What is a planetary nebula?

A planetary nebula is correctly defined as a shell of gas ejected from a star at the end of its evolution. This phenomenon occurs when a red giant star exhausts its nuclear fuel and undergoes changes that lead to the expulsion of its outer layers. The exposed core of the star, now a white dwarf, emits intense ultraviolet radiation that ionizes the ejected gas, causing it to glow and form the colorful shells associated with planetary nebulae.

This specific phase of stellar evolution is crucial for the recycling of materials in the universe. The ejected gases enrich the interstellar medium with elements such as carbon, nitrogen, and oxygen, which can later contribute to the formation of new stars and planets.

In contrast, the other options refer to different astrophysical phenomena. Regions where new stars are born refer to stellar nurseries, dense areas of stars in a galaxy cluster describe clusters themselves, and a cloud of dust surrounding a black hole pertains to the material surrounding accreting black holes, such as those found in active galactic nuclei. Each of these terms describes distinct processes and structures within the universe, highlighting the unique nature of a planetary nebula as a critical stage in the life cycle of stars.