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

  1. Oct 20, 2014 #1
    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?
     
  2. jcsd
  3. Oct 20, 2014 #2

    D H

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    I would think you're wrong. Even if your conjecture is correct, those intermediate mass stars don't need to produce solar flares to produce dangerous radiation. These are the blue stars, classes A, B, and O. They produce dangerous radiation all the time thanks to their higher surface temperatures. I also doubt your conjecture is correct.

    Finally, there's one last problem with those classes of stars. They don't stay on the main sequence long enough for life to have a chance to evolve. Extrapolating from a sample size of one (Earth), it takes a planet about 4.0 billion years to produce life of any reasonable complexity, 4.5 billion years to produce an intelligent species. That would pretty much rule out all but the smallest class A stars.
     
  4. Oct 21, 2014 #3
    to clarify, CNO starts playing major role is stars of 1.3 solar masses and heavier,
    so forget about heavy stars, lets compare Sun and a star with 1.3 Msun
     
  5. Oct 21, 2014 #4

    SteamKing

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    This article is very interesting. It discusses the time spent on the main sequence by stars of various mass:

    https://www.astro.umd.edu/~ssm/ASTR100/lecture20.pdf

    A star like the sun can stay on the main sequence for approx. 10 billion years. A star like Sirius A, which has twice the mass of the sun, will stay on the main sequence only about a billion years. Procyon A has a mass of about 1.4 times that of the sun, and there is evidence that it may be ending its time on the main sequence, having burned most of its hydrogen already. Procyon A is estimated to be about 2 billion years old and will start expanding into a red giant in about 10-100 million years:

    http://en.wikipedia.org/wiki/Procyon

    Sirius and Procyon are both double stars, with smaller white dwarf companions. Procyon B orbits rather more closely to its companion than Sirius B does, so the probability of any planets existing with stable orbits in the Procyon system is low.

    The evolution of life depends on many factors other than the type of fusion occurring in the central star. The amount of time it takes for life to evolve into something complex enough to be aware of itself and of the universe appears to be on the order of several billion years. This requirement alone means that life as we know it does not have sufficient time to evolve in solar systems where the central star is just a bit larger than the sun.

    It's the old Goldilocks principle: Everything has to be just right.
     
  6. Oct 21, 2014 #5
    But based on this article:
    http://en.wikipedia.org/wiki/Main_sequence#Lifetime
    lifetime decreases as power of 2.5, so for a star 1.3 times heavier than Sun we get, based on this formula, we get lifetime 1.92 times shorter than Sun, slightly longer than 5by. This is enough for life.

    As the same time, if a star is just 1.3 times heavier than Sun:
    http://en.wikipedia.org/wiki/Convection_zone

    So shouldn't such star be much better place to live?
     
  7. Oct 21, 2014 #6

    SteamKing

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    According to the article I referenced, the exponent is approximately = -3, so there is a range of estimated lifespans for larger stars. It is clear, however, that the larger the star, the shorter amount of time it spends on the main sequence.

    Whether or not living next to a larger star is 'better' is a question whose answer will be highly subjective. There are many other factors at play. If out solar system had multiple stars, would we even be around to discuss this question?
     
  8. Oct 21, 2014 #7
    Let me make it more objective.
    Multiple parameters (including parameters of the Standard Model) form N-dimensional space. In that space there is a "cloud", where life is possible. No matter how tiny this cloud is ("fine tuning problem"), we can ask: are we right at the center of that cloud or not?

    If we interpret home star mass as a parameter (other parameters could be metalicity, for example), we can ask the same question. And I am not sure we are in the center of this cloud.
     
  9. Oct 21, 2014 #8

    SteamKing

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    We may not be in the center of this 'cloud', whatever it is, but we are here nonetheless, which is a far more important fact.

    Could humans have evolved in a solar system whose central star was slightly larger than the sun? Possibly, but the odds of such an event occurring might have been markedly lower.

    Given the fact that so far we have been unable to detect the presence of a civilization capable of beaming electromagnetic signals into space indicates that while the number of worlds capable of supporting life might be considerable, deducing the actual number where life can be proven to exist is another problem altogether.
     
  10. Oct 21, 2014 #9

    D H

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    You are contradicting yourself. You want to focus on stars where the CNO cycle dominates over the pp chain. Those are the class A, B, and O stars. Your 1.3 solar masses (some authors say 1.5 solar masses) is what distinguishes class F from class A stars. Class A stars are the least massive classes of stars where the CNO cycle dominates. If the CNO cycle can kick in, it pushes stars to a much higher fusion rate. Class G and F stars are whitish. Higher mass stars (class A, B, and O) are bluish. It's a very sharp change because of the very sharp temperature sensitivity of the CNO cycle.

    For stars between 1/3 and 10 solar masses, there is a fourth power relation between mass and luminosity, which in turn means that there is inverse cube relation between mass and longevity on the main sequence. See the first link in SteamKing's post. Having at least four billion years on the main sequence means an upper limit of about 1.35 solar masses. That's right in the middle of that 1.3 to 1.5 solar mass range where CNO cycle dominates over the pp chain.

    That 1.3 to 1.5 solar mass range has to be a range. There is no clear cut boundary. There can't be. Given two stars of the same mass, the older star will burn hotter than will the younger one. The accumulated helium is the culprit. The core temperature rises as helium accumulates. For a young star at the upper end of that range, it's the pp chain that will dominate, resulting in a high mass class F star. For an old star at the lower end of that range, it's the CNO cycle that will dominate, resulting in a low mass class A star.

    This presents a serious problem with regard to friendliness to life for that 1.35 solar mass star. It will start it's life as a class F star. At some point in its life, it will reach a core temperature the CNO cycle becomes dominant. The star will suddenly become much hotter, a class A star, and a planet that was in the goldilocks zone is now toast.
     
  11. Oct 23, 2014 #10

    Ken G

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    Actually, that isn't true. There is no sudden change in any of the stellar parameters when CNO fusion takes over, because the details of the fusion process play essentially no role in any of the stellar parameters along the main sequence. Indeed, main-sequence luminosities of the warmer stars, and even the Sun, were derivable to a good approximation by Eddington, without even knowing there was any such thing as nuclear fusion! It is a surprising and fascinating aspect of stars that as long as they transport energy primarily by radiative diffusion, their luminosity depends pretty much only on their mass, and is a smooth (though fairly steep) function of that mass-- independently of what type of fusion is going on, or even whether there exists any process of nuclear fusion.
    Yes, that is the smooth relation to which I refer, and it does not depend much at all on the details of nuclear fusion.
    Not to be argumentative, but that isn't true. The spectral type, A or F, is determined by the surface temperature, not the core temperature. Evolution within the main sequence does increase the luminosity of a star a little bit, but it also puffs the star out a little, so the surface temperature actually drops as the star ages-- though not very much.
    That's what is not true, the star will get more luminous but slightly redder, not bluer, just as our own Sun is doing. Crossing a boundary from pp to CNO would not produce any sudden changes in luminosity, nor have any great significance to the habitability of a planet, though the gradual increase in luminosity that accompanies aging, regardless of whether pp crosses over into CNO, might-- just as it might for our own solar system in the far future.

    But it should also be noted that minor flares like our Sun has probably present little challenge to life, as long as the planet has a magnetic field like Earth. The nasty flare stars, way more active than the Sun, tend to be redder stars, like M stars. Keep away from the most active of the M stars, and life should be fine.
     
    Last edited: Oct 23, 2014
  12. Oct 24, 2014 #11

    Matterwave

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    To be fair guys, the OP did not specify intelligent life in his post. Signs of early microbial life on Earth date back 3 billion years or more, suggesting perhaps 1 billion to 2 billion years might be enough to evolve some sort of microbial life. Stromatolites date back to ~3.5 billion years, and some other, less conclusive, signs of life date back even farther.

    But cutting the lifespan of the host star much shorter than 1 billion years is probably pushing it even for microbial life, since it takes time for planetoids to clear their orbits and you may have very large collisions happening (e.g. the great bombardment, or the event that created our moon) which is very well not conducive to life at those early stages of planetary formation.
     
  13. Oct 24, 2014 #12
    Ok, forget about CNO, let's interpret my question wider
    For stars in a range of 0.5-1.5 solar masses, what mass is "ideal" for life?
     
  14. Oct 24, 2014 #13

    Matterwave

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    If by life, you mean life as we know it, then probably "ideal" would be close to 1 solar mass. But of course this is not to be unexpected. Life as we know it evolved around a star of mass 1 solar mass, and so is ideally suited for such a star.
     
  15. Oct 24, 2014 #14
    So if I am born in some country, I am "ideally suited" for that particular country?
    Many immigrants won't agree with you.
    AP guarantees that life is possible, but not that we are in the best place for it, so life could evolve against all odds (AP guarantees that you are "in the cloud" I've described above, but not giving any particular position in that cloud)
     
  16. Oct 24, 2014 #15

    SteamKing

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    Your question can't be answered objectively or subjectively, because we have only one data point: our own solar system. We can make some remote observations of nearby solar systems, but actual exploration appears to be possible only in the remote future, if at all. There is no evidence that actual living organisms, intelligent or otherwise, are present on any planets in these nearby systems, or even in solar systems more remote.
     
  17. Oct 24, 2014 #16
    I agree, it is too complicated to talk about life, but it is much easier to talk about stellar evolution. So it is a dilemma:

    more mass - less flares - shorter lifespan
    vs
    less mass - more flares - longer lifespan

    If we go another direction (lighter stars), when flares become "nasty"?
     
  18. Oct 24, 2014 #17

    Matterwave

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    Well, I said "around 1 solar mass" I didn't say "has to be our sun". Your analogy would work if you say you're ideally suited to your country, and not ideally suited for Antarctica...or the middle of the Sahara dessert, because the variation in solar properties is just that great.

    Let's take a smaller view. Let's say that for all your life you've ever only lived in the lush rainforests of the Amazon. You observe the life there and ask "what kind of climate is ideally suited for the life here?", you would say "well, we would like a lot of rain, we need humidity, we need rivers to provide water, we need it to be quite warm." I come and ask you "what about Antarctica? Is that suited for life as you know it?" (assume you've never been to Antarctica, but you've heard about it, or perhaps studied it from afar, but never found life). I think the only reasonable answer would be "well, we can't know for sure if Antarctica might have life or not, but it is not ideally suited for life as we know it". That's basically the only answer you can come up with, with your current knowledge and understanding. That's the answer I'm giving you.

    In addition, more mass - less flares - shorter lifespan is overly simplistic. I would first need to see some sources on the claim that "more mass=less flares" because I have not encountered this claim previously. Assuming it is correct, we must still consider "more mass = more ultraviolet, and high energy radiation" which is bad for life because they tend to break up the molecules that make up life as we know it. Less mass = longer life is true, but also consider that a small mass star needs you to be closer to it in order to be warm. If you are too close to your parent star, you become tidally locked and only face of your planet turns very hot while the other face turns very cold. Again, we can't rule out life on such a planet, but it is at least not conducive to life as we know it.
     
  19. Oct 24, 2014 #18

    Bandersnatch

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    This paper:
    http://www.as.utexas.edu/astronomy/education/spring02/scalo/heath.pdf
    analyses habitability of tidally-locked planets around red dwarfs. The impact of flares is discussed in section 6.
    In general, the authors give it a thumbs up.
     
  20. Oct 24, 2014 #19

    Matterwave

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    I'm sorry, I'm not entirely sure which part of my comment you were replying to? Section 6 seems to suggest that flares depend on age and rotation of the star and does not mention a mass-flare relationship.
     
  21. Oct 25, 2014 #20

    Bandersnatch

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    Apologies for not being clear enough(or at all). Late night posting does that to you.

    I meant to show that tidal locking needen't preclude "life as we know it", and, in a similar vein, that flaring associated with low-mass stars(i.e., red dwarfs) may not be a problem, at least as far as UV output is concerned.
    It would seem that the condition of being a tidally-locked planet around a red dwarf might actually extend the habitable zone.

    I know not of any global mass-flare relationship, but figured the suggestion of an earlier poster about low mass=more flares was actually about the specific case of red dwarfs flaring a lot, which they tend to do. At least I haven't heard it mentioned in any other context.
     
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