Any of a class of bright nebula e that may somewhat resemble planet s 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 (see white dwarf star ) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.