An example of an environment where electron-capture plays a prominent role is the core-collapse supernova. In this
scenario, we find a star at the end of its fusion burning life and becoming dependent on (outward) electron-degeneracy
pressure to withstand (inward) pressure from its own gravity.
the onion-like layers of a massive star, credit: http://www.solstation.com/x-objects/xte-bh.htm
The core-collapse supernova occurs in massive stars which have managed to fuse
elements much heavier than carbon due to the temperatures and densities created by their size. However, regardless
of mass, the elemental end point of all fusion processes that release energy is iron.
Near the end of the stellar life cycle, massive stars will house an inert core of iron ashes, surrounded by layers of
lighter ashes. This iron core is an electron-degenerate environment, held up largely by the
degeneracy pressure, much like the white-dwarf star in the Type Ia supernova case. The difference is that there
is ample material above the iron core to overcome the degeneracy pressure and cause a collapse. A collapse will
be averted as long as fusion of lighter elements in the upper atmosphere can provide heat and outward pressure to
balance the inward pressure from gravity. Once that ceases, however, nothing can prevent collapse and the core
material begins a freefall toward the star’s center.
An image of SN 1987a, credit http://hubblesite.org/
Once the density becomes too large, the core bounces back and a shockwave is created: the star explodes.
Electron-capture plays an important role in pre-supernova evolutions because the high abundance of medium-heavy
nuclei and the degenerate electron sea make the reactions energetically favorable. As electrons are captured, the
degeneracy is lifted and the collapse can be accelerated due to the reduction in outward pressure. In the capture
process neutrinos are emitted and they carry away energy. Charge-exchange experiments are a useful tool in calculating reliable
electron-capture rates, which can then be used in simulations to describe the evolutionary track of the star.
