See Also: planetary nebula(encyclopedia)
planetary(medicine)
planetary(dictionary)
evolution, planetary(medicine)
Nebula(medicine)
nebula(encyclopedia)
Gum Nebula(encyclopedia)
nebula(dictionary)
nebula(dictionary)
Orion Nebula(encyclopedia)

planetary nebula (sh)




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 (see white dwarf star) and ionizes the surrounding shell of gas, which is expanding at tens of miles per second.