What happens to the central star after it ejects a planetary nebula?

After a star ejects its outer layers to form a planetary nebula, the remaining core—often called the planetary-nebula nucleus—undergoes a significant transformation. Initially, this core is extremely hot, dense, and luminous, having been exposed by the loss of the star's overlying atmosphere.12

As the nebula expands and eventually dissipates into the interstellar medium, the central star continues to contract and evolve. Over a period of approximately 10,000 years, the star becomes increasingly faint and eventually cools to become a white dwarf star.3

These white dwarfs are compact, dense objects roughly the size of Earth that have exhausted their nuclear fuel. Supported by electron degeneracy pressure, they slowly radiate away their remaining thermal energy over billions of years, eventually cooling to become inert stellar remnants, sometimes referred to as black dwarfs.23