Next time you look up at the big nuclear fusion plant in the sky, know this.
The Sun is actually too cold - too cold to make the amount of energy it makes.
Well, it would be too cold, if we only believed the older, “classical” physics of the
19th century.
In this Think video we’re turning to the weird and wonderful world of quantum physics
and quantum tunnelling to understand how the Sun keeps making ALL that energy while being
too cold and without burning out super quickly, like a matchstick...
Sometimes we forget that we live next door to a star.
The Sun allows complex chemical reactions to happen all over Earth, including the complex
chemical reaction we call life.
Our planet would be a dark, lifeless ball of icy rock without it.
Our weather, plant life, warmth, light - almost everything depends on sunshine, so much so
that many of the first gods were based on the sun itself.
All of these things that make the Earth what it is are powered through the burning heat
of the sun, right?
Well...
About that.
If the sun actually generated energy through heat alone, like a giant fireball, it actually
would've burnt out millions of years ago!
So if that big ball of fire and gas up there doesn't provide energy through heat alone,
how does it do it?
Our resident Think astronomy expert Dr. Dipen Bhattacharya breaks it down for us.
Like other stars, the Sun shines through the energy generated in the middle, in its core.
Far from being a ball of fire, it is more like a frantic, insanely powerful nuclear
fusion plant.
It generates its incredible energy and heat through what’s called nuclear fusion.
This is when atoms combine together to form different atoms, and often release subatomic
particles like neutrons, protons or gamma rays while doing so.
We’re sure you’ve studied this at school, but these subatomic particles are the bits
that make up all atoms.
The temperature in the core of the Sun is about 15 million Kelvin (which is 15 million
Celsius or 27 million Fahrenheit).
We understand numbers like 15 million Kelvin might not mean anything to you, so imagine
touching molten lava.
Imagine how hot it would feel, then try to imagine a temperature 12,000 times hotter.
Something like that.
The pressure in the sun’s core is 250 billion times greater than our own atmospheric pressure
- the pressure you’re sitting in right now.
And even at that absurdly high temperature and pressure, it isn’t even close.
See, The Sun is made up of 90% hydrogen - which is its source of energy.
Every hydrogen atom has one proton.
All protons carry a positive charge - and so they repel each other.
The closer they get to each other the more they repel - a bit like very strong magnets!
Scientists call this the Coulomb repulsion force.
A force so strong that at the temperature of the sun’s core - no hydrogen protons
should ever fuse together.
So what would you need to get past this force?
A LOT of heat.
No no, not 15 million Kelvin.
More like 10 billion Kelvin!
But the Sun’s temperature is not even close to that number.
So what’s really happening here?
This is where we have to bring in the strange world of quantum mechanics...at this fundamental
level - a level so tiny that we’re talking about the insides of individual atoms - where
subatomic particles like electrons and protons also behave like waves.
It might make your head scratch, but you can imagine each proton is also a wave and the
waves of two protons can sometimes overlap a bit.
There exists a teeny tiny teensy weensy veerrrryy small probability that two protons overlapping
can “ignore” and overcome the repulsive force through what scientists call quantum
“tunneling”.
This will means that a proton will appear next to the other proton so they can fuse
together..
For tunneling to happen you don’t need 10 billion Kelvin - actually 15 million Kelvin
will do.
Tunneling can happen at this low temperature, but extremely rarely - a single proton needs
to wait billions of years before it can fuse with another to make Helium this way.
But because the sun is massive and it has sooooo many protons, statistically fusion
happens even with this super low probability - just at a very slow rate.
So what really happens once the protons tunnel through each other?
…..
When protons of the hydrogen atoms fuse together they make helium - this also releases energy
as gamma photons.
This process is called the proton-proton reaction chain.
All photons are particles of light, but within photons Gamma photons are much more powerful
and we cannot see them.
The gamma photons created in this proton-proton chain are the source of the energy of most
of the stars in the Universe.
This gamma energy is mostly created in the core, and then starts a long journey to the
surface of the sun.
When I say long, I mean more than one hundred thousand years.
A few hundred thousand years for one gamma photon to reach the surface of the sun - going
through all the different layers of the Sun.
This means that by the time they reach the surface their energy is reduced and they mostly
emerge as photons that make up visible light.
It then takes a little more than 8 minutes to reach us on Earth.
That means the light you feel on your face when you’re outside?
Maybe that was made in the Sun’s core before the first humans were coming out of Africa
100,000 years ago.
There’s another critical process that makes fusion even slower - the weak force.
It’s a fundamental force - And it is essential in making helium atoms by turning protons
into neutrons in this reaction.
It’s also very, very slow.
It’s the low probability of quantum tunneling and the super slow rate of the weak force
process which makes the sun burn bright for a long, long time.
It’s just amazing how much you can still learn about something every single human being
and every single living thing owes its existence to.
Imagine, it’s been only about 80 years since we’ve known how the Sun even works.
The Sun converts 4 billion kilograms of mass into energy every single second.
If it wasn’t for this fusion process being so slow because of quantum tunneling and the
weak force, the sunlight we all know and love would have disappeared a very long time ago.