# 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:** 20
**Page:** 2
### Author:  [@BlackNGold](https://boards.straightdope.com/u/BlackNGold)
#### Post date: [November 23, 2003, 11:39am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/21 "2003-11-23T11:39:48Z")
> [@](#):
>
> \*Originally posted by Achernar \*
> \*\*Theoretically, Iron (Fe) is the limit, but stars like the sun never make it past Carbon/Oxygen. More massive stars get up to Fe, again skipping a few steps. I think it goes C, Ne, O, Si, Fe. I think there might be a couple of intermediate elements too, like Mg and Ni. \*\*
I thought I remembered that Fe was the upper limit. Could someone explain (or point to a webiste) the changes in appearance of a star based on each primary fuel?
I’d like to think that Ne stars are wildly colored and flicker on and off, but I’m guessing not.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [November 23, 2003, 11:44am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/22 "2003-11-23T11:44:29Z")
Right, I don’t have time to give a full and detailled answer,
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [November 23, 2003, 11:45am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/23 "2003-11-23T11:45:47Z")
D’oh! And I obviously can’t see the difference between “submit” and “preview”, but [this link](http://hyperphysics.phy-astr.gsu.edu/hbase/astro/herrus.html#c4) shows how a star evolves in terms of luminosity and temperature through its life, essentially as its fuel changes. Hope this helps.
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### Author:  [@BlackNGold](https://boards.straightdope.com/u/BlackNGold)
#### Post date: [November 23, 2003, 2:50pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/24 "2003-11-23T14:50:52Z")
Very cool, thanks. I take it the “hydrogen flash (few years)” referred to is a fairly spectacular event?
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### Author:  [@ianzin](https://boards.straightdope.com/u/ianzin)
#### Post date: [November 23, 2003, 3:56pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/25 "2003-11-23T15:56:01Z")
Interesting question, and agreat answer from you **Angua.** I love it when people who know their stuff are on hand to deliver the Straight Dope like this.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [November 23, 2003, 4:10pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/26 "2003-11-23T16:10:59Z")
> [@](#):
>
> \*Originally posted by Achernar \*
> \*\*Yeah, it skips a few steps. As **Angua** said, it goes from H to He in proton-proton and CNO. Then it skips all the way from He to C in triple-alpha. And then sometimes you can have He + C -\> O.
>
> Theoretically, Iron (Fe) is the limit, but stars like the sun never make it past Carbon/Oxygen. More massive stars get up to Fe, again skipping a few steps. I think it goes C, Ne, O, Si, Fe. I think there might be a couple of intermediate elements too, like Mg and Ni. \*\*
Yes, H -\> He is proton/proton fusion. Then, He is converted to C via the triple alpha process. Following that, low mass stars, will undergo a phase where some C atoms will fuse with a fourth He to produce O (as **Achernar** describes above.)
In low mass stars, fusion will stop at this point. In heaviest stars (with masses greater than 4 solar masses), the core is composed primarily of C and O, and as the temperature rises, we get C burning. This is followed by a similar ‘flash’ as in the H-\>He burning transition, and neon burning begins, and increases the concentration of O and Mg. Once this is over, the core again contracts, and O burning begins, which gives us sulphur. The core contracts again once this is over, and Si burning begins which produces elements upto Fe.
Anything above Fe is not energetically viable to be produced via fusion, and are produced when high mass stars go supernova.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [November 23, 2003, 4:51pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/27 "2003-11-23T16:51:40Z")
> [@](#):
>
> \*Originally posted by BlackNGold \*
> \*\*Very cool, thanks. I take it the “hydrogen flash (few years)” referred to is a fairly spectacular event? \*\*
I’m not certain what you mean by this.
If you mean the point at which H core burning stops, and H shell burning starts, then, yes, it will be fairly spectacular - the luminosity of the sun will increase to about 1000 times the current solar luminosity.
If you mean the helium flash which initiates He burning in low mass red giant stars, then again, this will be a fairly explosive event. Its not a sudden flash of light - that would be incredible, but instead for a brief period of time, the He burning core is about 10[sup]11[/sup] times brighter than the current luminosity of the sun. However, there are no visible consequences, because firstly most of the energy released in this process goes into heating the core, as opposed to emitting visible light, and secondly, the photons that do escape the core cannot penetrate the star’s dense outer layers.
Does that clear things up?
Yikes. You can tell I teach, with all the review questions after my explanations
Oh, and thank you **ianzin** , I try my best.
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### Author:  [@Jinx](https://boards.straightdope.com/u/Jinx)
#### Post date: [November 29, 2003, 2:01am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/28 "2003-11-29T02:01:21Z")
I follow the gist of the answer to my question, and then got lost in the shuffle of the banter. Did anyone mention that stars cool down as they age? Aren’t younger stars hotter than middle aged stars? To the best of my knowledge this is true examining the H-R diagram and spectral classes.
If this is correct, then how can the core be heating up as the star ages? - Jinx 😕
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### Author:  [@lightingtool](https://boards.straightdope.com/u/lightingtool)
#### Post date: [November 29, 2003, 2:47am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/29 "2003-11-29T02:47:03Z")
Sorry to go back to the first response (by the lovely **Angua** ), but there’s something in there that I don’t get. Here are some snipped quotes to help make my question clear:
> [@](#):
>
> As the number of atomic nuclei in a star’s core decreases with time (because it takes 4 H atoms to make one He atom), the pressure in the star’s core decreases, so gravity allows the star to collapse, which makes the star hotter.
Ok, this I get. But the next paragraph confuses me.
> [@](#):
>
> This shell burning heats up the core, and causes the core to contract.
How does the shell burning heat cause the core to contract? I would think the core would want to expand. Does the higher temp allow for more He to be formed? If so, why doesn’t it get out of control (or is it out of control, but on a scale that I don’t see)?
> [@](#):
>
> Because He is heavier than H, the He formed in the shells drops down to the core, which makes the core heavier, which causes the core to contract more.
Also, why does the weight of the core cause it to contract? Is it simply a gravity thing that I’m not grasping?
Thanks in advance for helping a guy who works in theatre understand how stars work (I love this place).
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### Author:  [@Polycarp](https://boards.straightdope.com/u/Polycarp)
#### Post date: [November 29, 2003, 3:08am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/30 "2003-11-29T03:08:23Z")
Couple of questions, **Angua** :
I remember years ago reading that for some stars, the “carbon cycle” (which transforms four H to a He via a carbon \> nitrogen \> oxygen \> carbon-plus-alpha-particle) reaction predominates over the basic hydrogen \> deuterium \> tritium or tralphium \> helium[sub]4[/sub] reaction. What proportion of stars do this to any significant extent, and what kind of star has this as the dominant reaction?
I also recall reading that it’s only a truly brief time (minutes or hours) when silicon \> iron is the core reaction, before supernova or collapse status sets in. Can you clue us in to relative time frames for fusion of the elements in the hydrogen \> helium \> carbon \>oxygen \>neon … \> Silicon \> iron sequence? (I.e., how long will a star “burn” helium, once it starts to, before it moves on to the next stage?
Finally, and probably connected with the last question, what’s the deal with Wolf Rayet stars? I know they have extremely high surface temperatures, and that’s connected with core temperatures hot enough to burn helium (which normally produces giants)? What are they, and **why** are they?
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### Author:  [@aerodave](https://boards.straightdope.com/u/aerodave)
#### Post date: [November 29, 2003, 3:29am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/31 "2003-11-29T03:29:56Z")
> [@](#):
>
> \*Originally posted by TheLoadedDog \*
> \*\*Blimey. What on earth has **Angua** done beyond
> providing a detailed reply, and then politely rebutting a silly and somewhat rude next post? \*\*
His rebuttal was not polite, nor was the post in question rude. It would appear that you’ve gotten the two mixed up.
I thought the “daughter” remark was quite obviously the talk of a proud father. I saw nothing meant to slight **Angua**.
And Angua’s reply diturbed me greatly. Far too defensive about professional and educational credentials. Anytime you have to “whip out” your diplomas, it’s taken by most to be a sign of great insecurity. No one was questioning Angua’s professional credibility, but it would appear that he was looking pretty hard for it.
That said, I found his original answer very enlightening, particularly because I have a great amateur interest in astronomy and astrophysics. But his hostility left a bad taste in my mouth. I can only hope I don’t need his expertise anytime soon.
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### Author:  [@Naive\_Theory](https://boards.straightdope.com/u/Naive_Theory)
#### Post date: [November 29, 2003, 6:55am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/32 "2003-11-29T06:55:55Z")
> [@](#):
>
> \*Originally posted by TheLoadedDog \*
> \*\*Blimey. What on earth has **Angua** done beyond
> providing a detailed reply, and then politely rebutting a silly and somewhat rude next post? \*\*
To clarify: What Odinoneeye meant to say was (approximately)
“My daughter talks about stars and such all the time. Of course, she doesn’t go into such detail, but she does know the order of stellar life!”
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### Author:  [@Ryle\_Dup](https://boards.straightdope.com/u/Ryle_Dup)
#### Post date: [November 29, 2003, 7:05am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/33 "2003-11-29T07:05:11Z")
audiolover, how can you have read this post and not noticed that Angua is a _woman_?
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### Author:  [@aerodave](https://boards.straightdope.com/u/aerodave)
#### Post date: [November 29, 2003, 2:52pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/34 "2003-11-29T14:52:45Z")
Ryle Dupe, how can you have read this post and not noticed my name is audilover, not audi_o_lover?
And I saw no indication that she was a female. The flirting comments above aren’t telling either way. And besides, I used the neutral “he,” which is perfectly proper grammar when you don’t know—or in this case, don’t care—the gender of the subject.
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### Author:  [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)
#### Post date: [November 29, 2003, 3:48pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/35 "2003-11-29T15:48:55Z")
I’m an astronomy student too, almost finished my degree. The proud science father was being just that-- proud that his daughter could give almost the same explanation as a professional astronomer. I think that’s damn cool, because you and I were probably like that as kids too. You’re not the only one in the world who is smart, and I saw nothing in what he said to say that you were _wrong_, only pride that his daughter was _right_.
Move over and give other people some room. Nobody’s threatening your livelihood, and I think you need to take your little piece of paper less seriously.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [November 30, 2003, 10:44pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/36 "2003-11-30T22:44:11Z")
Right, OK. I am sorry for taking things the wrong way. Can we drop that now please, and get to the science?
> [@](#):
>
> You’re not the only one in the world who is smart, and I saw nothing in what he said to say that you were wrong, only pride that his daughter was right.
>
> Move over and give other people some room. Nobody’s threatening your livelihood, and I think you need to take your little piece of paper less seriously.
Fine. I never said I was the only one in the world who is smart. I know that he was proud that his daughter was right, however, I have said before, that in a fit of insanity, I mis-interpreted, and took **odinoneeye** to be saying with the “_but_ my daughter got the order of evolution right” (emphasis mine) part of his post, that I had somehow got it wrong.
I’m too sleep addled from the LonDope to answer the science right now, but, I will address the points tomorrow morning, unless someone else wants to.
**Audilover** , I am sorry if I appeared to be hostile, it was not meant that way at all. And maybe I am insecure, but that’s because I’ve had to work incredibly hard to get to where I am now - astronomy is _still_ a very male dominated world, and believe me, it is hard for a woman in this field. I am sorry if I appeared insecure, like I said, I mis-interpreted **Odinoneeye’s** comment, and I apologise sincerely for that mis-interpretation, and my subsequent over-reaction.
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### Author:  [@TheLoadedDog](https://boards.straightdope.com/u/TheLoadedDog)
#### Post date: [November 30, 2003, 10:54pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/37 "2003-11-30T22:54:57Z")
**audiolover** , and **Naive Theory** , I’m sorry but I don’t buy what you are saying. Not unless “you sound like an eleven year-old” is considered anything less than rude in your circles. Anyhow, 'nuff said. This ain’t the Pit.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [December 1, 2003, 10:58am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/38 "2003-12-01T10:58:58Z")
Back to the science…
> [@](#):
>
> How does the shell burning heat cause the core to contract? I would think the core would want to expand. Does the higher temp allow for more He to be formed? If so, why doesn’t it get out of control (or is it out of control, but on a scale that I don’t see)?
So, in H shell burning, there are no thermonuclear fusion processes going on in the core, but there are in the shell of H immediately surrounding the core. There is no heat being generated in the core. So, the core cools, and the pressure in the core decreases. So, the core contracts under the weight of the outer layers. This increases the core’s temperature, and so heat flows out from the core again towards the shell layers. This warms the gases around the core, and increases the rate of shell H burning. He being heavier than H falls into the core, which continues to contract and heat up as it gains mass. Basically, the reason the core heats up here is because of gravtiational energy being converted into heat energy. There is a catastrophic sequence essentially, but because of the timescales involved, we don’t see it.
> [@](#):
>
> I remember years ago reading that for some stars, the “carbon cycle” (which transforms four H to a He via a carbon \> nitrogen \> oxygen \> carbon-plus-alpha-particle) reaction predominates over the basic hydrogen \> deuterium \> tritium or tralphium \> helium4 reaction. What proportion of stars do this to any significant extent, and what kind of star has this as the dominant reaction?
I’m not sure about this at all. I will try to look it up and post an answer asap. Same with Wolf-Rayet stars.
As for timescales, for a large mass star, which is the only type which gets past He burning, they are:
H burning - 7x10[sup]6[/sup]years
He burning - 7x10[sup]5[/sup]years
C burning - 600 years
Ne burning - 1 year
O burning - 6 months
Si burning - 1 day
Core collapse - 1/4 second
Core bounce - milliseconds
Explosion (supernova) - 10 seconds.
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### Author:  [@Angua](https://boards.straightdope.com/u/Angua)
#### Post date: [December 1, 2003, 11:55am UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/39 "2003-12-01T11:55:56Z")
> [@](#):
>
> I remember years ago reading that for some stars, the “carbon cycle” (which transforms four H to a He via a carbon \> nitrogen \> oxygen \> carbon-plus-alpha-particle) reaction predominates over the basic hydrogen \> deuterium \> tritium or tralphium \> helium4 reaction. What proportion of stars do this to any significant extent, and what kind of star has this as the dominant reaction?
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.
**Wolf-Rayet stars**
A Wolf-Rayet star is a hot (25,000-50,000K), massive (M\>20M\_solar) star which has a high rate of mass loss due to strong stellar winds. These stellar winds essentially pull off the hydrogen “shell” or envelope, leaving the helium core exposed. They are often found as the more evolved member of a binary pair of stars, which leads us to the idea that probably the gravitational attraction between the two stars has something to do with forming the stellar winds. However, we also see Wolf-Rayet stars as single stars, so there’s probably something to do with radiation pressure as well. In short, we have very little idea as to what causes WR stars.
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### Author:  [@xash](https://boards.straightdope.com/u/xash)
#### Post date: [December 1, 2003, 6:27pm UTC](https://boards.straightdope.com/t/why-dont-stars-burn-all-fuel-instantly/214653/40 "2003-12-01T18:27:17Z")
Please stick to the General Question. Failure to do so may invite a formal warning.
Thank you.
- **xash**
General Questions Moderator
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