Ring nebula, planetary nebula in the northern constellation Lyra; cataloged as M57 or NGC 6720. It is perhaps the most famous and beautiful nebula of this type. Its name describes the appearance of the expanding shell of gas. The nebula is estimated to be more than 5,000 light-years distant.
"Ring nebula." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (February 20, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/ring-nebula
"Ring nebula." The Columbia Encyclopedia, 6th ed.. . Retrieved February 20, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/ring-nebula