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.
Right, I don’t have time to give a full and detailled answer,
D’oh! And I obviously can’t see the difference between “submit” and “preview”, but this link shows how a star evolves in terms of luminosity and temperature through its life, essentially as its fuel changes. Hope this helps.
Very cool, thanks. I take it the “hydrogen flash (few years)” referred to is a fairly spectacular event?
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.
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.
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.
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 
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:
Ok, this I get. But the next paragraph confuses me.
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)?
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).
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?
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.
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!”
audiolover, how can you have read this post and not noticed that Angua is a woman?
Ryle Dupe, how can you have read this post and not noticed my name is audilover, not audiolover?
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.
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.
Right, OK. I am sorry for taking things the wrong way. Can we drop that now please, and get to the science?
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.
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.
Back to the science…
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’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.
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.
Please stick to the General Question. Failure to do so may invite a formal warning.
Thank you.
-xash
General Questions Moderator