What is an expanding shell of bright gas surrounding a small white dwarf created when the outer layers of a red giant drift into space?

The correct answer is a planetary nebula, which refers to the luminous shell of gas that expands outward from a dying red giant star, particularly when it sheds its outer layers. As the core of the red giant collapses into a white dwarf, the outer layers are released into space, forming this colorful and glowing shell of gas.

This phenomenon is significant in the life cycle of stars, as it contributes to the enrichment of the interstellar medium with heavier elements. The light emitted from the gas is primarily due to ionization by the intense ultraviolet radiation from the exposed core, which gives the nebula its bright appearance.

In contrast, a supernova remnant refers to the aftermath of a massive star exploding in a supernova event, which is a very different process and involves the destruction of the star rather than the shedding of outer layers. A red giant shell is not a recognized astronomical term for such phenomena, and a globular cluster consists of tightly bound collections of stars rather than gas expelled from a star, making it unrelated to the concept of a planetary nebula.