What is Cno cycle: Definition and 11 Discussions

The CNO cycle (for carbon–nitrogen–oxygen; sometimes called Bethe–Weizsäcker cycle after Hans Albrecht Bethe and Carl Friedrich von Weizsäcker) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain reaction (p-p cycle), which is more efficient at the Sun's core temperature. The CNO cycle is hypothesized to be dominant in stars that are more than 1.3 times as massive as the Sun.Unlike the proton-proton reaction, which consumes all its constituents, the CNO cycle is a catalytic cycle. In the CNO cycle, four protons fuse, using carbon, nitrogen, and oxygen isotopes as catalysts, each of which is consumed at one step of the CNO cycle, but re-generated in a later step. The end product is one alpha particle (a stable helium nucleus), two positrons, and two electron neutrinos.
There are various alternative paths and catalysts involved in the CNO cycles, all these cycles have the same net result:

4 11H + 2 e−
→ 42He + 2 e+ + 2 e− + 2 νe + 3 γ + 24.7 MeV
→ 42He + 2 νe + 7 γ + 26.7 MeVThe positrons will almost instantly annihilate with electrons, releasing energy in the form of gamma rays. The neutrinos escape from the star carrying away some energy. One nucleus goes on to become carbon, nitrogen, and oxygen isotopes through a number of transformations in an endless loop.

The proton–proton chain is more prominent in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reaction starts at temperatures around 4×106 K (4 megakelvin), making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts at approximately 15×106 K, but its energy output rises much more rapidly with increasing temperatures so that it becomes the dominant source of energy at approximately 17×106 K.The Sun has a core temperature of around 15.7×106 K, and only 1.7% of 4He nuclei produced in the Sun are
born in the CNO cycle.
The CNO-I process was independently proposed by Carl von Weizsäcker and Hans Bethe in the late 1930s.
The first reports of the experimental detection of the neutrinos produced by the CNO cycle in the Sun were published in 2020. This was also the first experimental confirmation that the Sun had a CNO cycle, that the proposed magnitude of the cycle was accurate, and that von Weizsäcker and Bethe were correct.

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  1. VictorMedvil

    A What is the energy release of the CNO cycle in carbon fusion?

    I would like details on CNO Cycle Carbon Fusion about the exact energy release of an entire cycle through the reaction and other details you would think would be important about the CNO cycle.
  2. The forgetful one

    I Why is H burning not a B+ decay

    Hello physics gurus out there. Can someone please point me in the correct direction. I am looking for reasons why Hydrogen burning is NOT considered to be B+ decay in the standard CNO cycle? Thanks The forgetful one
  3. J

    CNO fusion using particle accelerator?

    The CNO cycle (see https://en.wikipedia.org/wiki/CNO_cycle) is a catalytic fusion reaction that produces energy in stars larger than the sun. It converts four protons into a helium-4 nucleus using a cycle of carbon, nitrogen and oxygen isotopes as catalysts and releases 26.7 MeV of energy mostly...
  4. Stephanus

    Star Nuclear Reaction: P+P, D+P, He3 and CNO Cycle

    Dear PF Forum, I'm just wondering about P+P reaction in star. Because of its pressure, in the core of the star, hydrogens fuse to become deuterium. Its the complete reaction P + P -> D D + P -> He3 He3 + He3 -> He3 + P + P and if the star is big enough, it can undergo CNO cycle, But what if...
  5. Stephanus

    Understanding the CNO Cycle: Exploring the Accuracy of a Commonly Shared Video

    Dear PF Forum, Is this picture correct? Or it is not CNO Cycle? I get that video from this Thanks for any answer
  6. sunrah

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

    The last reaction in CNO-I sees 15N + 1H --> 12C + 4He I'm interested why it doesn't produce 16O, which is also stable.
  7. A

    In the CNO cycle, how does Nitrogen-15 become Carbon-12 and Helium-4?

    Homework Statement I understand the rest of the cycle, the way my textbook has it, which is: 12C→13N→13C→14N→15O→15N→12C+4He The portion in bold is what I don't understand. Homework EquationsThe Attempt at a Solution I do understand that when the mass number increases (like from Carbon-13 to...
  8. T

    Is CNO cycle more life friendly than H->He?

    Wiki claims in stars, heavier than 1.3 solar masses (where CNO plays major role) there is an inner convection zone near the core, but no outer convection zone. Hence, such stars should be "calmer" - no solar flares, less radiation... Am I right?
  9. S

    Cno cycle dense plasma focus fusion

    hello, new to site. This is my first post. I just was wondering what any ones thoughts were on using the carbon, nitrogen, oxygen cycle in a dense plasma focus device to produce aneutronic fusion power.
  10. A

    Calculating Core Nuclear Reaction Rates for Stars Undergoing the CNO Cycle

    Problem: Two stars with identical core composition and density are undergoing nuclear fusion by the CNO cycle. Star A has a core temperature of 10% higher than star B. What is the ratio of their core nuclear reaction rates? My solution: as the energy generation rate q is proportional...
  11. C

    Energy released CNO cycle

    Homework Statement The first step of the CNO cycle is p \rightarrow C_{6}^{12} \rightarrow N_{7}^{14} + \gamma Estimate the energy of the CNO cycle, state any assumptions. Homework Equations - The Attempt at a Solution I was looking at mass difference, (1.00728u...
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