Multiple Star

A multiple star consists of three or more stars which appear from the Earth to be close to one another in the sky. This may result from the stars being physically close and gravitationally bound to each other, in which case it is physical, or this closeness may be merely apparent, in which case the multiple star is optical. Physical multiple stars are also commonly called multiple stars or multiple star systems.

Most multiple star systems are triple stars, also called trinary or ternary. Larger systems, such as quadruple stars (4 components), quintuple stars (5 components), sextuple stars (6 components), and so on are statistically less likely to occur.

Multiple stars have sizes intermediate between binary systems, with two stars in a stable orbit, and open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars. They can be divided into two classes corresponding dynamically to these two extremes. Most multiple stars are organized in a hierarchical manner, with smaller orbits nested inside larger orbits. In these systems there is little interaction between the orbits and, as in binary stars, the orbits are stable. Other multiple stars, termed trapezia, are usually very young, unstable systems. These are thought to form in stellar nurseries, and quickly fragment into stable multiple stars, which in the process may eject components as galactic high velocity stars. An example of such a system is the Trapezium in the heart of the Orion nebula.