# Why Don't Stars Burn All Fuel Instantly?
**URL:** https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653
**Category:** Factual Questions
**Created:** [November 22, 2003, 8:53am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653 "2003-11-22T08:53:04Z")
**Posts on this page:** 7
**Page:** 3
### Author:  [@Akatsukami](https://boards.straightdope.com/u/Akatsukami)
#### Post date: [December 1, 2003, 6:56pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/41 "2003-12-01T18:56:22Z")
> [@](#):
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> \*Originally posted by Angua \*
> **This reaction is energetically viable in stars with a mass greater than 1.1 times the solar mass, and with temperatures greater than 1.6x10[sup]7[/sup]K. As a reference, the sun’s temperature is about 6000K.**
Surely this conflates core and surface temperatures?
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [December 1, 2003, 7:00pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/42 "2003-12-01T19:00:20Z")
> [@](#):
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> \*Originally posted by Akatsukami \*
> \*\*Surely this conflates core and surface temperatures? \*\*
Yes, sorry. The higher temperature is a core temperature for a star. The 6000K is the surface temperature of the sun. The core temperature of the sun is 1.55x10[sup]7[/sup]K. Obviously 6000K is nowhere near enough to allow H fusion to occur.
However, the conclusion that the CNO cycle is only energetically viable for stars that are more massive and hotter than the sun still stands.
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### Author:  [@Rusalka](https://boards.straightdope.com/u/Rusalka)
#### Post date: [December 1, 2003, 8:28pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/43 "2003-12-01T20:28:47Z")
Angua - hope you see this. What does a professional astrophysicist do? I didn’t realize you could do that as a profession with a Master’s degree the way you become a lawyer or architect. I always assumed the only jobs for astrophysicists where astrophysicists actually do astrophysics required a doctorate and were research positions. Obviously I was wrong. The practice of astrophysics? Please enlighten - it sounds exciting.
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### Author:  [@Polycarp](https://boards.straightdope.com/u/Polycarp)
#### Post date: [December 1, 2003, 8:30pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/44 "2003-12-01T20:30:51Z")
**Angua** – Thank you very much for the explanations.
**Xash** or another Mod. – I presume the “Mod. Hat” comment was related to the ongoing discussion of the (apparently unintentional) slur on **Angua** ’s credentials, not to the “That brings to mind a related question” posts by myself and others, which is a longstanding tradition in GQ. Correct?
**Jinx** – To quickly clarify a question you posed in (b) in your OP, which **Angua** passed over briefly in discussing helium “burning,” the reason there is such a significant gap between hydrogen fusion and helium fusion is this: When two helium-4 nuclei fuse, they produce beryllium-8, which fissions on a microsecond time scale into two alpha particles (i.e., helium-4 nuclei), leaving you where you started. So to get to a functional helium fusion, you need temperatures and pressures that cause fusion of Be-8 and He-4 to form C-12 before the Be-8 breaks down again. Lithium and Beryllium (and I believe Boron) nuclides that are stable at STP are not stable at solar-core temperatures and pressures; they are produced predominantly in supernovae, IIRC, where they are flung out of the temperature/pressure regime necessary to create them before they have time to break down. – which accounts for their relative rareness.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [December 1, 2003, 9:06pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/45 "2003-12-01T21:06:32Z")
> [@](#):
>
> \*Originally posted by Rusalka \*
> \*\*Angua - hope you see this. What does a professional astrophysicist do? I didn’t realize you could do that as a profession with a Master’s degree the way you become a lawyer or architect. I always assumed the only jobs for astrophysicists where astrophysicists actually do astrophysics required a doctorate and were research positions. Obviously I was wrong. The practice of astrophysics? Please enlighten - it sounds exciting. \*\*
The only jobs for astrophysicists which require astrophysics are research positions. However, I’m currently doing my doctorate, but, here in the UK, its essentially a junior research position, and one is essentially a professional astrophysicist. One works under a permanent member of staff, doing one’s own research, with guidance from your supervisor (advisor).
**Polycarp** - no problem. I just hope that they were useful and not too technical.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [December 1, 2003, 9:08pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/46 "2003-12-01T21:08:22Z")
> [@](#):
>
> However, the conclusion that the CNO cycle is only energetically viable for stars that are more massive and hotter than the sun still stands.
I should remember this, but it’s been a long time since my last astrophysics class… I note that while the threshhold for the carbon cycle is larger and hotter than the Sun, it’s not much larger and hotter. Is it safe to say, then, that the C cycle is nonnegligible in the Sun, even if it’s not dominant? Or does the C cycle turn on abruptly at the threshhold?
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [December 1, 2003, 9:30pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/47 "2003-12-01T21:30:57Z")
> [@](#):
>
> \*Originally posted by Chronos \*
> \*\*I should remember this, but it’s been a long time since my last astrophysics class… I note that while the threshhold for the carbon cycle is larger and hotter than the Sun, it’s not much larger and hotter. Is it safe to say, then, that the C cycle is nonnegligible in the Sun, even if it’s not dominant? Or does the C cycle turn on abruptly at the threshhold? \*\*
I’m not entirely certain. However, thinking about it, we have a C[sup]12[/sup] nucleus fusing with a H nucleus. Since the electrical repulsive force is stronger between the C[sup]12[/sup] nucleus and the H nucleus than simply between 2 H nuclei, then presumably higher energies are required for the H nucleus to get close enough to fuse with the C[sup]12[/sup] nucleus than for two H nuclei to fuse. Hence one expects the pp process to dominate. Which it does - most of our Sun’s He comes from pp fusion, with only about 1% coming from the CNO cycle.
So, whilst we do have _some_ CNO fusion occuring in the Sun, its not dominant. I think this is because of the different temperature dependancies of the process. According to my trusty copy of Universe, the CNO cycle is proportional to temperature to the seventeenth power, whereas pp fusion is proportional to temperature to the fourth power. Which is why even though the threshold for CNO fusion is not too far off the mass and temperature of our sun, it is not even remotely dominant.
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