5 ms·

CNO fusion does happen but at a fraction of the rate of pp fusion at the core temperature of the Sun. CNO fusion is more efficient in the sense that it is much

CNO fusion does happen but at a fraction of the rate of pp fusion at the core temperature of the Sun. CNO fusion is more efficient in the sense that it is much more strongly temperature dependent. For pp fusion, the rate goes with T^4, but for CNO it is T^20. See eg. https://websites.pmc.ucsc.edu/~glatz/astr_112/lectures/notes8.pdf https://websites.pmc.ucsc.edu/~glatz/astr_112/lectures/notes... for a more detailed explanation.

enkid 6y ago
Does that mean hydrogen would be used up much more quickly in high temperature stars? If I remember correctly, that's why large stars have shorter lifespans.
- haiguise 6y ago
  Yup, the more massive a star the short it lives. The Sun for example will live around 10 billion years, whereas a star 10 times the mass of the Sun will only live for 30 or so million years. Using some hand-wavy arguments you could say that fusion pauses a star's collapse, so the more massive a star is, the more energy generation it needs to stay in equilibrium during this pause.
  - mnw21cam 6y ago
    There's a negative feedback loop there. That is, a star of a certain mass needs a certain amount of energy production in order to keep it from collapsing, and that amount of energy production is automatically achieved, because if it were too low then the star would collapse a bit, and increase it. Therefore, the rate of energy production isn't a consequence of the temperature. The rate of energy production is regulated, so effectively the temperature is a consequence of the required energy production instead.
    - xattt 6y ago
      Would this be one hypothesis for the solar sunspot cycle?
      - bananabreakfast 6y ago
        No, the balance of gravity to outward radiation pressure from the core is static in nearly all stars that aren't some of the largest we have observed. The solar sunspot cycle is caused by the periodic inversion of the polarity of the sun's magnetic field.
raxxorrax 6y ago
Ah, so it is basically even worse for their lifetime. Thank you for the answer. Would that mean that massive first generation stars could live longer than their current brethren, since there wasn't any C, N, or O yet?
- davidcuddeback 6y ago
  No. Size is the dominant factor. Lifespan and what stages of fusion it undergoes are dependent on its size. The more massive the star, the shorter its life span. Red dwarf stars can live for trillions of years, but massive stars may live less than a billion years. Notice that haiguise wrote "at the core temperature of the Sun." A more massive star has a higher core temperature, and thus haiguise's sentence about fusion rates would no longer apply. Fusion rates are faster at higher temperatures, and that's why more massive stars burn out faster. Notice haiguise wrote "T^4" and "T^20." Our sun is roughly 5000K. Massive stars can exceed 10000K. At twice the temperature, T^4 and T^20 imply 16x and 1,048,576x fusion rates, respectively. Edited to add: Wikipedia has an HR diagram with labels showing lifespans for stars at different temperatures: https://commons.wikimedia.org/wiki/File:Hertzsprung-Russel_StarData.png https://commons.wikimedia.org/wiki/File:Hertzsprung-Russel_S....
  - Zaak 6y ago
    But what is the effect on lifetime of not having any CNO present? If a present-day massive star would have a lifetime of 10 million years, how long would it live if it was a population III star with no CNO?
    - davidcuddeback 6y ago
      I'm not sure. I think a lack of metals makes a star less stable and burn out more quickly, but I could be wrong on that. There's some episodes of Astronomy Cast and Ask a Spaceman that I think would answer your question about pop3 stars more reliably than I could.
    - skykooler 6y ago
      Carbon is formed in small amounts by normal stellar fusion, and since it is a catalytic process the carbon is conserved. So even an early star would probably have some fusion happening via the CNO cycle.
rymohr 6y ago
You seem to know a lot about this stuff. I have a question for you. If fusion creates the potential for fission (radioactive waste) and radioactive waste can be used to build atomic bombs, how have we not figured out how to make mini perpetual-energy reactors?
- 7thaccount 6y ago
  Thermodynamics has some laws (First or Second, I can't remember) that point out perpetual energy or motion machines are impossible.
  - jahabrewer 6y ago
    I really don't know, but aren't there some caveats about those assuming that space is flat or something about the rate of expansion?
    - jschwartzi 6y ago
      Nope. The three laws are unequivocal. The universe can only increase or maintain entropy through physical processes. It can never return to a lower-entropy state. The laws say nothing about the topography of the universe and it wouldn’t matter anyway.
      - LeegleechN 6y ago
        When you dig into it more you realize the second law of thermodynamics is more of a statistical statement and doesn't have the same status as say the laws of quantum mechanics or relativity. It's possible to create hypothetical situations where all of the must fundamental laws are being followed but the second law of thermodynamics is violated (for example if there are many more 'ordered' states than 'disordered' ones). And there is some vanishingly small chance that it will be violated in our universe for a macroscopically observable length of time. In practice you won't go wrong by treating it as absolute.
      - mensetmanusman 6y ago

enkid 6y ago

Does that mean hydrogen would be used up much more quickly in high temperature stars? If I remember correctly, that's why large stars have shorter lifespans.

haiguise 6y ago
Yup, the more massive a star the short it lives. The Sun for example will live around 10 billion years, whereas a star 10 times the mass of the Sun will only live for 30 or so million years. Using some hand-wavy arguments you could say that fusion pauses a star's collapse, so the more massive a star is, the more energy generation it needs to stay in equilibrium during this pause.
- mnw21cam 6y ago
  There's a negative feedback loop there. That is, a star of a certain mass needs a certain amount of energy production in order to keep it from collapsing, and that amount of energy production is automatically achieved, because if it were too low then the star would collapse a bit, and increase it. Therefore, the rate of energy production isn't a consequence of the temperature. The rate of energy production is regulated, so effectively the temperature is a consequence of the required energy production instead.
  - xattt 6y ago
    Would this be one hypothesis for the solar sunspot cycle?
    - bananabreakfast 6y ago
      No, the balance of gravity to outward radiation pressure from the core is static in nearly all stars that aren't some of the largest we have observed. The solar sunspot cycle is caused by the periodic inversion of the polarity of the sun's magnetic field.

raxxorrax 6y ago

Ah, so it is basically even worse for their lifetime. Thank you for the answer. Would that mean that massive first generation stars could live longer than their current brethren, since there wasn't any C, N, or O yet?

davidcuddeback 6y ago
No. Size is the dominant factor. Lifespan and what stages of fusion it undergoes are dependent on its size. The more massive the star, the shorter its life span. Red dwarf stars can live for trillions of years, but massive stars may live less than a billion years. Notice that haiguise wrote "at the core temperature of the Sun." A more massive star has a higher core temperature, and thus haiguise's sentence about fusion rates would no longer apply. Fusion rates are faster at higher temperatures, and that's why more massive stars burn out faster. Notice haiguise wrote "T^4" and "T^20." Our sun is roughly 5000K. Massive stars can exceed 10000K. At twice the temperature, T^4 and T^20 imply 16x and 1,048,576x fusion rates, respectively. Edited to add: Wikipedia has an HR diagram with labels showing lifespans for stars at different temperatures: https://commons.wikimedia.org/wiki/File:Hertzsprung-Russel_StarData.png https://commons.wikimedia.org/wiki/File:Hertzsprung-Russel_S....
- Zaak 6y ago
  But what is the effect on lifetime of not having any CNO present? If a present-day massive star would have a lifetime of 10 million years, how long would it live if it was a population III star with no CNO?
  - davidcuddeback 6y ago
    I'm not sure. I think a lack of metals makes a star less stable and burn out more quickly, but I could be wrong on that. There's some episodes of Astronomy Cast and Ask a Spaceman that I think would answer your question about pop3 stars more reliably than I could.
  - skykooler 6y ago
    Carbon is formed in small amounts by normal stellar fusion, and since it is a catalytic process the carbon is conserved. So even an early star would probably have some fusion happening via the CNO cycle.

rymohr 6y ago

You seem to know a lot about this stuff. I have a question for you. If fusion creates the potential for fission (radioactive waste) and radioactive waste can be used to build atomic bombs, how have we not figured out how to make mini perpetual-energy reactors?

7thaccount 6y ago
Thermodynamics has some laws (First or Second, I can't remember) that point out perpetual energy or motion machines are impossible.
- jahabrewer 6y ago
  I really don't know, but aren't there some caveats about those assuming that space is flat or something about the rate of expansion?
  - jschwartzi 6y ago
    Nope. The three laws are unequivocal. The universe can only increase or maintain entropy through physical processes. It can never return to a lower-entropy state. The laws say nothing about the topography of the universe and it wouldn’t matter anyway.
    - LeegleechN 6y ago
      When you dig into it more you realize the second law of thermodynamics is more of a statistical statement and doesn't have the same status as say the laws of quantum mechanics or relativity. It's possible to create hypothetical situations where all of the must fundamental laws are being followed but the second law of thermodynamics is violated (for example if there are many more 'ordered' states than 'disordered' ones). And there is some vanishingly small chance that it will be violated in our universe for a macroscopically observable length of time. In practice you won't go wrong by treating it as absolute.
    - mensetmanusman 6y ago