Astronomy Colloquium

Our framework of cosmic explosions no longer has a glaring six-magnitude luminosity "gap" between the brightest novae and faintest supernovae. We now know of at least four new classes of transients that bridge this gap: .Ia explosions, calcium-rich halo transients, intermediate luminosity red transients, and luminous red novae. As we better characterize these recent discoveries, we find that these classes represent different missing pieces in our understanding of stellar evolution: electron capture induced collapse in low mass stars, fallback onto black holes in high mass stars, helium shell detonation in ultracompact binaries, and stellar mergers. Moreover, Calcium-rich gap transients may explain the old conundrum of the observed overabundance of Calcium in clusters.  The next frontier in gap transients is uncovering elusive binary neutron star mergers; a goal which may soon be within reach with coordination between the next generation of synoptic surveys and advanced gravitational wave interferometers. I conclude with the preparation for this march towards discovering even fainter, even faster and even rarer transients in the local universe.