Links zu Messier 57 
Planetary Nebula M57 (NGC 6720) bei SEDS
Deep-Sky Corner / Lyr / M57: Ring Nebula & IC 1296
Astronomical Drawings: Messier 57 - Ring Nebula
NASA - Ring Holds a Delicate Flower
Ringnebel - Wikipedia
Deep Sky-Archiv: Messier 57 "Ringnebel"