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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?