[raattgift]

The electron half of a pair produced very centrally within a star is very likely to be captured very quickly into a nuclear reaction.

This can produce central regions of pair-producing-but-election-capturing fusion, and a substantial excess of positrons. There are plenty of electrons away from these central regions for these positrons to meet by mutual attraction (and positron-positron repulsion). When they meet they annihilate, producing a gamma which can go in any direction, and which most likely will quickly deposit its momentum mostly-elastically into a nearby nucleus.

Very centrally produced gammas push nuclei outwards from the centre of the star; and the less centrally the gammas are produced, the greater the chance that the nuclei are pushed in some other direction (including inwards).

More detail at https://news.ycombinator.com/item?id=20726189