The chances of collision are low enough that there are few enough of them to fail to offset the now-missing original radiation pressure, which then allows a gravitational collapse to begin and progress.
What do you mean? Taking a guess at answering that, electrons and positrons aren't electrically neutral: they are very very strongly attracted to one another, especially compared to
> gravitational collapse
so the probability of electron-positron annihilation immediately after pair production is in general extremely high! (In lab settings you need strong magnetic traps to avoid that.)
The "trick" in the star's core is to remove the electrons locally, or alternatively to convert the mostly-elastic photon-nucleus scattering with a much more inelastic photon-nucleus scattering.
I've described the former in sibling comments -- oxygen and silicon are present in these stellar cores and aggressively capture electrons. The positrons then are pulled outwards by electrons outside the core, and annihilate there. I omitted that an electron-positron annihilation produces two (or more) gammas rather than one, and that the photons can go in (different) arbitrary directions.
The latter is also an important contributor. The gammas in question are not even close to being in free space. They're in a region densely populated by high-atomic-number nuclei, and the Z^2 contribution in https://en.wikipedia.org/wiki/Quantum_mechanical_scattering_... dominates. If the region were all lighter nuclei (hydrogen, helium) the probability of pair production would be much lower.
Roughly speaking, in the absence of immediate electron capture by the nucleus the pair-producing gamma "hits", the momentum of the gamma is split three ways: into each of the electron and positron, and into the nucleus. Electron capture is in effect just an extreme inelastic collision.
In the no-electron-capture case, the heavy nucleus, having absorbed the "recoil" proportion of the gamma into its internal degrees of freedom, has several ways to get rid of that momentum, re-emission of one or more photons with lower energy than the gamma, or transmutation (which might produce neutrinos).
If the electron and positron pair immediately annihilate, they do so minus the "recoil" energy to start with; additionally they produce more than one gamma, and in arbitrary directions. Consequently, there is less momentum available for subsequent elastic collisions.
What do you mean? Taking a guess at answering that, electrons and positrons aren't electrically neutral: they are very very strongly attracted to one another, especially compared to
> gravitational collapse
so the probability of electron-positron annihilation immediately after pair production is in general extremely high! (In lab settings you need strong magnetic traps to avoid that.)
The "trick" in the star's core is to remove the electrons locally, or alternatively to convert the mostly-elastic photon-nucleus scattering with a much more inelastic photon-nucleus scattering.
I've described the former in sibling comments -- oxygen and silicon are present in these stellar cores and aggressively capture electrons. The positrons then are pulled outwards by electrons outside the core, and annihilate there. I omitted that an electron-positron annihilation produces two (or more) gammas rather than one, and that the photons can go in (different) arbitrary directions.
The latter is also an important contributor. The gammas in question are not even close to being in free space. They're in a region densely populated by high-atomic-number nuclei, and the Z^2 contribution in https://en.wikipedia.org/wiki/Quantum_mechanical_scattering_... dominates. If the region were all lighter nuclei (hydrogen, helium) the probability of pair production would be much lower.
Roughly speaking, in the absence of immediate electron capture by the nucleus the pair-producing gamma "hits", the momentum of the gamma is split three ways: into each of the electron and positron, and into the nucleus. Electron capture is in effect just an extreme inelastic collision.
In the no-electron-capture case, the heavy nucleus, having absorbed the "recoil" proportion of the gamma into its internal degrees of freedom, has several ways to get rid of that momentum, re-emission of one or more photons with lower energy than the gamma, or transmutation (which might produce neutrinos).
If the electron and positron pair immediately annihilate, they do so minus the "recoil" energy to start with; additionally they produce more than one gamma, and in arbitrary directions. Consequently, there is less momentum available for subsequent elastic collisions.