13:55- We can't see infrared - the thermal - radiation emitted by our bodies.
0:13:59 > 0:14:03- However, we can use - thermal imaging cameras...
0:14:03 > 0:14:07- ..to see the infrared radiation - emitted by our bodies.
0:14:10 > 0:14:15- Using special cameras, we can see - the infrared light that humans emit.
0:14:16 > 0:14:21- This infrared light has less energy - than the visible light we see.
0:14:23 > 0:14:29- Some light is too energetic to see, - such as ultraviolet and X-rays.
0:14:36 > 0:14:40- The vast, hot sun - generates all forms of light.
0:14:43 > 0:14:47- At NASA's Solar Dynamics Observatory - in America...
0:14:48 > 0:14:51- ..they can see the sun - in its true light.
0:14:51 > 0:14:55- SDO measures light - in ten different band passes...
0:14:55 > 0:14:58- ..most of which - are in the far ultraviolet.
0:15:06 > 0:15:11- They cut through the glare to reveal - a different perspective on the sun.
0:15:15 > 0:15:20- The images from SDO are just - amazingly spectacular to look at.
0:15:22 > 0:15:26- And they were the day - we turned on the instruments.
0:15:29 > 0:15:32- By looking at the sun - in different forms of light...
0:15:33 > 0:15:35- ..we can see the complete picture.
0:15:43 > 0:15:46- But in the nuclear furnace - of the sun's core...
0:15:46 > 0:15:49- ..every photon starts out - as a gamma ray.
0:15:49 > 0:15:53- The most energetic form of light - in the cosmos.
0:15:54 > 0:15:57- In the sun's core, - a nuclear reaction happens...
0:15:57 > 0:16:00- ..and hydrogen turns into helium.
0:16:01 > 0:16:04- As part of that process, - gamma ray photons are created.
0:16:04 > 0:16:08- They emit a million times the energy - of visible photons.
0:16:09 > 0:16:12- Gamma rays can transform...
0:16:13 > 0:16:15- ..and even kill.
0:16:15 > 0:16:20- Gamma rays have so much energy, - they destroy atoms and molecules.
0:16:20 > 0:16:23- If millions of photons - get into the body...
0:16:23 > 0:16:26- ..they can destroy DNA - and cause cancer.
0:16:29 > 0:16:31- Fortunately for us...
0:16:31 > 0:16:35- ..our gamma ray photon - can't head straight out of the sun.
0:16:40 > 0:16:46- Processes in the sun's core create - these energetic gamma ray photons.
0:16:47 > 0:16:51- Fortunately for us, they are not - released straight from the sun...
0:16:52 > 0:16:54- ..or we'd be fried in an instant.
0:16:57 > 0:16:59- After its birth as a gamma ray...
0:17:00 > 0:17:05- ..our photon races out from the - sun's core at the speed of light.
0:17:07 > 0:17:09- But it encounters an obstacle.
0:17:12 > 0:17:16- A journey that should take seconds - slows down to a cosmic crawl.
0:17:19 > 0:17:24- Photons should take a few seconds - to escape from the sun's core.
0:17:24 > 0:17:28- It actually takes - hundreds of thousands of years.
0:17:31 > 0:17:36- What could possibly slow a photon - moving at the speed of light?
0:17:39 > 0:17:41- Something slams on the brakes.
0:17:42 > 0:17:45- Something hot, dense and huge.
0:17:48 > 0:17:53- So powerful, it doesn't just - slow light, it transforms it.
0:17:58 > 0:18:00- The sun is a massive ball of gas.
0:18:01 > 0:18:06- The photon must travel through - several layers within the sun...
0:18:06 > 0:18:09- ..before it can be released - into space.
0:18:14 > 0:18:18- Our newborn photon leaves the core - at the speed of light...
0:18:22 > 0:18:25- ..only to run into a dense soup - of hydrogen atoms.
0:18:25 > 0:18:27- A photon's nightmare.
0:18:27 > 0:18:31- A nightmare that stretches - for over 400,000 miles.
0:18:39 > 0:18:43- The radiation zone - surrounds the sun's core.
0:18:45 > 0:18:50- This zone is made of hydrogen gas, - but not a gas as we think of it.
0:18:51 > 0:18:56- The weight of all the material above - compresses the radiation zone...
0:18:56 > 0:19:01- ..until it's denser than lead and - nearly impossible to pass through.
0:19:05 > 0:19:10- If a photon travelled straight from - the sun's centre to its surface...
0:19:11 > 0:19:13- ..it would take ten seconds.
0:19:13 > 0:19:16- But it takes - between 100,000 and 200,000 years.
0:19:17 > 0:19:19- It goes on a very long journey.
0:19:19 > 0:19:22- The gases are dense and hot.
0:19:24 > 0:19:28- Superheated to over - 12.5 million degrees Fahrenheit...
0:19:28 > 0:19:30- ..the gas in the radiation zone...
0:19:31 > 0:19:35- ..is transformed into plasma - - the fourth state of matter.
0:19:37 > 0:19:41- Atoms in normal matter have - a nucleus with orbiting electrons.
0:19:44 > 0:19:48- In plasma, - the atoms have been torn apart...
0:19:48 > 0:19:50- ..and the electrons ripped away.
0:19:53 > 0:19:56- We're familiar with - the three main states of matter...
0:19:57 > 0:19:59- ..liquids, solids and gases.
0:19:59 > 0:20:03- We drink liquids, we breathe gases - and we eat solids.
0:20:03 > 0:20:08- But there's a fourth, which is - the most common in the universe...
0:20:08 > 0:20:10- ..and it's called plasma.
0:20:15 > 0:20:20- Compared to the rest of the cosmos, - earth is a calm and safe place.
0:20:24 > 0:20:28- But plasma is also created here - inside a lightning strike.
0:20:37 > 0:20:42- Temperatures inside a lightning bolt - can reach 53,000 degrees Fahrenheit.
0:20:42 > 0:20:46- Enough to momentarily - rip a few atoms apart.
0:20:48 > 0:20:50- The plasma lasts an instant...
0:20:51 > 0:20:54- ..then the electrons rebind - and it's gone.
0:20:58 > 0:21:03- But in our massive sun, the plasma - lasts for billions of years...
0:21:03 > 0:21:07- ..and makes up the entire - 200,000 mile deep radiation zone.
0:21:10 > 0:21:14- Worse for our photon, - the plasma is electrically charged.
0:21:14 > 0:21:16- This forms a cosmic trap.
0:21:19 > 0:21:23- A photon collides with a proton - or electron...
0:21:24 > 0:21:26- ..and some of the energy - of the photon...
0:21:27 > 0:21:30- ..is transferred to the proton - or electron.
0:21:30 > 0:21:35- What that means is the plasma - then becomes hotter and hotter.
0:21:35 > 0:21:39- Light interacts more strongly with - electronically charged objects...
0:21:39 > 0:21:41- ..than with neutral objects.
0:21:41 > 0:21:44- When you have a neutral atom, - light can pass by.
0:21:44 > 0:21:47- But separate - the positive and negative charges...
0:21:47 > 0:21:49- ..and light can't make it through.
0:21:51 > 0:21:56- Our photon, in the form of a gamma - ray, has entered the radiation zone.
0:21:56 > 0:22:00- It then smashes - into charged plasma particles.
0:22:02 > 0:22:06- For a fraction of a second, - the particle absorbs the photon...
0:22:06 > 0:22:08- ..then spits it back out...
0:22:09 > 0:22:12- ..and the photon - collides with another particle.
0:22:12 > 0:22:17- The charged plasma particles - make it difficult for the photon...
0:22:18 > 0:22:21- ..to work its way out of the sun - to the surface.
0:22:24 > 0:22:27- The photon loses - some of its original energy.
0:22:27 > 0:22:32- The dense plasma makes it difficult - for the photon to reach the surface.
0:22:35 > 0:22:37- It's an atomic game of basketball.
0:22:40 > 0:22:42- The court is the radiation zone.
0:22:43 > 0:22:46- The players - are the particles in the plasma...
0:22:48 > 0:22:50- ..and the ball is the photon.
0:22:50 > 0:22:53- You can imagine the photon - as the ball.
0:22:54 > 0:22:58- The ball is passed - from one player to the next.
0:23:00 > 0:23:06- This happens very quickly - and in a really random pattern.
0:23:11 > 0:23:16- But the ball will gradually make its - way to the other side of the court.
0:23:22 > 0:23:25- The photon wants the quickest route - up the court...
0:23:26 > 0:23:29- ..but it's thrown, - slowing its progress.
0:23:32 > 0:23:36- This process transforms the photon.
0:23:40 > 0:23:45- As the photon is thrown from - one electron or proton to another...
0:23:46 > 0:23:47- ..it loses energy.
0:23:48 > 0:23:52- It turns from being - a very powerful gamma ray...
0:23:52 > 0:23:56- ..to something with less energy, - like X-ray or ultraviolet.
0:24:00 > 0:24:03- Over hundreds of thousands - of years...
0:24:03 > 0:24:08- ..the photon keeps bouncing - through the dense radiation zone.
0:24:09 > 0:24:12- Each collision - takes away some energy...
0:24:15 > 0:24:18- ..transforming it - from a lethal gamma ray photon...
0:24:19 > 0:24:21- ..to a lower energy X-ray.
0:24:27 > 0:24:31- Almost a million years - after its creation in the core...
0:24:31 > 0:24:35- ..the photon has made it - through the radiation zone.
0:24:36 > 0:24:41- But there's no escape and it's about - to start another difficult journey.
0:24:45 > 0:24:50- This happens in a place so violent, - it makes the sun roar.
0:24:55 > 0:24:56- .
0:25:01 > 0:25:01- Subtitles
0:25:01 > 0:25:03- Subtitles- - Subtitles
0:25:14 > 0:25:19- We're following - a tiny piece of light a photon...
0:25:19 > 0:25:21- ..on its journey towards earth.
0:25:22 > 0:25:25- A journey - that will sustain all our lives.
0:25:27 > 0:25:29- A million year journey...
0:25:33 > 0:25:35- ..which is nearing its end.
0:25:37 > 0:25:39- Battered and sapped of energy...
0:25:40 > 0:25:44- ..it has changed - from a deadly gamma ray to an X-ray.
0:25:46 > 0:25:50- But it now enters - a mysterious layer of the sun.
0:25:52 > 0:25:53- The convection zone.
0:25:56 > 0:25:58- The convection zone...
0:25:58 > 0:26:02- ..lies between the radiation zone - and the sun's surface.
0:26:05 > 0:26:08- We can't see this zone - with our telescopes.
0:26:10 > 0:26:11- But we can hear it.
0:26:17 > 0:26:23- NASA's Solar Dynamics Observatory - listens to the sun.
0:26:23 > 0:26:27- The sound it picks up - is too deep for humans to hear.
0:26:27 > 0:26:33- But if you speed up forty days' - recording into a few seconds...
0:26:33 > 0:26:35- ..this is what you get.
0:26:40 > 0:26:41- The sound of chaos.
0:26:49 > 0:26:53- As we can't see into the sun, - its sound is vital to scientists.
0:26:56 > 0:27:01- When a gun fires, the bullet - rushes out and smashes into air...
0:27:01 > 0:27:04- ..creating waves of turbulence.
0:27:05 > 0:27:10- We hear the gun firing when - these waves vibrate our eardrums.
0:27:15 > 0:27:17- What we can see here is sound.
0:27:20 > 0:27:23- The sun works the same way, - on a much larger scale.
0:27:26 > 0:27:28- When we see waves moving across...
0:27:29 > 0:27:34- ..we see material moving up from - inside the sun and it makes noise.
0:27:34 > 0:27:38- It's like it ran into a wall - and it generates sound...
0:27:38 > 0:27:42- ..and we see that sound - moving all around the sun.
0:27:43 > 0:27:48- Sound waves crashing through plasma - create ripples in the sun's surface.
0:27:49 > 0:27:52- All we see are these ripples.
0:27:52 > 0:27:54- Those are the sound waves - of the sun.
0:27:55 > 0:27:59- They move around inside the sun - and back up to the surface.
0:28:03 > 0:28:07- By tracking the sound, - scientists can see into the sun.
0:28:08 > 0:28:12- They pick up sound waves smashing - against the sun's surface...
0:28:12 > 0:28:15- ..and resonating - throughout the interior.
0:28:18 > 0:28:21- This reveals a violent, - boiling convection zone.
0:28:23 > 0:28:28- In a similar way to earthquakes, you - can feel the waves travelling along.
0:28:30 > 0:28:33- The same happens on the sun.
0:28:33 > 0:28:36- It's possible - to look at one point...
0:28:36 > 0:28:41- ..and measure in which direction - these waves are travelling.
0:28:43 > 0:28:48- By mapping it, we can measure waves - travelling on the sun's surface...
0:28:49 > 0:28:51- ..so there's a noise inside the sun.
0:28:53 > 0:28:56- Columns of gases rise and fall.
0:28:59 > 0:29:04- The sun's convection zone is very - similar to a pan of water boiling.
0:29:06 > 0:29:10- Some bubbles of gas or plasma - are hotter than others...
0:29:10 > 0:29:12- ..and those rise to the top.
0:29:13 > 0:29:16- The colder bubbles - sink to the bottom.
0:29:18 > 0:29:21- At the bottom - of the convection zone...
0:29:22 > 0:29:25- ..photons smash into atoms - in the plasma...
0:29:25 > 0:29:27- ..heating them to a boil.
0:29:27 > 0:29:30- Next, the atoms - absorb the photons...
0:29:32 > 0:29:36- ..and they ride the boiling current - to the top.
0:29:37 > 0:29:40- The hot atoms - drag the photons with them.
0:29:42 > 0:29:44- Within the convection zone...
0:29:45 > 0:29:50- ..photons that collide with atoms - are carried up with the bubbles.
0:29:53 > 0:29:56- Suddenly, their journey is easier.
0:29:56 > 0:30:01- It's still a random route but it - does take them to the sun's surface.
0:30:05 > 0:30:10- Our photon is now in the hands of - a single player making a fast break.
0:30:12 > 0:30:17- In the convection layer, it's as if - one player has held onto the ball...
0:30:17 > 0:30:20- ..and is going - straight for the basket...
0:30:21 > 0:30:23- ..carrying the ball with it.
0:30:25 > 0:30:30- The atom carries the ball - up the court rapidly.
0:30:33 > 0:30:38- The photon's journey through the - convection zone takes just a week.
0:30:38 > 0:30:41- But, again, the photon transforms.
0:30:44 > 0:30:47- The bottom of the convection zone - is incredibly hot.
0:30:51 > 0:30:53- At the top, the temperature falls.
0:30:56 > 0:31:00- So as the photon rises up, - it cools, losing energy...
0:31:01 > 0:31:06- ..and changes from an energetic - X-ray to a light we can see.
0:31:10 > 0:31:13- It loses some of its energy - due to the temperature drop.
0:31:14 > 0:31:18- The nature of the photon - depends on the temperature.
0:31:18 > 0:31:23- It goes from being an X-ray photon - to being an ultraviolet photon...
0:31:23 > 0:31:26- ..or a photon - we can see with the naked eye.
0:31:29 > 0:31:34- At the top of the convection zone, - the atom releases the photon...
0:31:34 > 0:31:37- ..which shoots out as visible light.
0:31:39 > 0:31:44- This cools the atoms, which fall - back down to absorb more photons...
0:31:45 > 0:31:48- ..then they heat up and rise again.
0:31:51 > 0:31:55- The photon is ready to be released - from the sun's surface...
0:31:55 > 0:31:59- ..but the convection zone - unleashes another force.
0:32:00 > 0:32:04- A force that powers huge storms, - detonates bombs...
0:32:05 > 0:32:08- ..and threatens - to stop our light from escaping.
0:32:14 > 0:32:20- Finally, it has reached the shell - of the sun - the photosphere.
0:32:25 > 0:32:28- The photosphere - is the sun's outer layer.
0:32:29 > 0:32:33- When we look at the sun, - what we see is the photosphere.
0:32:35 > 0:32:41- Behind this outer shell - lies a spectacular and savage world.
0:32:46 > 0:32:51- This image of the sun was taken - in an extreme ultraviolet light...
0:32:51 > 0:32:53- ..which is close to X-ray.
0:32:54 > 0:32:58- Suddenly, instead of being - a calm white ball...
0:32:58 > 0:33:01- ..we see a surface - that's full of activity.
0:33:03 > 0:33:06- Look closely at the photosphere - and you'll see...
0:33:06 > 0:33:11- ..it's like the surface - of boiling water in a saucepan.
0:33:11 > 0:33:14- We can see cells - constantly rising and falling.
0:33:16 > 0:33:19- It's terrifying - to look at the sun like this.
0:33:20 > 0:33:24- The realization hits you that we - exist close to a very active star.
0:33:26 > 0:33:29- It has so much energy - it's terrifying.
0:33:30 > 0:33:34- It's a huge hydrogen bomb - in our back garden.
0:33:39 > 0:33:42- The surface of the sun - is a turbulent barrier.
0:33:44 > 0:33:48- Once again, - our photon is taken hostage.
0:33:55 > 0:34:00- At Kitt Peak Observatory, Arizona, - solar astronomer Matt Penn...
0:34:00 > 0:34:05- ..studies the solar surface to see - how photons of light get trapped.
0:34:15 > 0:34:20- Using the McMath-Pierce Telescope, - he focuses the sun's light...
0:34:21 > 0:34:24- ..to scan the photosphere in detail.
0:34:25 > 0:34:27- What we've got - are a few small sunspots.
0:34:28 > 0:34:31- Two large sunspots - are heading to the edge of the sun.
0:34:32 > 0:34:33- To the limb of the sun.
0:34:34 > 0:34:38- But they're all accompanied - by groups of smaller sunspots.
0:34:41 > 0:34:45- Sunspots mark out areas - where light is trapped.
0:34:46 > 0:34:49- A sunspot forms a dark spot...
0:34:49 > 0:34:52- ..by removing energy - from that part of the sun.
0:34:52 > 0:34:56- It blocks convective fluids - that transport heat and light...
0:34:56 > 0:34:58- ..from inside the sun to space.
0:34:59 > 0:35:02- We see a cooler region - that appears dark to us.
0:35:06 > 0:35:10- A powerful force stops our photon - from leaving the sun's surface.
0:35:22 > 0:35:27- Sunspots take shape where magnetism - from deep inside the sun...
0:35:27 > 0:35:30- ..blasts up through the photosphere.
0:35:32 > 0:35:36- Where the magnetic field - comes through the photosphere...
0:35:37 > 0:35:41- ..there are cooler areas which are - 5,000 degrees rather than 6,000.
0:35:42 > 0:35:46- They then appear darker, - creating these sunspots.
0:35:48 > 0:35:50- They can be huge.
0:35:51 > 0:35:55- The largest are more than - ten times the size of the earth.
0:35:59 > 0:36:02- The magnetism - that generates sunspots...
0:36:02 > 0:36:04- ..forms field lines - covering the sun.
0:36:06 > 0:36:10- On our solid earth, - the whole planet rotates together...
0:36:11 > 0:36:13- ..including its magnetic field.
0:36:15 > 0:36:20- But the sun is a big ball of gas - and the same rules don't apply.
0:36:21 > 0:36:23- The sun is a ball of gas...
0:36:23 > 0:36:27- ..so different parts - rotate at different speeds.
0:36:27 > 0:36:32- When a magnetic field moves out - of the photosphere and into space...
0:36:32 > 0:36:35- ..some parts travel - at different speeds.
0:36:37 > 0:36:42- This stretches and pulls the - magnetic field, creating energy.
0:36:46 > 0:36:50- The sun rotates faster - at the equator than at the poles...
0:36:51 > 0:36:55- ..twisting and tangling its - magnetic field with each rotation.
0:36:57 > 0:36:58- The result is mayhem.
0:37:04 > 0:37:07- The movements - of the magnetic fields...
0:37:07 > 0:37:12- ..being dragged by movement - in the photosphere creates energy.
0:37:13 > 0:37:16- It builds up and it twists - until it almost breaks.
0:37:17 > 0:37:20- It's like a complicated knot.
0:37:22 > 0:37:24- Where the magnetic lines twist...
0:37:25 > 0:37:30- ..flows of plasma containing - our photon can't reach the surface.
0:37:35 > 0:37:40- A patch of the sun goes dark - where light can't escape.
0:37:42 > 0:37:44- A sunspot is created.
0:37:46 > 0:37:51- But in the tangle of magnetic lines, - something has to snap.
0:37:51 > 0:37:55- Sometimes there's too much tension - and something gives...
0:37:57 > 0:37:59- ..creating an explosion.
0:38:01 > 0:38:05- Huge loops of magnetic energy - arc out over the sunspots...
0:38:08 > 0:38:11- ..twisted and ready - to be untangled...
0:38:12 > 0:38:15- ..like a magnetic bomb - which is ready to explode.
0:38:18 > 0:38:21- The energy and tension - in the magnetic field...
0:38:23 > 0:38:26- ..is desperate to be released.
0:38:26 > 0:38:29- This usually happens - in an explosion.
0:38:43 > 0:38:47- The sun's surface explodes. - We call it a coronal mass ejection.
0:38:48 > 0:38:53- These explosions from the sun's - surface create the Northern Lights.
0:39:00 > 0:39:05- Solar flares erupt into space, - releasing massive amounts of energy.
0:39:12 > 0:39:16- Each explosion is like tens of - thousands of huge nuclear bombs...
0:39:16 > 0:39:19- ..being detonated simultaneously.
0:39:21 > 0:39:27- They can contain - billions of tons of hot plasma...
0:39:28 > 0:39:32- ..at a temperature - of over one million degrees.
0:39:33 > 0:39:36- All this is thrown out into space...
0:39:36 > 0:39:39- ..at speeds of around - 1,000 kilometres per second.
0:39:41 > 0:39:45- You can imagine this huge thing - being hurled into space.
0:39:49 > 0:39:54- After nearly a million years, our - packet of light makes its escape.
0:39:58 > 0:39:59- It is finally free...
0:40:00 > 0:40:05- ..catapulting out along with - trillions of other photons.
0:40:05 > 0:40:08- But their journey is far from over.
0:40:09 > 0:40:14- Most photons hurtle onwards - to the far reaches of the cosmos.
0:40:14 > 0:40:16- To new worlds.
0:40:20 > 0:40:21- .
0:40:26 > 0:40:26- Subtitles
0:40:26 > 0:40:28- Subtitles- - Subtitles
0:40:34 > 0:40:39- It takes nearly a million years for - tiny parcels of light photons...
0:40:40 > 0:40:43- ..to finally burst - from the sun's surface.
0:40:56 > 0:40:59- Suddenly, it's free - to travel through space...
0:41:00 > 0:41:02- ..and to travel - through the universe.
0:41:11 > 0:41:15- Photons shoot out into empty space - at the speed of light.
0:41:19 > 0:41:23- After just eight minutes, - our photon reaches earth.
0:41:24 > 0:41:26- Each step of the life of a photon...
0:41:27 > 0:41:30- ..from the moment it's created - in the sun's core...
0:41:31 > 0:41:35- ..to the moment it reaches our eyes - here, on earth...
0:41:36 > 0:41:37- ..is amazing.
0:41:38 > 0:41:42- It's formed through a nuclear fusion - in the sun's core.
0:41:43 > 0:41:47- It takes a random route - through the sun's core...
0:41:47 > 0:41:50- ..for hundreds of thousands - of years.
0:41:51 > 0:41:54- It then exits the photosphere.
0:41:54 > 0:42:00- It travels through space for - eight minutes and reaches our eyes.
0:42:00 > 0:42:03- That's how - we can see everything around us.
0:42:06 > 0:42:09- Our photon finally arrives...
0:42:09 > 0:42:13- ..smashes into a leaf - and passes on its energy.
0:42:15 > 0:42:20- This is photosynthesis - the - fundamental link in our food chain.
0:42:23 > 0:42:26- Carrying energy - born in the sun's core...
0:42:26 > 0:42:29- ..our photon starts life - on our planet.
0:42:32 > 0:42:36- The sun's light creates our fuel - and drives our weather.
0:42:44 > 0:42:47- Every single day, - enough photons hit the earth...
0:42:47 > 0:42:51- ..to power our civilization - for 27 years.
0:42:53 > 0:42:55- The sun's light - is more than warmth and heat.
0:42:59 > 0:43:01- Without light, we would not survive.
0:43:03 > 0:43:07- Without the sun's heat, - which reaches us through photons...
0:43:08 > 0:43:11- ..any kind of life on earth - would not be possible.
0:43:15 > 0:43:18- Trillions of photons - hit the earth every second.
0:43:22 > 0:43:27- Many more pass us by, and for them, - the journey is far from over.
0:43:30 > 0:43:35- Around 80 minutes after - leaving the sun, they reach Saturn.
0:43:36 > 0:43:39- After four hours, - they reach Neptune.
0:43:43 > 0:43:47- Only a small part of the light - that comes from the sun hits earth.
0:43:47 > 0:43:52- The remainder carries on, going - further and further into the galaxy.
0:43:55 > 0:43:58- In 18 hours, - they leave our solar system.
0:44:00 > 0:44:03- The sun is now a dim speck - in the distance.
0:44:05 > 0:44:09- But the photons keep going, - out into deep space.
0:44:13 > 0:44:18- In 1,000 years, some of the photons - reach the Kepler 62 system...
0:44:19 > 0:44:21- ..another solar system...
0:44:22 > 0:44:26- ..with potentially habitable - earth-like planets.
0:44:26 > 0:44:31- There are billions - of earth-like planets in our galaxy.
0:44:32 > 0:44:33- If there is life here...
0:44:34 > 0:44:38- ..it's possible they could also - see the light from our sun.
0:44:41 > 0:44:42- The sun is a star.
0:44:42 > 0:44:47- It takes eight minutes for - the sun's light to reach the earth.
0:44:47 > 0:44:49- But other photons - continue into space.
0:44:50 > 0:44:52- People who live on other stars...
0:44:52 > 0:44:55- ..may be able to look back - and see our sun.
0:44:58 > 0:45:03- In 1,500 years, the photon reaches - other distant parts of the cosmos...
0:45:04 > 0:45:06- ..like the Horsehead Nebula.
0:45:10 > 0:45:14- In 7,000 years, - it passes the Pillars Of Creation.
0:45:17 > 0:45:19- At these distances...
0:45:19 > 0:45:24- ..the light from our star can only - be seen with a powerful telescope.
0:45:28 > 0:45:33- Modern telescopes have the capacity - to create huge mirrors...
0:45:33 > 0:45:36- ..to collect as much light - as possible...
0:45:36 > 0:45:40- ..so we can look further and further - into the universe.
0:45:42 > 0:45:46- Because the light has taken - such a long time to reach us...
0:45:46 > 0:45:49- ..we can look deeper and deeper - in time.
0:45:50 > 0:45:53- With a strong enough telescope...
0:45:53 > 0:45:59- ..we could see the light from a star - at the far end of the universe.
0:45:59 > 0:46:03- Our tiny star is visible - across the universe...
0:46:03 > 0:46:05- ..if someone is looking.
0:46:07 > 0:46:11- In exactly the same way - we look at distant stars...
0:46:12 > 0:46:16- ..anybody living on those stars - could look back on our sun...
0:46:16 > 0:46:19- ..and see - an ordinary-looking star.
0:46:23 > 0:46:27- Deep inside our sun, the photon - started its remarkable journey.
0:46:33 > 0:46:36- It took a million years - to escape the sun's grip...
0:46:36 > 0:46:40- ..to bring energy, heat and life - to our world.
0:46:44 > 0:46:49- Like other stars, light from our sun - journeys through the universe...
0:46:50 > 0:46:53- ..spreading energy - throughout the cosmos.
0:47:29 > 0:47:32- S4C subtitles by Eirlys A Jones
0:47:32 > 0:47:32- .