What is a planetary nebula? Rings of icy rock which form around stars prior to fusion beginning. The remains of a high-mass star that has exploded. Interstellar gas from which planets are likely to form. Gas ejected from a low-mass star in the final stage of its life. Gas created from the remains of planets that once orbited a dead star.

A planetary nebula is (Gas ejected from a low-mass star in the final stage of its life.)

Explanation: The star exist in form of life cycles from its formation to the final stage of existence. Alot of hydrogen fusion reactions into helium occurs in the core for a long period of time. With time, its helium-rich core becomes hot and dense enough to become a fuel source itself. During the final stage of life, helium burns to forge an even heavier mix of carbon, nitrogen and oxygen, and this renewed burst of energy puffs the star out into a vastly larger red giant. The star stops producing energy and dies, but in those final stages it sheds its outer layers ( planetary medulla). This material blows out into interstellar space, carrying with it traces of the heavier elements it once formed, primarily carbon.