M 57 (aka NGC 6720) is commonly
known as the Ring Nebula. It is located approximately 2,300 light
years away in the constellation of Lyra. It spans only one arcminute
and shines at a magnitude of 8.8. It was discovered by Charles Messier
in 1779 and is one of only four planetary nebulae in his catalogue.
The others are M 27, the Dumbbell Nebula; M 76, the LIttle Dumbbell
Nebula; and M 97, the Owl Nebula.
A planetary nebula is created when
a star is nearing the end of its life cycle. The red giant star
expells a shell of ionized gas, eventually leaving a white dwarf
star in its center. M 57 is estimated to be expanding at a rate
of 12-18 miles per second. It expands one arcsecond per century.
The small spiral galaxy to the lower
left of M 57 is IC 1296