A triple star system is a physical system consisting of three stars that are gravitationally bound to each other. The term can also refer to an astronomical configuration where there is an optical illusion caused by the close proximity of the three stars. In physical systems, the two main types of triple star systems are those with stable orbits and those with unstable orbits. Unstable orbit systems are also known as hierarchical triple star systems.
The most well-known example of a stable triple star system is Alpha Centauri, which consists of two bright stars, Alpha Centauri A and Alpha Centauri B, orbiting each other at a distance of about 23 AU, and a faint red dwarf star, Proxima Centauri, orbiting them at a distance of about 0.5 AU. There are many other examples of stable triple star systems such as HD 131399AB in the constellation Centaurus.
In contrast, hierarchical triple star systems have much wider separations between the stars and usually consist of one giant or supergiant star orbited by two smaller companion stars. An example is Mu Cephei in the constellation Cepheus. Hierarchicaltriple star systems can also be unstable if the orbit of the companion stars around the primary is too wide or if they approach too close to each other during their orbit. In these cases, gravitational interactions can lead to orbital decay and eventual collision or merger of thestars.