M57

M57, also known as the Ring Nebula, is a planetary nebula in the northern constellation Lyra. It represents the glowing shell of gas ejected by a star at the end of its life, surrounding a hot white dwarf that remains at the center. The nebula’s light comes from ultraviolet radiation from the central star ionizing the expelled material, which then emits in characteristic nebular lines, most prominently doubly ionized oxygen (O III) and hydrogen.

The central star is a hot white dwarf with a surface temperature on the order of 100,000 kelvin, providing the energy that ionizes the surrounding gas. The nebula lies at a distance of roughly 2,000 to 3,000 light-years from Earth and has an apparent diameter on the order of about one arcminute. Its overall shape is a roughly circular to slightly elliptical shell, often perceived as a ring in long-exposure images or through a small telescope.

Formation and structure are discussed in the context of late stellar evolution for sun-like stars: a star expels its outer layers during the asymptotic giant branch phase, and the remaining hot core ionizes the ejected material as it contracts into a white dwarf. The Ring Nebula serves as a well-studied example of a planetary nebula, illustrating processes of mass loss, ionization, and shell expansion, with expansion velocities measured in tens of kilometers per second.

Discovered in 1779 independently by Antoine Darquier de Pellepoix and Charles Messier, M57 remains a popular target for amateur observers and a benchmark object in the study of planetary nebulae.