Supernovae are explosions of stars. Stars lose their mass in various ways during their evolution, and supernova progenitors are surrounded by circumstellar media formed during their evolution. When supernovae explode, supernova ejecta collide to their circumstellar media. The interaction between the supernova ejecta and circumstellar media leaves several observational signatures in supernovae, and we can investigate their progenitors' mass-loss history by studying such signatures. In this talk, I will first discuss observational signatures of supernovae caused by the interaction between supernova ejecta and circumstellar media. Then, I will show that these signatures are revealing that a significant fraction massive stars are experiencing unexpectedly active mass loss shortly before their explosions. I will also discuss some future directions to uncover the unknown mass-loss mechanisms of massive stars through multi-messenger signals.
Colloquium Committee