Dark Energy

[AGarvin]

Hi Pete,

No problems with the questions. If I can answer I'll will, if not .... I'll stay out of it.

The onset of the CNO cycle is related to the level of impurities in the star (ie, heavy elements) and temperature. I think (?) it's at around 2 solar masses that the CNO cycle begins to dominate in main sequence stars. It plays no role in a white dwarf as a white dwarf is a dead star. It's the left over core of a star after it's red giant stage. There is no fusion going on in the star and they're held up from collapse by what is call electron degeneracy pressure (a quantum thing based on the Pauli exclusion principle). It's generally believed that stars below 8 solar masses will end up as white dwarfs. Basically they're not big enough to produce enough pressure and heat to allow fusion beyond carbon. The core ends up a white dwarf and the the outer layers just slowly blow away and leave behind a wonderful planetary nebula for us to observe. The Sun will end up a white dwarf at the end of its life. Stars above 8 solar mass are believed to end up going supernova leaving behind a neutron star or black hole. They are large enough to allow fusion beyond carbon. When they hit iron fusion stops as energy has to be supplied to fuse beyond iron. Splitting atoms (fission) beyond iron is what produces energy, eg, atomic bomb.

Cheers,
Andrew.

[PeteMo (Pete)]

Hi Andrew
Thanks for that. According to my old notes we had CNO Fusion as the dominant process in Main Sequence stars >8 Solar Masses, and the elements heavier than Iron produced by Type II SN. I was quite surprised at how small a white dwarf was considering the amount of matter crammed into it can be up to 1.5 Solar Masses.

One thing that still puzzles me is how to determine whether a Type II SN will become a Black Hole (like the Type I 30-40 solar masses) or a Neutron Star. This is one thing I never fully understood.

I know a lot can happen in a decade, so appreciate your explanations.

Pete

[AGarvin]

According to my old notes we had CNO Fusion as the dominant process in Main Sequence stars >8 Solar Masses, and the elements heavier than Iron produced by Type II SN.

Now you've got me curious. I'll do a bit if research and see if I can clarify when the CNO cycle becomes dominant in main sequence stars. Certainly the heavy elements come from Type II SN and Type Ib/Ic SN.

One thing that still puzzles me is how to determine whether a Type II SN will become a Black Hole (like the Type I 30-40 solar masses) or a Neutron Star. This is one thing I never fully understood.

I don't think anyone knows or can predict this that I'm aware of. From what I understand it's also entirely possible that neither is left and that the entire star blows up, core and all.

We seem to have hijacked this thread away from its topic (sorry Alex).

Andrew.

[xelasnave]

Andrew and Pete,
there is no need to be sorry you are covering very interesting stuff that many will love to follow, I urge you both to continue. Maybe the outpouring of energy contributes to dark energy.. have you thought of how a star blasting itself apart may help form a pressure in space for example. Just a thought to keep you guys rolling. Thank you both for having a discussion on these matters as I feel it is indeed relevant to the matter at hand and if its not who cares its neat stuff.
alex

[DobDobDob (Ron)]

Digging up my 8 year old notes, I find this interesting summary of the CNO cycle.

For the CNO cycle to proceed, there must be 12C nucli present. While this is the most common isotope of carbon, it makes up only a tiny fraction of a star's composition. Not much is needed, however. Furthermore, the 12C nucleus is restored at the end of the cycle, so the process does not use up any carbon.