The Ring Nebula (M57)
Object Type: Emission Nebula
Scope: Orion 8" Newtonian
Exposure 24 minutes
The famously named "Ring Nebula" is located in the northern constellation of Lyra, and also catalogued as Messier 57, M57 or NGC 6720. It is one of the most prominent examples of the deep-sky objects called planetary nebulae (singular, planetary nebula), often abbreviated by astronomers as simply planetaries or PNe.
M57 is located in Lyra, south of its brightest star Vega. Vega is the northwestern vertex of the three stars of the Summer Triangle. M57 lies about 40% of the angular distance from Lyrae to Lyrae.
This nebula was discovered by Antoine Darquier de Pellepoix in January, 1779, who reported that it was "...as large as Jupiter and resembles a planet which is fading." Later the same month, Charles Messier independently found the same nebula while searching for comets. It was then entered into his catalogue as the 57th object. Messier and William Herschel also speculated that the nebula was formed by multiple faint stars that were unable to resolve with his telescope.
In 1800, Count Friedrich von Hahn discovered the faint central star in the heart of the nebula. In 1864, William Huggins examined the spectra of multiple nebulae, discovering that some of these objects, including M57, displayed the spectra of bright emission lines characteristic of fluorescing glowing gases. Huggins concluded that most planetary nebulae were not composed of unresolved stars, as had been previously suspected, but were nebulosities.
Planetary nebulae are formed after medium or low mass stars, such as the Sun, exhaust their hydrogen fuel in the stellar core. At this point the structure of the star changes so it can achieve a new equilibrium condition in which it can continue to burn; the outer layers of the star expand and it becomes a red giant. Further internal temperature instabilities develop from the fusion reactions, causing the outer atmosphere to be expelled by hot superwinds either continuously or in several energetic pulses. This expanding gaseous shell forms the spherical nebula, brightly illuminated by ultraviolet energy from the central star.
The central PNN was discovered by Hungarian astronomer Jen Gothard on September 1, 1886 from images taken at his observatory in Herény, near Szombathely (now part of Szombathely). Within the last two thousand years, the central star of the Ring Nebula has left the asymptotic giant branch after exhausting its supply of hydrogen fuel. Thus it no longer produces its energy through nuclear fusion and, in evolutionary terms, it is now becoming a compact white dwarf star.