Final answer:
A planetary nebula leaves behind a white dwarf when it dissipates, which eventually cools to become a black dwarf.
Step-by-step explanation:
When a planetary nebula dissipates, the core that remains is typically a white dwarf. This end state applies to stars whose core masses are less than approximately 1.4 solar masses at the time they run out of fuel. Planetary nebulae often show different structures that suggest multiple episodes of mass loss when the star was a red giant, with some exhibiting a ring or lobes of matter. The greenish color seen in some planetary nebulae is due to oxygen ions. Over time, these white dwarfs will cool down to eventually become black dwarfs, which are dark and difficult to observe. Dying stars with core masses between 1.4 and about 3 solar masses typically become neutron stars, and those with core masses greater than 3 solar masses end their lives as black holes.