The Ring Nebula, Messier 57, lies in the constellation Lyra. It is also catalogued as NGC 6720 and sits between the bright stars Beta and Gamma Lyrae, south of Vega, one of the vertices of the Summer Triangle.
M57 is a planetary nebula: the glowing shell of gas expelled by a Sun-like star near the end of its life. The exposed stellar core has become a white dwarf, whose ultraviolet radiation makes the surrounding gas shine. Its ring-like appearance is partly a matter of viewing angle; astronomers think the nebula has a three-dimensional, elongated structure.
Charles Messier catalogued the object in 1779 while compiling a list of hazy objects that could be mistaken for comets. With an apparent magnitude of about 8.8, it is too faint for unaided vision but is a popular small-telescope target. A common mix-up is confusing Lyra with nearby Cygnus, which contains other famous deep-sky objects, including the North America Nebula.