Final answer:
In an evolved high-mass star, electrons combine with protons to form neutrons and neutrinos through the reaction known as electron capture, which contributes to the collapse of the star and nucleosynthesis of elements heavier than iron.
Step-by-step explanation:
In an evolved high-mass star, when the electrons combine with protons to form a pure neutron core, the reaction is known as electron capture. During the collapse of the iron core in a massive star, the density becomes so great that electrons are forced into the atomic nuclei. There, electrons combine with protons to produce neutrons and neutrinos. This reaction can be represented as:
p + e- → n + νe,
where p is a proton, e- an electron, n a neutron, and νe a neutrino.
This process significantly increases the density of the core, as neutrons can be packed together more tightly than the atoms in ordinary matter. Eventually, if the outward pressure cannot resist the force of gravity, the star may collapse into a neutron star or, if massive enough, continue to collapse into a black hole. The collapse of the star and the accompanying electron capture play a critical role in a supernova explosion, which not only results in the creation of a neutron star but also the dispersal of heavy elements into the cosmos, contributing to the nucleosynthesis of elements heavier than iron.