I review our current understanding of superluminous supernovae, mysterious events 100 times brighter than conventional stellar explosions.
INTRODUCTION: THE DISCOVERY OF SLSNEThe study of supernovae spans centuries and cultures, with Chinese astronomers recording the oldest known 'guest star' in 185 AD. By the turn of the millenium, it was well established that core-collapse supernovae (CCSNe) signal the death of a star of more than 8 M (where M is the solar mass). When the nuclear reactor in its core has converted all its fuel into stable iron, and can no longer extract energy to support itself, it collapses to a neutron star and releases its gravitational energy. Type Ia supernovae (SNe Ia) occur when a white dwarf gains sufficient mass from a binary companion to encounter a runaway thermonuclear reaction. Both types of supernova release ∼ 10 51 erg in kinetic energy, peak with a luminosity up to 10 9 L , and produce many of the chemical elements needed for life in the Universe.A more extreme class of 'superluminous supernovae' (SLSNe), ∼ 10 − 100 times brighter again, escaped our notice until the 21st century. Their eventual discovery was intimately tied with advances in optical sky surveys in the mid-to-late 2000s. Figure 1 shows the growth in the supernova discovery rate from 1996-2020, with a sharp uptick around 2010 driven by wide-field robotic telescopes and automated source detection employed by surveys such as the Catalina Real-time Transient Survey (CRTS; Drake et al. 2009), the Panoramic Survey Telescope and Rapid Reponse System (PanSTARRS; Kaiser et al. 2010) and the Palomar Transient Factory (PTF; Rau et al. 2009).Nearly 20,000 supernovae were reported in 2020 -a 100-fold increase in just 20 years. The volumetric rate of SLSNe is only ∼1 in every few thousand supernovae (Quimby et al. 2013;Frohmaier et al. 2021), though because these brighter explosions can be detected out to greater distances, they should make up ∼1% of the supernovae detected by a given survey. Finding SLSNe in significant numbers therefore requires the discovery and classification of hundreds or thousands of supernovae per year.Yet Figure 1 also shows that the fraction (not just the absolute number) of SLSNe within a given survey also increased dramatically in 2010. Earlier surveys had targeted massive galaxies that have a correspondingly high rate of CCSNe and SNe Ia, but it turns out that SLSNe