Stars swap through different elements in their lifetime. They burn hydrogen, then helium, then carbon, then continue using the next lightest material available after which the previous has been exhausted. This process continues until the star attempts to burn iron (FE in the diagram), in which case marks the end of the world.
KY Cygni is a red supergiant 1,420 times the size of the sun.
A Thorne–Żytkow object is a star that hides another star within itself.