APOD: 2022 March 25 - The Medusa Nebula

Good Red Friday evening, Rallypoint, and welcome to the March 25th Astronomy Picture of the Day (AOD): "The Medusa Nebula." Also known as Abell 21 and Sharpless 2-274, the Medusa Nebula lies about 1,500 light years away in the constellation Gemini. It was discovered in 1955 by University of California, Los Angeles astronomer George O. Abell, who classified it as an 'old planetary nebula'. Until the early 1970s, this Medusa was thought to be a supernova remnant. But with the computation of expansion velocities and the thermal character of detected radio emissions, it was concluded in 1971 that Dr. Abell's original classification was correct.

So what is a 'planetary' nebula? It is a class of emission nebulae that consists of an expanding, glowing shell of ionized gas ejected a central red giant stars late in its life. In the 1780's, William Herschel described certain types of nebulae as resembling planets. The term 'planetary nebula' stuck.