A Planetary Nebula is a glowing gaseous shell thrown off by a star during the latter stages of its evolution. Eighteenth century astronomer William Herschel named this type of object a 'Planetary Nebula' because the faint green tint and round shape reminded him of his recent discovery, the planet Uranus. (Also note Darquier's 1779 description of M-57 as looking like 'a fading planet'.) Unfortunately, the name stuck, even though these objects have no relationship to the planets!
We now know that they result when moderate to small sized stars reach old age. When stars begin to run short of their hydrogen nuclear fuel, their cores shrink, heat up, and start 'burning' helium. The outer parts of the star greatly expand, forming a red giant star. When even the helium is gone, the core collapses into a White Dwarf star, and the outer parts escape into space, forming an expanding shell. Note that the formation of a Planetary Nebula is a rather gentle event, nothing like the titanic supernovae explosions that mark the passing of heavy stars.
Astronomers are still struggling to understand the true shapes of these gas shells, and how they evolve as they expand. Some Planetary Nebulae have several concentric shells, while some seem to have holes in the shells with jets of material streaming through.
The central stars of Planetary Nebulae are the cores of the preceding red giant stars, and are rapidly evolving into White Dwarf stars. These intrinsically faint stars are very dense and no larger than the earth. A single spoonful of their material weighs as much as a truck. Made mostly of helium, carbon, and heavier elements, they no longer have thermonuclear reactions producing energy. Newly formed White Dwarfs have surface temperatures of at least 85,000 degrees K, and thus radiate mostly ultraviolet light. Over billions of years, they will cool to the frigid background temperature of space.
Although there are several methods to estimate the distance of the Planetary Nebulae, none work very well. Thus astronomers can give only approximate distances that have much less precision than the distances for other deep sky objects.
To see Kopernik's Planetary Nebula Pictures, Check out these links:
George Normandin, KAS
November 1st, 1998
revised Sept. 14th, 2004