Multiple Star Systems

 

This artist's concept depicts a faraway solar system like our own, except for one big difference. Planets and asteroids circle around not one, but two suns.

Multiple star systems or physical multiple stars are systems of more than two stars. Multiple star systems are called triple, trinary or ternary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple with five stars; sextuple with six stars; septuple with seven stars; and so on.

In a triple star system, each star orbits the center of mass of the system. Usually, two of the stars form a close binary star and the third is further away; this configuration is called hierarchical. Multiple star systems containing more than three stars are also usually hierarchical.

 Examples

  • HR 3617 is a multiple star with three component stars, HR 3617A, HR 3617B, and HR 3617C. A and B form a physical binary star, while C is optical.

  • Alpha Centauri is a triple star composed of a main binary yellow dwarf pair (Alpha Centauri A and Alpha Centauri B), and an outlying red dwarf, Proxima Centauri. A and B are a physical binary star, with an eccentric orbit in which A and B can be as close as 11 AU or as far away as 36 AU. Proxima is much further away (~15,000 AU) from A and B than they are to each other. Although this distance is still small compared to other interstellar distances, it is debatable whether Proxima is gravitationally bound to A and B.

  • HD 188753 is a physical triple star system located approximately 149 light-years away from Earth in the constellation Cygnus. The system is composed of HD 188753A, a yellow dwarf; HD 188753B, an orange dwarf; and HD 188753C, a red dwarf. B and C orbit each other every 156 days, and, as a group, orbit A every 25.7 years. An Extrasolar Hot Jupiter like Planet is known to orbit very close to star A.

  • Polaris, the north star, is a triple star system in which the closer companion star is extremely close to the main star—so close that it was only known from its gravitational tug on Polaris A until it was photographed by the Hubble Space Telescope in 2006.

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