Goal: To understand how stars generate their energy

Objectives:

• To learn about the Proton – Proton Chain

• To learn about the Carbon – Nitrogen – Oxygen Cycle

(Horsehead Nebula - in Orion -Image from APOD)

What would happen to the core of the sun if the sun stopped

producing energy via fusion?

To prevent collapse

• Remember when we looked at the core of the sun that we saw that the sun held itself up with a combination of gas pressure and radiation pressure (light has energy)

• This was called “Hydrostatic Equilibrium”

Proton – Proton Chain

• Short answer: method by which a star converts protons (Hydrogen nuclei) to Helium nuclei (the electrons in the core of a star fly around on their own).

Proton – Proton Chain

• However it is a lot more complicated that I have made it seem.

• After all, how do we take 4 protons and make a helium atom when a helium atom has 2 protons and 2 neutrons?

Why don’t the atoms in this room fuse together?

Repulsion

• In the cores of stars all the nuclei have + charges.

• + charges repel other + charges.

• So, they won’t attract and fuse by accident.

• So, what do we need to be able to do it?

Energy

• It takes energy to overcome this repulsive force.

• Much like it takes energy to get up the stairs.

• So, how do we give a proton energy?

Temperature

• Temperature is just a measure of how fast the particles of a gas, liquid, or plasma move.

• Higher temperature = faster movement.• So, this means that the higher the

temperature, the more energy the particles have.

• Huzaah, the core of the sun is 100 million degrees!

Step one

• We take 2 protons in the core to the sun and try to slam them together.

• They get closer and closer.

• Here come the fireworks!

• And!

Step one

• We take 2 protons in the core to the sun and try to slam them together.

• They get closer and closer.

• Here come the fireworks!

• Nothing happens….

What happened?

• Even at 100 million degrees, protons STILL don’t have enough energy to overcome the repulsive barrier.

• So, how do we get fusion?

Quantum Mechanics!

• No, I will not do a lecture on Quantum.

• Just 1 basic principal: there is uncertainty in the position of each proton.

• In laymen’s terms that means that a proton is not just in a specific position, but has a small probability at being in a nearby position.

So,

• When 2 protons start to get close, there is a small probability they will actually be in the same spot.

• This is called quantum tunneling – basically tunneling through the repulsive barrier.

• This allows us to have fusion!

However,

• The probability of this tunneling is very small, and it depends very highly on how close they get.

• This means that how rapidly you fuse protons depends very highly on the temperature (and also on the density squared).

• Fusion in the proton – proton chain (sometimes call p-p chain) relies on temperature to the FORTH power!

Most of the time though

• Most of the time the protons will repel.

• In our sun it takes an average of a billion years for a proton to fuse with another proton.

• This is why the sun can last for 10 billion years (takes 10 billion years to use up its fuel).

Step 1 concluded

• So, eventually we get 2 protons to collide.• What do we get?• No, we don’t get a Helium atom with 2 protons

and no neutrons. Those don’t exist.• Another difficulty in the fusion process is that

you turn 2 protons into deuterium (which is hydrogen with a neutron in it) + stuff.

• So, that means a proton has to convert to a neutron. That is hard to do.

Step 2

• It would be easy to say 2 deuterium go to 1 helium.

• It would give you 2 protons and 2 neutrons.

• But, sadly, it does not work that way.

• Reason, there just is not enough deuterium.

Instead

• Deuterium fuses with what is the most common thing around, a proton.

• This creates Helium 3 (Helium which has a weight of 3; 2 from the 2 protons and the last from 1 neutron).

Step 3a

• 3a occurs 69% of the time in our sun.

• In time you will get some amount of Helium 3.

• If 2 of these fuse, then you get a Helium 4 and 2 protons.

Step 3b

• 31% step 3b occurs instead.

• In this case a Helium 3 fuses with a Helium 4 creating Beryllium 7.

• The Beryllium 7 combines with an electron (converts a proton into a neutron) to create Lithium 7.

• The Lithium 7 fuses with a proton to create 2 Helium 4 atoms.

Carbon – Nitrogen – Oxygen Cycle

• While the sun utilizes the p-p chain. Other stars use this (called hereafter the CNO cycle).

• Instead of fusing protons and protons we now fuse protons to carbon.

• Will this be easier or harder?

Charges

• Protons have 1 atomic charge.

• Carbon has 6 (6 protons).

• So, it is much harder.

• Therefore, it takes more energy, which means higher temperatures.

• This method depends on temperature to the 20TH power!!!

So, we need more heat

• A star with a hotter core will be able to generate more energy, and will use the CNO cycle far more.

• A start with a less hot core will use the p-p chain more and will generate less energy.

• What kind of stars are these? Stay tuned. We will examine this next hour.

Step 1

• Okay so now for the steps.

• Carbon 12 + a proton goes to Nitrogen 13

• A proton in the Nitrogen 13 converts to a neutron so that the atom converts to Carbon 13.

Steps 2 and 3

• Carbon 13 + a proton goes to Nitrogen 14.

• Step 3 adds another proton to get to Oxygen 15.

• The Oxygen 15 has a proton decay to a neutron and it goes to Nitrogen 15.

Step 4

• Add 1 more proton (the 4th total) and you get Carbon + a Helium atom.

• Note that we started with Carbon and end with Carbon.

• That makes Carbon a catalyst for this reaction.

• A catalyst is a substance that aids a reaction but remains unchanged by it.

Conclusion• Stars generate energy in their cores via fusion.• This energy keeps them from collapsing.• Fusion is only possible because of quantum

tunneling, therefore it highly depends on the temperature of the core.

• The temperature of the core is very important.• There are 2 methods to convert protons to

Helium.

• To think about during the break - Why does fusion create energy?