known as physical multiple stars, to distinguish them from optical multiple stars, which merely look close together when viewed from Earth. Multiple stars may refer to either optical or physical, but optical multiples do not form a star system.

Triple stars that are not all gravitationally bound (and thus do not form a triple star system) might comprise a physical binary and an optical companion (such as Beta Cephei) or, in rare cases, a purely optical triple star (such as Gamma Serpentis).