CNO Cycle

[KingGambit]

Dear PF Forum,

What is CNO Cycle?
Is it Carbon Nitrogen Oxygen cycle or Carbon Neon Oxygen?

Veritasium CNO cycle

In this video, Derek Muller says that Carbon fuses into Neon, and Neon fuses into Oxygen.
I can't imagine that Derek makes an error here. But for myself, I get a little confused, too.
If Carbon is bombarded by Alpha particle, then Carbon should become Oxygen, right. And Oxygen + Alpha particle = Neon?

So, can I ask what fusion happens in a star start from Carbon,
Is it Carbon, Nitrogen, Oxygen
- if this is, then what element that Carbon fuses to, to become Nitrogen, it can't be Alpha particle, right.
Is it Carbon, Neon (ehm), Oxygen??
- Well, I'm no chemist much less an astrophysics, but I do memorize the eleventh or tweleve elements in the periodic table to understand the sequence, Carbon, Nitrogen, Oxygen, Fluor, Neon, Sodium, ?? .

Thank you very much for any help here

 

[DaveC426913]

It is not a simple additive reaction. All the protons are not necessarily consumed by the process. It is catalytic, meaning there is an alpha particle produced as a byproduct.

Wiki has an easy primer on the process.

 

[pines-demon]

Dear PF Forum,

What is CNO Cycle?
Is it Carbon Nitrogen Oxygen cycle or Carbon Neon Oxygen?

CNO clearly stands for carbon-nitrogen-oxygen. In some cycles it can produce neon see CNO cycle (CNO IV). You also have the carbon-burning process (not to be confused with CNO) that produces neon by colliding two carbon atoms. The CNO cycle can take place even in hydrogen-core stars.

Veritasium CNO cycle

In this video, Derek Muller says that Carbon fuses into Neon, and Neon fuses into Oxygen.
I can't imagine that Derek makes an error here. But for myself, I get a little confused, too.
If Carbon is bombarded by Alpha particle, then Carbon should become Oxygen, right. And Oxygen + Alpha particle = Neon?

What Derek seems to be talking about it is the remaining core of the star. When he talks about neon, it refers to the process which is called the neon-burning process (which comes after the carbon-burning). It is followed by the oxygen-burning process.

So, can I ask what fusion happens in a star start from Carbon,
Is it Carbon, Nitrogen, Oxygen
- if this is, then what element that Carbon fuses to, to become Nitrogen, it can't be Alpha particle, right.

In order to produce nitrogen during the CNO phase, the star produces some oxygen-15 which is unstable and decays into nitrogen by beta decay.

 

[snorkack]

Dear PF Forum,

What is CNO Cycle?
Is it Carbon Nitrogen Oxygen cycle or Carbon Neon Oxygen?

In this video, Derek Muller says that Carbon fuses into Neon, and Neon fuses into Oxygen.
I can't imagine that Derek makes an error here. But for myself, I get a little confused, too.
If Carbon is bombarded by Alpha particle, then Carbon should become Oxygen, right. And Oxygen + Alpha particle = Neon?

Unless proton/s are expelled on reaction with alpha particle!

So, can I ask what fusion happens in a star start from Carbon,
Is it Carbon, Nitrogen, Oxygen
- if this is, then what element that Carbon fuses to, to become Nitrogen, it can't be Alpha particle, right.

One option of fusion cycle. That is fusion with protium. (Note that deuterium and tritium are rare in stars, deuterium because it rapidly fuses with protium).
While wikipedia CNO cycle article is nice, it does not address the branching well, by treating CNO as separate cycles.
Have a look at https://www-nds.iaea.org/relnsd/vcharthtml/VChartHTML.html. Hover the mouse cursor at an isotope, and it shows a number of ways to break up the nucleus.
Look at ¹³N. It shows:
...
Q_α=-9495 keV (and more digits I´m truncating out)
...
S_p=1943 keV
This means that there are the following "possible" reactions:
¹³N=¹²C+p-1943 keV
¹³N=⁹B+α-9495 keV
Note that S is (almost) always expressed as positive, it is the energy lost knocking out a nucleon or gained adding it (⁹B is one of the few exceptions - it immediately fissions to p+2α). Q may be positive or negative, if it is negative then the decay cannot happen unless energy is added.
So we see that reaction
¹²C+p=¹³N+γ
is possible.
Reaction
¹³N=⁹B+α=3α+p
will happen only if a huge amount of energy is somehow supplied. I marked it in italics that it is the ruled out alternative.
Continuing the chain...
¹³N will emit positron or capture electron to promptly convert to ¹³C. I'll imply these beta decays for all radioactive isotopes in the chain/cycle.
¹³C+p=¹⁴N+γ+7550 keV
¹⁴N=¹⁰B+α-11612 keV
next:
¹⁴N+p=¹⁵O+γ+7296 keV
¹⁵O=¹¹C+α-10218 keV
But:
¹⁵N+p=¹⁶O+γ+12127 keV
¹⁶O=¹²C+α-7161 keV
summing up to:
¹⁵N+p=¹²C+α+4966 keV
This I bolded because in contrast to previous 3 reaction pairs, this time adding proton to get ¹⁶O releases more energy than is needed to take α out and therefore this last reaction is possible.
A general rule is that when the same reagents of nuclear reaction can form either one nucleus and a gamma ray OR two nuclei then forming two nuclei happens much more often even though it releases less energy. This is why it is named STRONG interaction!
The bolded interaction closes one cycle. The reaction forming ¹⁶O is the minor branch.
What next?
¹⁶O+p=¹⁷F+γ+600 keV
¹⁷F=¹³N+α-5818 keV
next
¹⁷O+p=¹⁸F+γ+5607 keV
¹⁸F=¹⁴N+α-4415 keV
¹⁷O+p=¹⁴N+α+1192 keV
This closes cycle, again! And it returns to the middle of the previous cycle! If you heat any of the ¹⁵O, ¹⁶O or ¹⁷O with hydrogen, the oxygen is mainly destroyed, forming helium and carbon, or first helium and nitrogen (and then converting carbon to nitrogen and back - but NOT converting them to boron).
But the second cycle again has the minor exit forming ¹⁸F. What next?
¹⁸O+p=¹⁹F+γ+7993 keV
¹⁹F=¹⁵N+α-4013 keV
¹⁸O+p=¹⁵N+α+3980 keV
Third return of the loop, and also to the middle of the first cycle! All stable O isotopes are destroyed on heating with hydrogen.
Next:
¹⁹F+p=²⁰Ne+γ+12843 keV
²⁰Ne=¹⁶O+α-4729 keV
¹⁹F+p=¹⁶O+α+8114 keV
fourth return of the loop
the minor exit
²⁰Ne+p=²¹Na+γ+2431 keV
²¹Na=¹⁷F+α-6561 keV
next
²¹Ne+p=²²Na+γ+6738 keV
²²Na=¹⁸F+α-8479 keV
next
²²Ne+p=²³Na+γ+8794 keV
²³Na=¹⁹F+α-10467 keV
next
²³Na+p=²⁴Mg+γ+11692 keV
²⁴Mg=²⁰Ne+α-9316 keV
²³Na+p=²⁰Ne+α+2676 keV
And this is a new loop! It does NOT loop back to the previous loops - heating Ne and Na with hydrogen can convert them to each other, but can NOT convert them to F, and then back to C, N and O. This is why there is a CNO cycle that ends after F.
However, it is very hard to remove CNO from CNO cycle. In order to convert CNO into Ne, the reaction has to take the less likely branch 4 times in a row. Which is why CNO mostly cycle between each other, and don´t produce much Ne.