The planetary nebula called the Medusa Nebula has the catalog designation Abell 21.
Abell 21 is a large, faint planetary nebula in the constellation Gemini. Its popular name comes from its wispy, tangled appearance, which resembles the snake-covered head of the mythological Medusa.
Despite the name “planetary nebula,” Abell 21 has nothing to do with planets. The term is a historical label from early telescopic observations, when some of these objects appeared disk-like. Planetary nebulae are actually glowing shells of gas expelled by aging stars.
Abell 21 was cataloged by George O. Abell in his work on planetary nebulae. Its nickname is descriptive rather than an official mythological classification.