Probing the Extremes of Stellar Evolution in the Decade of 20 Million Transients

In the decade from 2025-2035, we expect to discover several tens of millions of astronomical transients, thanks to Vera C. Rubin Observatory and the International Gravitational-Wave Observatory Network. This presents both an enormous opportunity, in that previously rare classes of transients will soon be discovered routinely, but also a wake-up call that traditional techniques of classification and analysis are not cut out for the job. In this colloquium, I will describe the current and future landscape of transient astronomy: the aspects of stellar physics we can constrain using large samples of stellar explosions, as well as the data science methods our field must adopt in order to take full advantage of our observations. For example, initial samples of tens of core-collapse supernovae have provided surprising clues about extreme mass loss and pre-explosion eruptions in massive stars. Similarly sized samples of thermonuclear supernovae observed within days of explosion show unexpected behavior that could provide hints about their poorly understood progenitor systems and explosion mechanisms. Lastly, a single gravitational-wave-triggered kilonova suggests that these events may dominate the production of the heaviest elements in the Universe, but we have almost no information about diversity within that class. In all these cases, modern cyberinfrastructure (e.g., real-time discovery alerts, robotic follow-up observations, robust image subtraction, machine-learning classification) will be key to maximizing the science return from powerful new discovery engines.