"The bottom line is that this amount of material undergoing fusion, can only create a tiny fraction of the heat energy of the Sun."
At the risk of being a nuisance, I still don't agree. How do you explain that Kelvin–Helmholtz contraction can only sustain the sun at its present luminosity for about 15 million years? See http://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_mechanism.
There are two things here that I think may be at issue: temperature and energy output. Gravitational collapse is probably responsible (together with the opacity you mention) for maintaining the high temperature in the sun's core but I am sure that the energy output is due to fusion and not gravitational energy release.
Gosh, aint it interesting! I am sure Chris will have a comment or two soon.
I showed this to a someone currently still working in astronomy at university and this was their reaction:-
"'The gravitational collapse is balanced by the pressure created by the thermonuclear reactions in the stars core'. Quite - so there is no more collapse so you cannot extract any further gravitational energy."
I am not sure that I can eloquently explain the process nuances, with my limited knowledge on these matters, currently, other than to paraphrase what I have read.
So, I will quote from the set book, which will probably cause much debate, and I will seek some advice from my course tutor, who may be able to offer another view:
"You might be tempted at this stage to think that, even though the power released in fusion is very small, the energy released heats up the core of a star, and this is why stars are hot. The energy released is indeed important, but it is not the reason that stars heat up. In fact the opposite is true: fusion prevents a star from getting hotter!
In fact, a star's temperature and luminosity are not determined by nuclear burning, but they are maintained by nuclear burning. Nuclear fusion replenishes the slow leakage of energy from the star's core and eventually from its surface [...] If someone had asked you at the beginning of this book, 'what makes stars hot?', you could have been forgiven for answering 'It is because they have thermonuclear reactions in their cores.' That answer sounds plausible, but hopefully now you can see that it is incorrect. Stars are hot because they have collapsed from large diffuse clouds, and gravitational potential energy has been converted to kinetic energy. That is why stars are hot." Ryan, S.G, Norton, A.J, 2010. Stellar Evolution and Nucleosynthesis Cambridge: The Open University.
Yes, I can see the point being made but I think it can be misleading. It can lead you to think that the star's energy output is derived from the release of the star's potential energy which is not so in equilibrium. So, as I said before, gravitational collapse gets the core of the new star up to the right temperature for nuclear burning to start. Then when fusion has started an equilibrium is reached and no more potential energy is released, instead this comes from nuclear burning. It's all a bit chicken and egg!
"The bottom line is that this amount of material undergoing fusion, can only create a tiny fraction of the heat energy of the Sun."
ReplyDeleteAt the risk of being a nuisance, I still don't agree. How do you explain that Kelvin–Helmholtz contraction can only sustain the sun at its present luminosity for about 15 million years? See http://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_mechanism.
There are two things here that I think may be at issue: temperature and energy output. Gravitational collapse is probably responsible (together with the opacity you mention) for maintaining the high temperature in the sun's core but I am sure that the energy output is due to fusion and not gravitational energy release.
Gosh, aint it interesting! I am sure Chris will have a comment or two soon.
I showed this to a someone currently still working in astronomy at university and this was their reaction:-
ReplyDelete"'The gravitational collapse is balanced by the pressure created by the thermonuclear reactions in the stars core'. Quite - so there is no more collapse so you cannot extract any further gravitational energy."
It is great fun to explore this issue.
ReplyDeleteI am not sure that I can eloquently explain the process nuances, with my limited knowledge on these matters, currently, other than to paraphrase what I have read.
So, I will quote from the set book, which will probably cause much debate, and I will seek some advice from my course tutor, who may be able to offer another view:
"You might be tempted at this stage to think that, even though the power released in fusion is very small, the energy released heats up the core of a star, and this is why stars are hot. The energy released is indeed important, but it is not the reason that stars heat up. In fact the opposite is true: fusion prevents a star from getting hotter!
In fact, a star's temperature and luminosity are not determined by nuclear burning, but they are maintained by nuclear burning. Nuclear fusion replenishes the slow leakage of energy from the star's core and eventually from its surface [...] If someone had asked you at the beginning of this book, 'what makes stars hot?', you could have been forgiven for answering 'It is because they have thermonuclear reactions in their cores.' That answer sounds plausible, but hopefully now you can see that it is incorrect. Stars are hot because they have collapsed from large diffuse clouds, and gravitational potential energy has been converted to kinetic energy. That is why stars are hot." Ryan, S.G, Norton, A.J, 2010. Stellar Evolution and Nucleosynthesis Cambridge: The Open University.
Yes, I can see the point being made but I think it can be misleading. It can lead you to think that the star's energy output is derived from the release of the star's potential energy which is not so in equilibrium. So, as I said before, gravitational collapse gets the core of the new star up to the right temperature for nuclear burning to start. Then when fusion has started an equilibrium is reached and no more potential energy is released, instead this comes from nuclear burning. It's all a bit chicken and egg!
Delete