Messier 57, The Ring Nebula. Often regarded as the prototype of all planetary nebulae, it was not deemed so by its discoverers. These objects are the remains of Sun-like stars which have blown away their outer atmospheres, leaving planet-sized white dwarfs at their centers (you can see it here in the center).
The Ring Nebula was discovered by the French Astronomer Antoine Darquier de Pellepoix in January, 1779. He described it as “a dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet.” Only a few days later, Charles Messier independently found the same nebula while searching for comets, and entered it as the 57th object in his catalogue. Messier, and later William Herschel, speculated that the nebula was formed by multiple faint stars that they were unable to resolve.
William Herschel, who discovered the planet Uranus, found other nebulous objects resembling M57 (and his newly discovered planet), and introduced the term “Planetary Nebulae” for them. Yet, oddly, Herschel did not count this, their most prominent representative among them. He instead considered M 57 to be a peculiar object,
a “curiosity of the heavens”, and described it as “a perforated nebula, or ring of stars”; this was the first mention of its ring shape. Herschel also identified some of the superimposed stars, and correctly assumed that “none [of them] seems to belong to it.”