Star system

The most current estimates guess that there are 125 to 200 billion galaxies in the Universe, each of which has hundreds of billions of stars. A recent German supercomputer simulation put that numbe...

A star system or stellar system is a small number of stars which orbit each other, bound by gravitational attraction. A large number of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star system may also be used to refer to a system of a single star together with a planetary system of orbiting smaller bodies.

Binary star systems

A stellar system of two stars is known as a binary star, binary star system or physical double star. If there are no tidal effects, no perturbation from other forces, and no transfer of mass from one star to the other, such a system is stable, and both stars will trace out an elliptical orbit around the center of mass of the system indefinitely.

Multiple star systems

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. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.

Observation:

Most multiple star systems known are triple; for higher multiplicities, the number of known systems with a given multiplicity decreases exponentially with multiplicity. Because of the dynamical instabilities mentioned earlier, triple systems are generally hierarchical: they contain a close binary pair which has a more distant companion. Systems with higher multiplicities are also generally hierarchical. Systems with up to six stars are known; for example, Castor (Alpha Geminorum), which consists of a binary pair in a distant orbit of two closer binary pairs. Another system known with six stars is ADS 9731, which consists of a pair of two triple systems, each of which is a spectroscopic binary in orbit together with a single star.