molecular gas clouds that are contracting and fragmenting, leading to protostars and young stellar objects, becoming full-fledged stars with protoplanetary disks around them, conventional stars burning through their fuel with their own fully-formed planetary systems, stars evolving into subgiants, giants, and even supergiants, stars dying in planetary nebulae, supernovae, and other life-ending events, and stellar remnants of now-extinct stars like white dwarfs, neutron stars, and black holes.