CNO Cycle: Understanding the Last Reaction and the Mystery of 16O

In summary: No, if two particles go into one, you can't conserve both energy and momentum unless you can find internal degrees of freedom in the daughter to put that energy. So the daughter can be in an excited state, which eventually de-excites and releases a gamma ray. That can conserve both energy and momentum because you end up having two particles go into two particles.In summary, the CNO cycle uses Carbon, Oxygen, and Nitrogen as catalysts to create an alpha particle from 4 protons. Multiple particles fusing to create a single one is a rare process, and due to energy conservation, the energy has to be "just right" to produce the particle. The process is less likely to happen because if you
  • #1
sunrah
199
22
The last reaction in CNO-I sees 15N + 1H --> 12C + 4He

I'm interested why it doesn't produce 16O, which is also stable.
 
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  • #2
That does indeed happen, just far less often. So it's a matter of branching ratios.
 
  • #3
Specifically, the CNO cycle uses Carbon, Oxygen, an Nitrogen as catalysts for the reaction 4H->He, the "interesting" branches of the cycle are the ones which produces an alpha particle from 4 protons.
 
  • #4
Multiple particles fusing to create a single one is a rare process in general. Due to energy conservation, the energy has to be "just right" to produce the particle. It is never exactly right, so you have to hit some excited state that decays to the ground state afterwards.
Carbon-12 has this state for the triple-alpha process, oxygen-16 does not have a state at the right energy.

Matterwave said:
Specifically, the CNO cycle uses Carbon, Oxygen, an Nitrogen as catalysts for the reaction 4H->He, the "interesting" branches of the cycle are the ones which produces an alpha particle from 4 protons.
Well, our interest does not influence the reaction rates ;).
 
  • #5
mfb said:
Well, our interest does not influence the reaction rates ;).

But they might influence which branches get emphasized in a textbook and which branches are neglected.
 
  • #6
I think it's fair to say that the processes that are of most interest are the ones that have the fastest rates. There are states of Oxygen 16 that have the right energy to be obtained from Nitrogen 15 and a proton, they just spit out a gamma ray, but they are just less likely to happen because if you spit out an alpha particle instead, you have a higher momentum particle for the same energy (it's less relativistic), so you get access to more momentum states. There's also different internal processes involved, so they all affect the branching ratio. This means there are just more ways the processes with a higher branching ratio can happen. I'll bet it's not at all uncommon for three alpha particles to come together but then fly apart again before they release a gamma ray and drop into a bound state, it's just that we don't count that as a kind of reaction so it just doesn't show up. But of the reactions that change the nature of the nucleus, the ones that happen the most ways are the ones that we need to track.
 
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  • #7
mfb said:
Multiple particles fusing to create a single one is a rare process in general. Due to energy conservation, the energy has to be "just right" to produce the particle. It is never exactly right, so you have to hit some excited state that decays to the ground state afterwards.
Carbon-12 has this state for the triple-alpha process, oxygen-16 does not have a state at the right energy.
My understanding of the processes involved is probably a very naive one, but couldn't the extra energy simply go into kinetic energy of the daughter nucleus?
 
  • #8
No, if two particles go into one, you can't conserve both energy and momentum unless you can find internal degrees of freedom in the daughter to put that energy. So the daughter can be in an excited state, which eventually de-excites and releases a gamma ray. That can conserve both energy and momentum because you end up having two particles go into two particles.
 
  • #9
Ken G said:
I think it's fair to say that the processes that are of most interest are the ones that have the fastest rates. There are states of Oxygen 16 that have the right energy to be obtained from Nitrogen 15 and a proton, they just spit out a gamma ray, but they are just less likely to happen because if you spit out an alpha particle instead, you have a higher momentum particle for the same energy (it's less relativistic), so you get access to more momentum states. There's also different internal processes involved, so they all affect the branching ratio. This means there are just more ways the processes with a higher branching ratio can happen. I'll bet it's not at all uncommon for three alpha particles to come together but then fly apart again before they release a gamma ray and drop into a bound state, it's just that we don't count that as a kind of reaction so it just doesn't show up. But of the reactions that change the nature of the nucleus, the ones that happen the most ways are the ones that we need to track.

Fair enough.
 
  • #10
Bandersnatch said:
My understanding of the processes involved is probably a very naive one, but couldn't the extra energy simply go into kinetic energy of the daughter nucleus?
You can view the reaction in the system of the final product - no kinetic energy.
Alternatively: the kinetic energy is fixed by the momentum, there is no degree of freedom to use here.
 
  • #11
Thanks Ken G and mfb. And thanks to the OP for asking the question. I remember wondering about it back in secondary school when I still thought I'd go into astronomy for a career. Coldn't find the answer back then, and completely forgot about it until now.
 

What is the CNO cycle and why is it important?

The CNO cycle, also known as the carbon-nitrogen-oxygen cycle, is a nuclear reaction that takes place in the cores of stars. It is responsible for the conversion of hydrogen into helium, which is the main source of energy for stars. This cycle is important because it allows stars to maintain a balance between the inward force of gravity and the outward pressure caused by nuclear reactions, allowing them to shine for billions of years.

How does the CNO cycle work?

The CNO cycle involves a series of nuclear reactions that convert hydrogen into helium. It starts with the fusion of four hydrogen nuclei (protons) to form a helium nucleus (alpha particle). This process releases energy in the form of gamma rays. The alpha particle then combines with another proton to form a deuterium nucleus, which then fuses with another proton to form a helium-3 nucleus. This process repeats until the final product is a stable helium-4 nucleus.

What role does carbon play in the CNO cycle?

Carbon is a catalyst in the CNO cycle, meaning it helps to speed up the reaction without being consumed itself. In the first step of the cycle, carbon combines with a proton to form nitrogen. This nitrogen then undergoes several transformations, releasing gamma rays and eventually producing a carbon atom again, which can be used to repeat the cycle.

Why is the production of 16O a mystery in the CNO cycle?

In the CNO cycle, one of the final products is a stable oxygen-16 nucleus. However, scientists have observed that the amount of oxygen-16 produced in stars is much higher than what is predicted by the CNO cycle. This discrepancy, known as the "oxygen problem," is still not fully understood and is an active area of research in nuclear astrophysics.

What are the implications of the CNO cycle for our understanding of the universe?

The CNO cycle is crucial in our understanding of how stars produce energy and how they evolve over time. It also plays a significant role in the formation of heavier elements in the universe, as the fusion of hydrogen into helium is a necessary step for the creation of all other elements. Additionally, the CNO cycle has implications for our understanding of the early universe and the processes that led to the formation of the first stars and galaxies.

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