Hot subdwarf stars are a class of stars that are much hotter and more luminous than main sequence stars of similar mass. These stars have evolved from red giants, losing much of their outer layers in the process. Hot subdwarfs are typically small and dense, with high surface temperatures and strong stellar winds. They are important objects for studying stellar evolution and the dynamics of binary star systems. Hot subdwarf stars are also thought to be potential progenitors of Type Ia supernovae, which are important for understanding the universe's expansion.