A multiple star is a star system consisting of three or more stars that appear from Earth close to each other in the sky.
This may be due to the physical proximity of the stars and their gravitational interaction, in which case a multiple star is considered physical. In the second case, their proximity can only be domestic, then it is an optical system. Physical multiple stars are generally also called multiple star systems.
For the most part, these systems are often labeled as trinary or temporary. Larger systems, such as four-stars, five-stars, six-stars, and so on, are statistically less common.
The size of a multiple star system varies midway between binary systems, with two stars in stable orbits and open star clusters that have complex dynamics and contain from 100 to 1000 stars.
Since they must be divided into two classes, which correspond to these two extremes. Most multiple stars are arranged in a hierarchical manner with a smaller orbit located inside the larger orbit. In systems, there are few interactions between orbits and, as with binary stars, they are stable.
Other multiple stars, termed trapezoids, are very young, unstable systems. They are thought to have formed in stellar “nurseries” and quickly separated into multiple stars. An example of such a system is the Trapezium in the Orion Nebula.
In a physical three-star system, each star orbits the center of gravity of the system. Usually, two stars form a closed binary system, and the third star orbits the pair at a distance much greater than the orbits of the first two stars. This arrangement is called hierarchical. The reason for such an arrangement is that if the sizes of the inner and outer orbits are comparatively large, the system may become dynamically unstable, leading to the ejection of a star from such a system.
A three-star in which not all stars are gravitationally connected may contain a physical binary and another optical star (companion), such as Alfirk. Rarely, a purely optical triplet such as Gamma Serpentis may occur, where each of the three is at a completely different distance from Earth.
The second known class of multiple stars consists of young trapezoids, named after a multiple star known as the Trapezium in the heart of the Orion Nebula. Such systems are not uncommon, but often occur near or inside clear nebulae. These stars do not have standard hierarchical relationships, but complete unstable orbits, where the center of gravity is not fixed at one point, but moves depending on how the stars change their common positions. Such stars eventually calm down and form a nearby binary with a distant companion, another star that was once part of the system but was ejected at high speed into interstellar space. Examples of such ejected stars are AE Aurigae, Mu Columbae, and 53 Arietis, which reached the Trapezium in the Orion Nebula several million years ago.
Examples of multiple star systems:
HR 3617 is a multiple star with three stars, HR 3617A, HR 3617B, and HR 3617C. A and B form a physical binary, while C appears to be optical.
Alpha Centauri is a three-star formed by a pair of yellow dwarfs (Alfa Centauri A and Alfa Centauri B) and a distant red dwarf Proxima Centauri. The moderately non-central orbit of the binary causes the smallest distance between the stars to 11 AU and the largest to 36 AU. Proxima is from them approximately 15,000 AU away. This distance is still incomparably small with interstellar distances.
HD 188753 is a physical three-star star located approximately 149 ly from Earth in the constellation Cygnus. It consists of a yellow dwarf HD 188753A, an orange dwarf HD 188753B, and a red dwarf HD 188753C.
The northern star Polaris or Alpha Ursa Minoris (α UMi) is a three-star system where the nearest star is extremely close to the main star – so close that it could only be detected by gravitational influences on the Polaris A (α UMi A) until it was in 2006 photographed by the Hubble Space Telescope.