  A significant fraction of stars between 7-11 solar masses are thought to
become supernovae, but the explosion mechanism is unclear. The answer depends
critically on the rate of electron capture on $^{20}$Ne in the degenerate
oxygen-neon stellar core. However, due to the unknown strength of the
transition between the ground states of $^{20}$Ne and $^{20}$F, it has not
previously been possible to fully constrain the rate. By measuring the
transition, we have established that its strength is exceptionally large and
enhances the capture rate by several orders of magnitude. This has a decisive
impact on the evolution of the core, increasing the likelihood that the star is
(partially) disrupted by a thermonuclear explosion rather than collapsing to
form a neutron star. Importantly, our measurement resolves the last remaining
nuclear physics uncertainty in the final evolution of degenerate oxygen-neon
stellar cores, allowing future studies to address the critical role of
convection, which at present is poorly understood.
