Kicking off of this question, I have a short follow-up. The fusion of hydrogen into helium in the Sun's core requires the emission of two positrons per final helium nucleus, because we start off with four protons and end up with two protons and two neutrons confined inside the helium product, and the process therefore needs to get rid of two units of positive charge.
These positrons, of course, are quickly annihilated by two electrons from the environment, squaring the final balance in the books - we take four proton-electron pairs and produce a helium atom with only two electrons.
My question is what, exactly, "quickly" means in this situation. The process is probably instantaneous with respect to most physically relevant phenomena (including, for the purposes of the previous question, transport phenomena), but it will have some nonzero timescale (zeptoseconds? nanoseconds? hours?). So: what is the mean lifetime of one of these positrons? Does it depend on conditions such as the ambient temperature and pressure or the positron's energy? Does it change if we go from the Sun to stars of other masses or in other stages of development?
This question feels to me like it is pretty elementary given sufficient knowledge of solar physics, and that it must have been answered in the 1960s at the latest. Heuristic arguments are therefore reasonable as long as they justify their hypothesis, but I'm ideally looking for an answer explicitly rooted in solid solar physics.