In the manga, a guardian Senshi who protects their respective planet possesses a special Star Seed called a Sailor Crystal. Star Seeds are the life force of individuals - they are the soul of a person. Star Seeds are born in the Galaxy Cauldron, but some of these Star Seeds develop into Sailor Crystals, which grow into planets and guardians for those planets. Sailor Crystals are not mentioned in the anime; It is only mentioned that Senshi have true Star Seeds which don't turn black when removed and continue to glow for all time.