The CNO cycle.

[AGarvin]

A brief discussion was raised in a previous thread regarding the CNO cycle in stars. I've rummaged through my notes and found the exact cycles (for those interested). For those unfamiliar with the process, CNO stands for Carbon-Nitrogen-Oxygen, and it's one of two fusion cycles that power the cores of stars (the other being the proton-proton cycles). It's not a significant process in the Sun as the core is not hot enough but comes into play in larger stars.

H - Hydrogen
C - Carbon
N - Nitrogen
O - Oxygen
He - Helium
e+ - positron
nu - neutrino
g - gamma photon

The numbers represent the isotope of the element, eg, the carbon atom has 6 protons in the nucleus. If it has 6 neutrons, it's carbon-12 (6 + 6 = 12). If it has 7 neutrons, it's carbon-13.

The CNO main cycle is:

12C + 1H > 13N + g (10^6 years)
13N > 13C + e+ + nu (beta+ decay, 14 minutes)
13C + H1 > 14N + g (3x10^5 years)
14N + H1 > O15 + g (3x10^8 years)
O15 > 15N + e+ + nu (beta+ decay, 82 seconds)
*15N + H1 > 12C + 4He (10^4 years)

Times scales from Blohm-Vitense 1992.

*There's a small chance (0.0004) that instead of 15N + 1H producing 12C + 4He it will actually produce a 16O. This will trigger the CNO bi-cycle from the O16 atom:

*15N + 1H > 16O
16O + H1 > 17F
17F > 17O + e+ + g
O17 + 1H > 14N + 4He

No time scales found for the bi-cycle.

Andrew.

[xelasnave]

Andrew I am so very interested I will step forward at the risk of appearing stupid and ask if you could explain your notes a little more. If could take say one star and explain how the thing moves from there I for one would be very much obliged. I gather there is a relationship here that has been quanitfied, I dont follow it as well as others but I dont want to miss the point here. If it is too much I will understand but coming from the outside I need a starter here.
Alex

[AGarvin]

Hi Alex,

I wasn't quite sure what to say with such a large subject. I'm just an amateur astronomer too, I’ve just managed to accumulate papers, books and what not on this sorta stuff over the years. Hopefully I've got my facts correct (and not to may errors , it's been a while since I looked at this stuff). I don't know what your level of undertsanding is on stellar evolution so I thought a summary with some links would be a good start. I’ve used Wikipedia in most links as it was easier with everything in the one place.

A good start would be the Hertszprung-Russell diagram. In plots spectral type and luminosity class with temperature and luminosity. It also plots forming pre-main sequence T-Tauri and Herbig stars as they move down what are called the Hayashi and Henyey tracks, and older stars as they move off the main sequence along what are called the horizontal and asymptotic giant branches.

As for the actual fusion cycles; pre main sequence stars generally begin burning primordial deuterium (hydrogen-2) as they’re collapsing as deuterium fuses at a lower temperature than hydrogen-1. Along with the CNO cycle, the other main sequence cycle is the proton-proton cycle. Once this kicks in the star will be on the main sequence. There are four variations of this cycle, the PPI, PPII, PPIII and PPIV. Once a star run out of hydrogen and begins burning helium itmoves off the main sequence. The helium burns by the triple alpha process.

Stars smaller than about 8 solar masses go through a red giant stage and pretty much die at carbon ending up as white dwarfs. The smallest stars (red dwarfs) can theoretically end at the helium stage and leave behind a helium white dwarf, although I don’t think (?) the universe is old enough for any of these to have occurred yet. Larger stars become red supergiants and burn down through the elements; carbon and oxygen fuses with helium producing neon and magnesium down through silicon, sulphur and eventually iron, with each producing a burning shell around the core and feeding the shell below it. Since iron won’t fuse (unless energy is put into the system) the core ends up collapsing producing a supernova and leaving behind a neutron star, black hole or nothing at all. Elements heavier than iron are produced in the supernova explosion mainly by the neutron capture processes.

White dwarfs and neutron stars are supported from collapse by degeneracy pressure.

Hopefully this is the sort of thing you were after.

Andrew.

[xelasnave]

Hey Andrew that is fantastic. You have embarrassed me that I have put you to so much trouble but I thank you from the bottom of my heart.
If you ever have a legal or real estate related problem please pm me and I will try and repay the time you have put into your reply and I can assure you that although it will be provided as free advice others could present you with a stagering fee. Keep it in mind there is no time limit on my offer. I hope others will benefit from all you have outlined.I can see that tonight I will be as happy as I can expect following up on all you have covered. I think describing yourself as only an amateur astronomer takes away from your excellent understanding of matters I have seen you provide input.
You may notice in the thread on galaxy line up I have tracked that down I find those areas particularly interesting also ..the link I provided offers other very interesting links if you are interested.
Thank again and again.
alex

[PeteMo (Pete)]

Hi Andrew
Thanks for the detailed CNO explanation. It was only briefly mentioned on my course as the dominant process in stars with greater than 8 solar masses. I'm more familiar with the PPI and PPII (Proton - Proton) reactions as my course focused more on our Solar System.

The larger (more massive) the star the higher the core temperatures, the faster the nuclear processes convert lighter elements into heavier elements. I was quite surprised that some of the Super Giants might only be around for a few million years before boom day. Like the old saying goes "A candle that burns twice as bright, burns for half as long".

I was quite intrigued by the times for some of these processes take. Like Alex, I certainly appreciate you taking the time to explain this. Here's some of my notes about the Proton-Proton (PP) chains, so so we have a more comprehensive outline of the nuclear processes that take pace in stars.

The Proton-Proton PPI, PPII and PPIII chains are how Hydrogen is convereted to Helium in the the core of a star. A protostar must reach a mass of at least 0.08 Solar Masses before there will be enough matter and heat to kick off the first PPI chain reaction whereby 2 Hydrogen atoms fuse to create a Deuterium atom. If the mass is less than 0.08 Solar Masses, there is insufficient mass and heat for nuclear reactions to start, so it is a 'failed star' or Brown Dwarf. Makes you wonder about Jupiter being a potential Brown Dwarf.

Deuterium is an isotope of Hydrogen (like Carbon 12 and Carbon 14, or Uranium 235 and Uranium 238) having the same number of protons but a different number of neutrons. Whereas Hydrogen has an atomic weight of 1 (single proton in the nucleus), Deuterium has an atomic weight of 2 (single Proton and single Neutron in the nucleus). You can also get Tritium, another Hydrogen isotope with an atomic weight of 3 (1 Proton and 2 Neutrons in the nucleus).

My notes are a bit simplistic, but give the general outline of the PPI, PPII and PPIII chains. There are several permutations of how Hydrogen can be converted into Helium, hence the 3 PP chains. The PPI chain is the most important of these Proton-Proton chains.

e+ = positron
e- = electron
v = neutrino
y = gamma ray

H = Hydrogen
D = Deuterium
He = Helium (AKA Alpha particle)
Li = Lithium
Be = Beryllium
B = Boron

PPI Chain
H + H ==> D + e+ + v * takes 14,000,000,000 years
D + H ==> 3He + y * takes 6 seconds
3He + 3He ==> 4He + H + H * takes 1,000,000 years

PPII Chain
3He + 4He ==> 7Be + y
7Be + e- ==> 7Li + v (decays back to Lithium)
7Li + H ==> 4He + 4He

PPIII Chain
7Be + H ==> 8B + y
8B ==> 8Be + e+ + v (decays back to Beryllium)
8Be ==> 24He (decays back to Helium)

You will notice that after the second reaction in the PPI chain, the 3He created can either join with another 3He to form 4He (last step in the PPI chain), or, join with 4He (created in the last PPI step) to form 7Be (become the first step in PPII chain). If the temperature is above 14,000,000K there is enough heat for 3He to fuse with the 4He.

[DobDobDob (Ron)]

Great work Andrew, well done

[AGarvin]

No worries (and thanks). I hope folks get something out of it. It's refreshing my memory as well, I've forgotten heaps.

Your notes look spot on to me Pete. I listed four PP chains as when I was getting the links I noticed Wiki also listed He3 + H1. I've never seen that reaction listed in any doco on the PP chains before, but figured Wiki was more upto date. They do however say "never been observed".

Wiki also lists the pep reaction to produce deuterium (1H + e- + 1H > 2H). In this case, rather than one proton undergoing beta+ decay, a proton undergoes electron capture.

Andrew.

[xelasnave]

I am still working thru all this but its leaving me with more questions than when I started.
The good thing is I feel I am developing an understanding although I hav emany loose ends to sort out.
But it is a priveledge to be able to delve into this stuff. I find it so interesting even if I have not got it firmly under my belt yet..which I freely admitt.
alex

[ispom]

I found here

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

the same

or is that not the same???

do I can not appreciate the work of Andrew?

[PeteMo (Pete)]

Hi Andrew
I've never heard of the PP IV reaction until you mentioned the WIKI article on it. There's no mention of it in any of my reference books. It certainly looks like a simpler way to get a light Helium atom 3He (2 Protons + 1 Neutron) up to a full blown Helium atom 4He (2 Protons + 2 Neutrons).

Has the PP IV reaction been recently discovered? This may explain the omission from all the books I've got. The only other reason I can think of is that it has not been observed, so was deliberately omitted for that reason.