carbon-nitrogen cycle

Overview of the CNO-I Cycle

The CNO cycle (for carbon–nitrogen–oxygen) is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars more massive than about 1.3 times the mass of the Sun. The proton–proton chain is more important in stars the mass of the Sun or less. This difference stems from temperature dependency differences between the two reactions; pp-chain reactions start occurring at temperatures around 4×10⁶ K,^[1] making it the dominant energy source in smaller stars. A self-maintaining CNO chain starts occurring at approximately 15×10⁶ K, but its energy output rises much more rapidly with increasing temperatures.^[2] At approximately 17×10⁶ K, the CNO cycle starts becoming the dominant source of energy.^[3] The Sun has a core temperature of around 15.7×10⁶ K and only 1.7% of 4
He nuclei being produced in the Sun are born in the CNO cycle. The CNO-I process was independently proposed by Carl von Weizsäcker^[4] and Hans Bethe^[5] in 1938 and 1939, respectively.

In the CNO cycle, four protons fuse, using carbon, nitrogen and oxygen isotopes as a catalyst, to produce one alpha particle, two positrons and two electron neutrinos. Although there are various paths and catalysts involved in the CNO cycles, simply speaking all these cycles have the same net result:

4 1
1H  →  4
2He  +  2 e+
 +  2 ν
e  +  3 γ  +  26.8 MeV

The 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. The carbon, nitrogen, and oxygen isotopes are in effect one nucleus that goes through a number of transformations in an endless loop.

Cold CNO Cycles

Under typical conditions found in stellar plasmas, catalytic hydrogen burning by the CNO cycles is limited by proton captures. Specifically, the timescale for beta-decay of radioactive nuclei produced is faster than the timescale for fusion. Because of the long timescales involved, the cold CNO cycles convert hydrogen to helium slowly, allowing them to power stars in quiescent equilibrium for many years.

CNO-I

The first proposed catalytic cycle for the conversion of hydrogen into helium was at first simply called the carbon–nitrogen cycle (CN cycle), also honorarily referred to as the Bethe–Weizsäcker cycle, because it does not involve a stable isotope of oxygen. Bethe's original calculations suggested the CN-cycle was the Sun's primary source of energy, owing to the belief at the time that the Sun's composition is 10% nitrogen;^[5] the solar abundance of nitrogen is now known to be less than half a percent. This cycle is now recognized as the first part of the larger CNO nuclear burning network. The main reactions of the CNO-I cycle are 12
6C→13
7N→13
6C→14
7N→15
8O→15
7N→12
6C:^[6]

12 6C	+	1 1H	→	13 7N	+	γ			+	1.95 MeV
13 7N			→	13 6C	+	e+	+	ν e	+	1.20 MeV	(half-life of 9.965 minutes)
13 6C	+	1 1H	→	14 7N	+	γ			+	7.54 MeV
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	1.73 MeV	(half-life of 122.24 seconds)
15 7N	+	1 1H	→	12 6C	+	4 2He			+	4.96 MeV

where the Carbon-12 nucleus used in the first reaction is regenerated in the last reaction. After the two positrons emitted annihilate with two ambient electrons producing an additional 2.04 MeV, the total energy released in one cycle is 26.73 MeV; it should be noted that in some texts, authors are erroneously including the positron annihilation energy in with the beta-decay Q-value and then neglecting the equal amount of energy released by annihilation, leading to possible confusion. All values are calculated with reference to the Atomic Mass Evaluation 2003.^[7]

The limiting (slowest) reaction in the CNO-I cycle is the proton capture on 14
7N; it was recently experimentally measured down to stellar energies, revising the calculated age of globular clusters by around 1 billion years.^[8]

The neutrinos emitted in beta decay will have a spectrum of energy ranges, because although momentum is conserved, the momentum can be shared in any way between the positron and neutrino, with either being emitted at rest and the other taking away the full energy, or anything in between, so long as all the energy from the Q-value is used. Because the mass of the electron and neutrino are much less than the mass of the daughter nucleus, for the precision of values given here, the recoil of the nucleus can be neglected. Thus the neutrino emitted during the decay of nitrogen-13 can have an energy from zero up to 1.20 MeV, and the neutrino emitted during the decay of oxygen-15 can have an energy from zero up to 1.73 MeV. On average, about 1.7 MeV of the total energy output is taken away by neutrinos for each loop of the cycle, leaving about 25 MeV available for producing luminosity.^[9]

CNO-II

In a minor branch of the reaction, occurring in the Sun's inner part, the core, just 0.04% of the time, the final reaction shown above does not produce carbon-12 and an alpha particle, but instead produces oxygen-16 and a photon and continues 15
7N→16
8O→17
9F→17
8O→14
7N→15
8O→15
7N:

15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)
17 8O	+	1 1H	→	14 7N	+	4 2He			+	1.19 MeV
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)

Like the carbon, nitrogen, and oxygen involved in the main branch, the fluorine produced in the minor branch is merely catalytic and at steady state, does not accumulate in the star.

CNO-III

This subdominant branch is significant only for massive stars. The reactions are started when one of the reactions in CNO-II results in fluorine-18 and gamma instead of nitrogen-14 and alpha, and continues 17
8O→18
9F→18
8O→15
7N→16
8O→17
9F→17
8O:

17 8O	+	1 1H	→	18 9F	+	γ			+	5.61 MeV
18 9F			→	18 8O	+	e+	+	ν e	+	1.656 MeV	(half-life of 109.771 minutes)
18 8O	+	1 1H	→	15 7N	+	4 2He			+	3.98 MeV
15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)

CNO-IV

Like the CNO-III, this branch is also only significant in massive stars. The reactions are started when one of the reactions in CNO-III results in fluorine-19 and gamma instead of nitrogen-15 and alpha, and continues 19
9F→16
8O→17
9F→17
8O→18
9F→18
8O→19
9F:

19 9F	+	1 1H	→	16 8O	+	4 2He			+	8.114 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F			→	17 8O	+	e+	+	ν e	+	2.76 MeV	(half-life of 64.49 seconds)
17 8O	+	1 1H	→	18 9F	+	γ			+	5.61 MeV
18 9F			→	18 8O	+	e+	+	ν e	+	1.656 MeV	(half-life of 109.771 minutes)
18 8O	+	1 1H	→	19 9F	+	γ			+	7.994 MeV

Hot CNO Cycles

Under conditions of higher temperature and pressure, such as those found in novae and x-ray bursts, the rate of proton captures exceeds the rate of beta-decay, pushing the burning to the proton drip line. The essential idea is that a radioactive species will capture a proton more quickly than it can beta decay, opening new nuclear burning pathways that are otherwise inaccessible. Because of the higher temperatures involved, these catalytic cycles are typically referred the hot CNO cycles; because the timescales are limited by beta decays instead of proton captures, they are also called the beta-limited CNO cycles.

HCNO-I

The difference between the CNO-I cycle and the HCNO-I cycle is that 13
7N captures a proton instead of decaying, leading to the total sequence 12
6C→13
7N→14
8O→14
7N→15
8O→15
7N→12
6C:

12 6C	+	1 1H	→	13 7N	+	γ			+	1.95 MeV
13 7N	+	1 1H	→	14 8O	+	γ			+	4.63 MeV
14 8O			→	14 7N	+	e+	+	ν e	+	5.14 MeV	(half-life of 70.641 seconds)
14 7N	+	1 1H	→	15 8O	+	γ			+	7.35 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)
15 7N	+	1 1H	→	12 6C	+	4 2He			+	4.96 MeV

HCNO-II

The notable difference between the CNO-II cycle and the HCNO-II cycle is that 17
9F captures a proton instead of decaying, and helium is produced in a subsequent reaction on 18
9F, leading to the total sequence 15
7N→16
8O→17
9F→18
10Ne→18
9F→15
8O→15
7N:

15 7N	+	1 1H	→	16 8O	+	γ			+	12.13 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F	+	1 1H	→	18 10Ne	+	γ			+	3.92 MeV
18 10Ne			→	18 9F	+	e+	+	ν e	+	4.44 MeV	(half-life of 1.672 seconds)
18 9F	+	1 1H	→	15 8O	+	4 2He			+	2.88 MeV
15 8O			→	15 7N	+	e+	+	ν e	+	2.75 MeV	(half-life of 122.24 seconds)

HCNO-III

An alternative to the HCNO-II cycle is that 18
9F captures a proton moving towards higher mass and using the same helium production mechanism as the CNO-IV cycle as 18
9F→19
10Ne→19
9F→16
8O→17
9F→18
10Ne→18
9F:

18 9F	+	1 1H	→	19 10Ne	+	γ			+	6.41 MeV
19 10Ne			→	19 9F	+	e+	+	ν e	+	17.22 MeV	(half-life of 122.24 seconds)
19 9F	+	1 1H	→	16 8O	+	4 2He			+	8.114 MeV
16 8O	+	1 1H	→	17 9F	+	γ			+	0.60 MeV
17 9F	+	1 1H	→	18 10Ne	+	γ			+	3.92 MeV
18 10Ne			→	18 9F	+	e+	+	ν e	+	4.44 MeV	(half-life of 1.672 seconds)

Use in astronomy

While the total number of "catalytic" CNO nuclei is conserved in the cycle, in stellar evolution the relative proportions of the nuclei are altered. When the cycle is run to equilibrium, the ratio of the carbon-12/carbon-13 nuclei is driven to 3.5, and nitrogen-14 becomes the most numerous nucleus, regardless of initial composition. During a star's evolution, convective mixing episodes bring material in which the CNO cycle has operated from the star's interior to the surface, altering the observed composition of the star. Red giant stars are observed to have lower carbon-12/carbon-13 and carbon-12/nitrogen-14 ratios than main sequence stars, which is considered to be convincing evidence for the operation of the CNO cycle.

The presence of the heavier elements carbon, nitrogen and oxygen places an upper bound of approximately 150 solar masses on the maximum size of massive stars^{[citation needed]}. It is thought^[who?] that the "metal-poor" early universe could have had stars, called Population III stars, up to 250 solar masses without interference from the CNO cycle at the beginning of their lifetime.

See also

• Triple-alpha process
• Proton–proton chain

References

Further reading

• Bethe, H. A. (1939). "Energy Production in Stars". Physical Review 55 (5): 434–56. Bibcode 1939PhRv...55..434B. doi:10.1103/PhysRev.55.434. 
• Iben, I. (1967). "Stellar Evolution Within and off the Main Sequence". Annual Reviews in Astronomy and Astrophysics 5: 571. Bibcode 1967ARA&A...5..571I. doi:10.1146/annurev.aa.05.090167.003035.