No one could figure out what was powering it.
In the 19th century, some scientists thought it could all be explained by gravity.
The Sun began as a giant cloud of dust and gas.
And this cloud collapsed under the force of gravity releasing lots of heat.
This might explain why the Sun is hot, but it doesn't explain why the Sun has been hot for so long.
A clear picture didn't emerge until we had a better model of the atom.
At the center of each atom is a tiny nucleus made of protons and neutrons.
Protons have a positive electric charge and this is a problem because their charges are pushing them apart.
The electric force is trying to rip apart the nucleus.
But there is a strong force holding it together: the nuclear force.
The nuclear force brings protons and neutrons together.
It doesn't really affect pairs of protons or pairs of neutrons.
To hold everything together you need to have just the right combination of protons and neutrons.
Too many protons and it falls apart.
Too many neutrons and it falls apart.
But when you get just the right combination, you produce a nuclear reaction.
Nuclear reactions are similar to chemical reactions in many ways.
There are certain combinations of chemicals that are more stable. When you form stable compounds, you make energy.
For example, if you combine hydrogen and oxygen to make H₂0, this is a more stable combination and you make energy.
Similarly, if you combine two hydrogen nuclei into Helium, this is a more stable combination and you make energy.
But nuclear reactions are different from chemical reactions.
They're much harder to produce, they're much more powerful, and they involve neutrons.
Chemical reactions are part of our everyday life, but nuclear reactions are not. The reason is that the nuclear force only works over very short distances.
The protons are all positively charged.
So if you bring hydrogen nuclei together, they just repel each other.
If you bring them closer, they repel each other more.
The nuclear force only kicks in when they are extremely close.
Normally, the nuclei never get near each other.
It is only under conditions of extreme pressure and extreme heat that the nuclei will collide.
And this only happens at the center of a very massive object like our Sun or inside a nuclear bomb.
Neutrons are very important in nuclear reactions, but for chemical reactions they're irrelevant.
This is a hydrogen nucleus and so is this.
For the purposes of chemistry, we don't care about the neutrons.
But for nuclear reactions, we do.
We have to keep track of them.
So this is a Hydrogen-1 nucleus.
And this is a Hydrogen-2 nucleus.
The number at the end is the total number of protons and neutrons.
Two protons and a neutron make Helium-3.
Two and two make Helium-4.
Hydrogen-2 is also known as Deuterium.
Nuclear reactions are very powerful.
Let's look at our two reactions again.
The chemical reaction releases 5 eV of energy.
The nuclear reaction releases 24 million eV of energy.
So the nuclear reaction is 5 million times more powerful.
There's enough energy in just a pound of deuterium to power 10,000 homes for a year.
This is an insane amount of energy.
It's why the Sun is so bright, why nuclear bombs are so scary, and why scientists have been working on fusion power for decades with little success.
There's another nuclear force that we need to talk about.
We've been talking about the strong nuclear force, but there's also a weak nuclear force.
The strong force is what holds the nucleus together.
The weak force causes radioactivity.
And it's called the weak force because it acts slowly.
The weak force can cause protons and neutrons to switch identities.
A proton can turn into a neutron.
Or a neutron can turn into a proton.
This is called beta decay.
Neutrons are heavier than protons just barely.
But what this means is that a free neutron can lose some mass and turn into a proton.
But a proton cannot turn into a neutron unless it's near other particles.
The reason this process is called beta decay is because it's one of the ways that a radioactive element can decay.
Now it turns out that radioactivity can be slow.
Imagine you have a box full of Uranium-235 atoms.
Eventually, the Uranium atoms will all turn into Lead, but they're not in any hurry to do this.
If you want to wait until half your Uranium atoms have turned into Lead, you'll be waiting 700 million years.
Radioactivity involves more than just beta decay.
There's actually more going on here.
But the point is that beta decay can be a slow process and it's a random process.
Now we finally have covered enough background to explain how the Sun shines.
This was worked out in the 30s by Hans Bethe.
The Sun is mostly made of Hydrogen-1 which is just a proton.
The Sun is very hot and very dense so these protons collide.
But this doesn't cause a nuclear reaction because there are no neutrons.
Most of the time when two protons collide they immediately fly apart producing no energy.
But it is possible, very very rarely, that something else happens: beta decay.
At the moment the two protons collide, one of them undergoes beta decay and turns into a neutron.
This gives us Hydrogen-2 and releases tons of energy.
The Hydrogen-2 nucleus continues moving around and when it bumps into a proton it turns into Helium-3 and this releases tons of energy.
As this process continues, the Sun produces more and more Helium-3.
When two Helium-3 nuclei collide, they can form an even better combination, Helium-4 and this releases tons of energy.
Hydrogen is converted into Helium in three steps.
First, two protons are combined into Deuterium through beta decay.
Next, a proton is added to make Helium-3 and finally, two Helium-3 nuclei collide creating
Helium-4 plus two leftover protons.
The first step is by far the slowest because you have to wait for beta decay.
For a typical proton in the center of the Sun, the first step takes on average one billion years.
Compare that to the second step which takes about 4 seconds.
Now in the final step, you have to find another Helium nucleus which is rare.
This takes about 400 years. These nuclear reactions are the source of the Sun's tremendous power.
The Sun is the ultimate energy source.
The Sun heats the air which causes wind.
The Sun heats water which causes rain giving us hydroelectric power.
Sunlight is the foundation of life.
The energy from ancient sunlight was deposited into the ground giving us our fossil fuels:
coal, oil, and natural gas.
Practically all the world's energy ultimately comes from either the Sun or nuclear power.
To summarize, we present Children Explaining Nuclear Physics.
Hydrogen-1 atoms.
The Sun is made up of most of these.
Now what if two of them collide?
Then one of them will turn into a neutron and you get Hydrogen-2.
Next, it collides with another atom and becomes Helium-3.
And then it collides with another one and you get Helium-4.
Beautiful!
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