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A question about helium flash


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Quick bit of context, I'm studying astronomy and planetary science with the Open University.

I've just covered a brief secion that explains helium flash. I understand why the process happens (degenerate gas, pressure unaffected by the temp until >>) and that most of the He will be consumed in a short timescale, however what I fail to see is that:

a) If the energy released by the core is absorbed by the core, due to the opacity of the star, which then raises the core temp sufficiently to change the state of the gas to a non degenerate state, stabilizing the core, then how does this differ to stars >2.25M (solar) which begin He fusion before the core becomes degenerate (obviously the onset of He fusion is different, but what other difference does it make to the star, such as will the He fusion stage be significantly shorter as most of the He is depleted, even though the energy released is absorbed by the core)

B) If there is no outward indication of helium flash, other than theoretically, how do we know this process takes place?

Just so you know, this is not related to any question; assignment, exam or otherwise. The topic was covered very briefly, and after having read a few articles on the subject, I still have these questions.

Thanks in advance!

Ryan

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a) If the energy released by the core is absorbed by the core, due to the opacity of the star, which then raises the core temp sufficiently to change the state of the gas to a non degenerate state, stabilizing the core, then how does this differ to stars >2.25M (solar) which begin He fusion before the core becomes degenerate (obviously the onset of He fusion is different, but what other difference does it make to the star, such as will the He fusion stage be significantly shorter as most of the He is depleted, even though the energy released is absorbed by the core)

B) If there is no outward indication of helium flash, other than theoretically, how do we know this process takes place?

Just so you know, this is not related to any question; assignment, exam or otherwise. The topic was covered very briefly, and after having read a few articles on the subject, I still have these questions.

I believe it's all theoretically calculated using models of solar fusion. There are some quite sophisticated models that can be run, that take account of all manner of phenomena.

For larger stars, they burn hotter anyway, so they reach the required He fusion temperature (~108K) before matter is crushed to degeneracy. So you dont get the thermonuclear SN1a like explosion.

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I believe it's all theoretically calculated using models of solar fusion. There are some quite sophisticated models that can be run, that take account of all manner of phenomena.

For larger stars, they burn hotter anyway, so they reach the required He fusion temperature (~108K) before matter is crushed to degeneracy. So you dont get the thermonuclear SN1a like explosion.

Thanks for the reply. That's kind of what I was getting at, I know the why and the how, but other than through modelling, is there any way to tell if helium flash has occured, and how would it affect the star (eg lifetime, size etc)

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Have you tried posting the question on your course forum on the OU web site? Maybe someone else on the course or even one of your tutors can explain it to you.

I don't really use the OU forums, I was put off on earlier modules where they were a waste of time. I have submitted my question there now though, and I'll see what happens. :smiley:

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Thanks for the reply. That's kind of what I was getting at, I know the why and the how, but other than through modelling, is there any way to tell if helium flash has occured, and how would it affect the star (eg lifetime, size etc)

Helium flashes will be rare events (low mass stars live a long time), and they only last a few seconds, so they would be hard to observe even if they were significantly visible. 

There are He shell flashes too - not sure if they are more visible.

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