What is a planetary nebula?

A planetary nebula is a luminous shell of gas and dust that is expelled from certain types of stars, particularly low- to medium-mass stars, during the later stages of their evolution. The correct answer describes it as a spherical cloud of particles made up of heavy elements. This formation occurs after a star has exhausted its nuclear fuel and has shed its outer layers. The core remains as a white dwarf, which emits ultraviolet radiation that ionizes the expelled gas, causing it to glow.

The heavy elements found in the nebula result from nuclear fusion processes that occurred within the star over its lifetime. Therefore, the description of a planetary nebula as a spherical cloud of particles made up of heavy elements accurately reflects its composition and formation process, distinguishing it from other astronomical structures like regions of star formation where new stars are actively forming.