Massive Stellar Endpoints

At their deaths, most massive stars will explode as luminous supernovae, seeding the universe with heavy elements and impacting stellar and galactic evolution. However, there is a wide diversity to these events with different explosion mechanisms, different circumstellar environments, and the possibility that some massive stars may die without an accompanying bright transient. In this thesis, I explore the transients that arise from the deaths of different massive stars and the methods by which we can learn about their progenitors. There has been an observed lack of supernovae arising from red supergiants with masses greater than ~ 17 solar masses. Models have shown that such stars may undergo direct core-collapse to form a black hole without the production of a bright transient in a process known as a failed supernova. I develop a pipeline to search for dim transients associated with these events, and apply this to a decade's worth of archival observations of nearby galaxies. I find no sources consistent with a failed supernova and use this non-detection to set an observational upper limit to the rates of such transients of 0.61, 0.33, 0.26, or 0.23 times that of core-collapse supernovae for absolute magnitudes of -11, -12, -13, and -14 respectively. Intermediate Luminosity Red Transients (ILRTs) are a class of `gap transient' with peak luminosity between that of classical novae and supernovae. They are believed to be produced by the electron-capture driven collapse of a super-asymptotic giant branch star producing a weak supernova in a dusty circumstellar environment. I perform a systematic analysis into the evolution of the Na I D absorption line in a sample of ILRTs. I show that evolution is ubiquitous and diverse in this sample and can be used as a means of probing the circumstellar medium of these events. I present a simple toy model to predict this evolution as arising from ejecta from a central supernova passing through a circumstellar environment whose ionisation state changes over time. The progenitors to only a handful of supernovae have been unambiguously identified from direct detections in pre-explosion imagery. Progenitor detection relies on having an archive of observations of nearby galaxies where individual sources can be resolved, and largely makes use of Hubble Space Telescope (HST) imaging at present. I investigate the potential for using the advanced observational capabilities of the James Webb Space Telescope (JWST) to build an archive of observations of nearby galaxies from which future supernovae may be associated with progenitors. I develop an observing strategy for a Survey program which allows for the recovery of the most important physical characteristics of red supergiants.