Stars, fusion, how exactly the weak interaction acts ?

[binbagsss]

I am currently reading E=Mc^2 and have a quick question regarding some of the nuclear fusion they describe that takes place in stars:

When proton fusion occurs, one of the protons decays into a neutron + positron + neutrino – due to the weak interaction.
And the proton and neutron , acted together on by the strong nuclear force,+ the positron and neutrino formed have a lower mass than the intial 2 protons, releasing Energy.
So from this, I conclude that 4 protons are required to form 1He with 2 neutrons and 2 protons.

Then, in terms of the nuclear fusion which occurs in stars, the book says that:
2 HE fuse together to form BE-8 – 4 neutrons and 4 protons – this is where I do not follow.

From the above I would then assume that together 2HE would fuse into 2 protons + 2 neutrons (from the protons and due to the weak interaction), + another 4 neutrons, giving 6 neutrons, 2 proton rather than 4 protons and 4 neutrons.

So, my question is, how does the weak interaction act?
- could it be the case that 2 of the neutrons undergo decay themselves into a proton, electron + antineutrino
or
- could no quark flavour change occur , (so proton fusion occurs without any weak interaction interference – is this possible and so that only sometimes when proton fusion occurs, does the weak interaction interfere) – book states “ ...Not always, but some of the time when two protons come close together, a proton turns intself into a neutron..”

Thank a lot

 

[Astronuc]

pp reactions are a bit different than others which would involve composite nuclei, i.e., nuclei of more than one nucleon.

http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/procyc.html
http://burro.cwru.edu/Academics/Astr221/StarPhys/ppchain.html
http://zebu.uoregon.edu/~rayfrey/321/lecture5.pdf

The only possibility for p+p is a d with the transformation of one p to n (and emission of a positron) in order for a stable product. It is possible to have a p+n=> d + γ, which happens continually in LWRs.

When composite nuclei are involved, e.g., p+d, d+d, or d+t, etc, then the composite nucleus can decay by emission of a gamma ray, neutron or proton, or some composite nucleus.

See - http://csep10.phys.utk.edu/astr162/lect/energy/cno.html (note p,γ and p,α reactions, but also p,β⁺). β⁺ means that a proton transforms to a neutron.

α+α => Be-8 pretty much doesn't happen because Be-8 is unstable. Triple α (forming C-12) is possible under certain circumstances.
http://csep10.phys.utk.edu/astr162/lect/energy/triplealph.html

More likely, in a normal star, one might find He3 + He4 => Be7