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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.
This entry comes from Encyclopædia Britannica Concise. For the full entry on planetary nebula, visit Britannica.com.