Ternary Star System

Stellar systems in which three stars are bound together by gravity are known by various names. Following the model of binary for pairs of bound stars, these triple systems are variously known as ternary, tertiary or trinary systems. The simple term 'triple star' is also commonly used, although formally this has a slightly distinct meaning (just as the term 'double star' can include optical doubles, a 'triple star' might in principle refer to three unrelated stars on a close line of sight as seen from Earth).

Ternary systems are not uncommon. Indeed, it has been estimated that about a tenth of all star systems have three members, and there are two main ways for such triple systems to come into existence. Systems like this can emerge through the usual process of stellar formation, in situations where a protostellar mass collapses into three distinct gravitational foci, and indeed triple formation events of this kind have been observed. Alternatively, the system can begin life as a binary or even as a single star, and later capture further members through gravitational interactions.

The most common hierarchy for the stars in a triple system has one pair interacting as a close binary, with the third star distinct from this pair. Often the binary pair forms the primary gravitational focus of the system, with the third star following an orbit around it. A notable example of this structure is the Alpha Centauri system, in which two relatively Sun-like stars form the central binary pair, while the third star, the red dwarf Proxima, follows a wide orbit around the inner binary. It is thought that Proxima may represent an example of binary star becoming a ternary star by capturing a third member. Another prominent example of this kind of hierarchy is Fomalhaut in Piscis Austrinus, which probably forms a binary pair with the star TW Piscis Austrini, with this pair of stars being orbited by a red dwarf.

The inverse of this structure is also seen. That is, there are systems in which a single (typically massive) star forms the core of the system, and is orbited by a binary pair. The most prominent example of this kind of structure is Rasalgethi or Alpha Herculis, in which the primary star is a red supergiant, which is orbited at a distance of some 500 AU by a close binary pair formed by giant and a dwarf. Another example of a system like this is Keid or Omicron² Eridani, in which an orange main-sequence star is orbited by a binary consisting of a red dwarf and a white dwarf.

Several extrasolar planets have been detected within ternary systems. Commonly these are found within systems with a single primary star, and are found in a close orbit around that star, with examples being found in the Kepler-13 system, and around Psi¹ Aquarii. A rarer situation is the appearance of a so-called 'circumtriple' planet, a planet that orbits around all three stars in a triple system. No such planet has yet been definitively confirmed (though the KX Librae system comes close, having a brown dwarf in orbit beyond the three stars of the system). A potential true example of such a planet may be found in the GW Orionis system, an unusual formation in which all three stars possess overlapping rings of dust, and distortions in these rings hint at the existence of a circumtriple planet.