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Definition of PLANETARY NEBULA
: a usually compact luminous ring-shaped nebula that is composed of matter which has been ejected from a hot star at its center
First Known Use of PLANETARY NEBULA
Any of a class of bright nebulae that may somewhat resemble planets when viewed through a small telescope but are, in fact, expanding shells of luminous gas around dying stars. A planetary nebula is the outer envelope shed by a red giant star not massive enough to become a supernova. Instead, the star's intensely hot core becomes exposed (seewhite dwarf star) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.