Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
· Get a Cloudy Nights T-Shirt · Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User

Speciality Forums >> Science! Astronomy & Space Exploration, and Others

Pages: 1
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
CNO cycle question new
      #5918117 - 06/13/13 01:42 AM

At the end of the standard CNO cycle, a Nitrogen nucleus of weight 15 takes on a proton and fissions into 12C and 4He, thus completing the cycle.

An alternative reaction is 15N + p -> 16O + gamma.

Why is the first reaction the dominant one?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Neutrino?
sage
*****

Reged: 12/14/09

Loc: Wasatch Front
Re: CNO cycle question new [Re: derangedhermit]
      #5919219 - 06/13/13 05:45 PM

Not really sure. I would suspect that the excited state of 16O that produces the gamma decay is due to narrow resonances in the nucleus. Since the proton that is captured by the 15N is spread over a wide distribution of energies, it is most likely that the proton doesn't have the needed energy to produce that resonance in the nucleus. Thus having sufficient energy, an alpha particle would most likely tunnel out.

Post Extras: Print Post   Remind Me!   Notify Moderator  
llanitedave
Humble Megalomaniac
*****

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: CNO cycle question new [Re: Neutrino?]
      #5919490 - 06/13/13 08:27 PM

This seems like a good candidate for accelerator experiments. If they've been done, do the dominant and alternative reactions occur a similar frequencies?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Joel F.
Pooh-Bah
*****

Reged: 03/19/06

Loc: Overland Park, Kansas
Re: CNO cycle question new [Re: derangedhermit]
      #5919531 - 06/13/13 08:55 PM

Icko Iben, Jr. (2013) page 285

"...once every ~2200 times that an 15N nucleus reacts with a proton, the resulting compound nuclus decays into an 16O with the release of a gamma ray rather than into a 12C nucleus and a alpa paticle...."

He also shows some tableswith other discusion on lifetime of varios isotopes.

Hope this helps!


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question new [Re: Neutrino?]
      #5919775 - 06/13/13 11:36 PM

Quote:

Not really sure. I would suspect that the excited state of 16O that produces the gamma decay is due to narrow resonances in the nucleus. Since the proton that is captured by the 15N is spread over a wide distribution of energies, it is most likely that the proton doesn't have the needed energy to produce that resonance in the nucleus. Thus having sufficient energy, an alpha particle would most likely tunnel out.




Well, the 16O is the O isotope that is 99+% of the O in the universe (meaning it is rather stable), and has a higher binding energy per baryon than 4He... But the question is that the reaction doesn't take place to begin with, not that the product isn't stable...


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question new [Re: Joel F.]
      #5919785 - 06/13/13 11:46 PM

Quote:

Icko Iben, Jr. (2013) page 285

"...once every ~2200 times that an 15N nucleus reacts with a proton, the resulting compound nuclus decays into an 16O with the release of a gamma ray rather than into a 12C nucleus and a alpa paticle...."

He also shows some tableswith other discusion on lifetime of varios isotopes.

Hope this helps!




Does it say what temp and pressure regimes? The % occurrence is low in stars of mass similar to ours (but I didn't think it was that low), but I think it is sensitive to temp and pressure, increasing corresponding to the increase of the mass of the star.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joel F.
Pooh-Bah
*****

Reged: 03/19/06

Loc: Overland Park, Kansas
Re: CNO cycle question new [Re: derangedhermit]
      #5919812 - 06/14/13 12:21 AM

Send me a PM with an address to send you copies of the sections you may want.

Post Extras: Print Post   Remind Me!   Notify Moderator  
Neutrino?
sage
*****

Reged: 12/14/09

Loc: Wasatch Front
Re: CNO cycle question new [Re: derangedhermit]
      #5919820 - 06/14/13 12:30 AM

Quote:



Well, the 16O is the O isotope that is 99+% of the O in the universe (meaning it is rather stable), and has a higher binding energy per baryon than 4He... But the question is that the reaction doesn't take place to begin with, not that the product isn't stable...




Well of course, 16O is stable. But an excited 16O that has just formed from capturing a proton with some decent energy is not just some random 160 floating around. Not sure what it is, but there is some temp when the probabilities of the CNO occuring will take dominance. Apparently, our Sun isn't quite there.


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question new [Re: llanitedave]
      #5919832 - 06/14/13 12:40 AM

Quote:

This seems like a good candidate for accelerator experiments. If they've been done, do the dominant and alternative reactions occur a similar frequencies?



I have no idea if they have been performed. I am sure this is something well-known, not something exotic; I just don't understand that step.

I am curious because 1) the CNO cycle happens in the Sun, although as a small %, and 2) for stars even slightly larger than ours, the CNO cycle can become the dominant reaction chain. All such larger stars would have different lives if the production of 4He was greatly reduced.

I know once 4He starts making 12C, that lots of reactions start to become possible. But this is still in the hydrogen fusion stage, before 4He fusion, before things get complicated.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Joel F.
Pooh-Bah
*****

Reged: 03/19/06

Loc: Overland Park, Kansas
Re: CNO cycle question new [Re: Neutrino?]
      #5919834 - 06/14/13 12:41 AM

CNO reactions dominate at temps above 16 x 10^6 K.

Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question new [Re: Neutrino?]
      #5919845 - 06/14/13 12:49 AM

Quote:

Well of course, 16O is stable. But an excited 16O that has just formed from capturing a proton with some decent energy is not just some random 160 floating around. Not sure what it is, but there is some temp when the probabilities of the CNO occuring will take dominance. Apparently, our Sun isn't quite there.



I am with you on the increasing rate of CNO cycle occurring with temp and pressure increasing. It's the low rate of 15N + p -> 16O + gamma that I don't understand. I think that rate is not temp+pressure sensitive.

Does the production of a photon change (lower) the probability of a reaction?


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question [Re: derangedhermit]
      #5935129 - 06/22/13 06:34 PM

I eventually found this evidently classic work: Synthesis of the Elements in Stars.

Also this more recent page has an overview and lots of interesting links.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Neutrino?
sage
*****

Reged: 12/14/09

Loc: Wasatch Front
Re: CNO cycle question new [Re: derangedhermit]
      #5984203 - 07/22/13 09:42 AM

I only quickly skimmed it, did it say what was going on?

Post Extras: Print Post   Remind Me!   Notify Moderator  
deSitter
Still in Old School


Reged: 12/09/04

Re: CNO cycle question new [Re: derangedhermit]
      #5984587 - 07/22/13 02:50 PM

Quote:

I eventually found this evidently classic work: Synthesis of the Elements in Stars.

Also this more recent page has an overview and lots of interesting links.




That is one of the great papers in history. Thanks for linking it. Geoffrey Burbidge was one of the few astrophysicists with the courage to defend Halton Arp when he was being personally attacked and denied telescope time. He is one of my personal heroes. Sadly passed a couple of years ago, but had a long and productive life.

-drl


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: CNO cycle question new [Re: deSitter]
      #5985510 - 07/23/13 05:13 AM

Synthesis of the Elements in Stars does not answer the question directly. After consolidating the understanding of the reaction chains in stars, it only gives some indications of how reaction rates are theoretically calculated and mentions some experimental measurements (but not of what I was specifically asking about here).

The page at the more recent link I gave doesn't either, but it does provide more information, and point to many papers in the area, and point to one specific recent paper that answers the question llaniteDave asked: have the relative reaction rates been directly measured in an accelerator? The answer is no, but they are getting closer - they have recently directly measured this reaction at the equivalent of higher temperatures (4 to 16 times that of the sun's core), and think understanding this reaction is of some importance for a few reasons, like helping sort the relative abundance of O and C in certain types of stars, and early processes in globs, to name a couple. (I'm learning all this as I go, so I may be making statements of gross error). The experimenters still have trouble getting conditions down low enough on earth to model our little star, because the environmental background noise is too high IIUC. The way they test this reaction and similar ones is since the reaction produces gamma rays, they bang the right stuff at the interesting range of energy levels and try to catch and count the gamma rays (that's about a crude a way to put it as I can imagine). Then, as near as I can tell, they guess which reactions produced which peaks at certain energy levels - ahem, they compare their predictions of reaction rates at given energies with the experimental results.

BTW, for those as illiterate as I am in these things, 15N(p,gamma)16O, the form the authors use, is what I was asking about originally: 15N + p -> 16O + gamma; just another way of writing it.

In summary:
- the test hasn't been run, but they are getting closer to being able to, and they have extensively now tested the reaction at somewhat higher energy levels
- I'll never get on top of the math required to do the calculations of the theoretical relative reaction probability "cross-sections" myself (they are laid out in the last paper I referenced)
- One responder above mentioned resonance (energy levels), which does come in as one of many factors into the equations

I've saved the best for last: an answer via e-mail:

"Your question is an intelligent one. The reaction 15N + p -> 16O + gamma is indeed possible and competes with 15N + p -> 12C + 4He. However, in addition to the strong force needed to bond the nuclei, the former involves the electromagnetic interaction (as signaled by the release of a photon - the gamma ray), needed to conserve energy and momentum) and therefore has a smaller "cross-section" or probability than the latter reaction, which involves only strong forces. Thus, the former reaction is ignored in the simplified discussions of the so-called "CNO cycle", but it is included as a correction in more accurate calculations, where successive captures of protons by 16O and heavier species finally also "terminates" in the fissioning of a heavy nucleus into something lighter plus 4He. These additional stages are at a competitive disadvantage relative to the one that starts and ends with 12C because of the greater Coulomb barrier associated with the heavier nuclei. I hope this answers your question.

Regards,
Frank Shu"

I then learned that these nuclear masses (CNO) are at the earlier, lower end of where the Coulomb barrier is a significant factor; but evidently, already a factor nonetheless. It also seems that Dr. Shu is saying that even on the infrequent occasions where 16O is produced, it, or heavier charged nuclei, don't stay in that form, but are reduced back to 4He. So C, N, and O are truly catalysts for these cycles.

It was very gracious of him to respond.

Lee


Post Extras: Print Post   Remind Me!   Notify Moderator  
llanitedave
Humble Megalomaniac
*****

Reged: 09/26/05

Loc: Amargosa Valley, NV, USA
Re: CNO cycle question new [Re: derangedhermit]
      #5985979 - 07/23/13 12:27 PM

Very nice answer, and it was also good of you to ask the question in the way you did, Lee. Thanks all around!

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1


Extra information
2 registered and 3 anonymous users are browsing this forum.

Moderator:  LivingNDixie, FirstSight, JayinUT 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 582

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics