What Happens After a Star Fuses Hydrogen in Its Core?

[rawhemi]

I am writing a program which simulates the evolution of stars for my A2 computing project, and need help understanding what happens after the hydrogen in the core has been fused, because I have read a lot of contradictory information.

My understanding is:
For a low mass star (~1 SM):

Hydrogen fuses to Helium,
Helium cannot fuse - outward pressure stops,
Star shrinks,
Hydrogen ignites in shell outside core,
Star expands into Red giant, <Hydrogen shell burning, Dead Helium core>
Core heats up because of shell burning
Helium ignites, <Helium flash because of low mass>
Star settles back to ~ normal size (Helium core burning, Hydrogen shell burning)
Helium fuses to Carbon and Oxygen in core,
Carbon cannot fuse - outward pressure stops,
Star shrinks,
Helium ignites in shell outside core, (along with the Hydrogen shell)
Star expands into Red giant AGAIN, <Hydrogen shell burning, Helium shell burning, Dead Carbon core>
Core heats up, but not enough to start Carbon fusion,
Eventually hydrogen and helium shell fusion stop and the star cools,
Outer layers given off in nebula, leaving the dead Carbon, Oxygen core.

Basically, is this right? :D

Also, does a star HAVE to start fusing Iron in order to supernova? Can a star fuse everything up to just before Iron, and then simply end up as a white dwarf? Because I don't see how a star massive enough to fuse silicon (but not Iron) would not surpass the Chandrasekhar limit and end up as a neutron star, but with a planetary nebula. I have never heard about this happening, but maybe it is possible...

Anyway really sorry about the long question, any input is appreciated because I have really confused myself this time :)

Cheers,
Alex

 

[phyzguy]

People spend their careers trying to get these simulations right. Trying to simulate what you've outlined here would probably take years. You would need accurate simulation models for many things, including hundreds of nuclear fusion cross sections, radiative transport, convective transport, etc, etc. Perhaps it is too ambitious for a computer science project? A good project might be to try and model a radially symmetric star powered by hydrogen fusion. "Stellar Structure and Evolution" by Kippenhahn and Weigert is a great reference.

 

[rawhemi]

Yeh I know, but I'm not modelling it in that much depth - I'm simplifying lots and assuming things ie. the molecular cloud is 100% hydrogen, the star's luminosity = (mass^3.5 / mass of sun ^ 3.5) * luminosity of the sun, etc and so far it's pretty accurate. It's only for my physics teacher to help him teach GCSE students about stellar evolution.

 

[Chronos]

There has never been such a thing as a 100% hydrogen star. BB nucleosynthesis contributed about 25% helium and small amounts of other metals to the mix.

 

[SpiffyKavu]

Your project doesn't sound like a simulation. Of course, you could do a simulation, but you only need an animated visualization. You could get some information on wiki, maybe. A quick google search will get you what you need. You'll need relative sizes and the time spent at each phase. Books on stellar evolution from your local library should also contain this information. Wiki has pretty good sections on stellar evolution.

There are some interesting thermal pulses during the AGB part, due to the extreme temperature sensitivity of helium fusion (triple-alpha process). Your outline for a stellar mass star is essentially correct. There are some subtleties, but you can imagine the picture can get complicated really quick.

And a star must fuse elements through iron to go supernova. There are still unanswered questions with regards to mass loss, but it is thought that most stars of 8 solar masses or less will produce a carbon/oxygen white dwarf.

Beyond that mass, the core will produce carbon and oxygen (C/O) like normal. The the core of C/O will become larger than the Chandrasekhar mass. The electron degeneracy pressure holding up this core can no longer support the mass, so the core contracts again. It heats up and becomes more dense until carbon fuses.

This process repeats until it can no longer sustain itself; when iron is produced. The core never becomes degenerate again until iron. Fun fact, as iron is being produced, the photons (thermal, I think, from charged particle scattering, but don't quote me on that) floating around are energetic enough to break the iron nuclei apart, a process known as photodissociation. Iron does fuse a little bit, but it yields no energy, and the product decays almost immediately.

 

[rawhemi]

Ah! Thank you! I think I understand now - just read about carbon detonation, so I guess it's like when a white dwarf in binary collapses from mass transfer, but it happens inside the star. So the star doesn't even need to heat the core up enough if it's above ~1.4 SM, because whatever happens it's going to keep fusing anyway from being above the Chandrasekhar limit :D Cheers!