The "Ring Nebula", also catalogued as Messier 57, M57 or NGC 6720, is one of the most prominent examples of the deep-sky objects called planetary nebulae. M57 is located in constellation Lyra, south of its brightest star Vega.
All the interior parts of this nebula have a blue-green hue that is caused by the doubly-ionized oxygen emission lines at 495.7 and 500.7 nm (OIII). In the outer region of the ring, part of the reddish hue is caused by hydrogen emission at 656.3 nm (H-alpha). Forbidden lines of ionized nitrogen (NII) contributes to the reddishness at 654.8 and 658.3 nm.