Star system

A star system, also known as a stellar system, is a collection of stars that orbit one another. These stars remain together because of gravitational attraction, which is the pull exerted by one mass on another. While astronomers sometimes use the term to refer to a single star, it usually describes a small group of stars bound by gravity. It is important to distinguish these from planetary systems, which include planets and comets. Star systems are also much smaller than star clusters or galaxies. While a star system might have a handful of stars, an open star cluster typically contains between 100 and 1,000 stars.

Astronomers use specific names to describe how many stars are in a system. A system with two stars is called a binary star or a physical double star. If there are three stars, it is a triple, ternary, or trinary system. Systems can continue to grow in complexity, with names like quadruple, quintuple, sextuple, septuple, octuple, and nonuple for systems containing four through nine stars. However, systems with four or more components are rare. The number of known systems decreases exponentially as the number of stars increases. For example, in a 1999 revision of Tokovinin's catalog, 551 out of 728 described systems were triple stars.

It is vital to distinguish physical multiple stars from optical multiples. Physical multiple stars are truly bound by gravity and orbit one another. Optical multiples are stars that only appear to be close together when viewed from Earth. An optical multiple does not form a true star system. Some systems are complex hybrids, such as a physical binary that has an optical companion. In these cases, the stars are not all gravitationally bound to each other.

To find these systems, scientists use several detection methods. They can observe the stars directly as they orbit each other or an empty space. This empty space might be where a dim star or a neutron star is located. They may also look for a varying Doppler shift in the light. Another method is observing fluctuations in brightness caused by eclipses. This requires Earth to be in the orbital plane of the stars. Scientists can also watch for brightness changes caused by stars reflecting light or gravitationally deforming one another.

Multiple star systems are generally divided into two dynamical classes: hierarchical systems and trapezia. Hierarchical systems are stable because they consist of nested orbits. In these systems, stars are organized into smaller groups that orbit a common center of mass. These groups can be further divided into even smaller subgroups. This organization is often described using mobile diagrams, which look like ornamental mobiles hanging from a ceiling. Each level of the hierarchy can be treated as a two-body problem. A famous example is the star Castor, which is a sextuple system with a hierarchy of 3.

Trapezia are very different because they are usually young and unstable. These systems often form in stellar nurseries, which are bright nebulae. Instead of organized orbits, the stars in a trapezium compete for stability through a process called interplay. This results in chaotic behavior modeled as an n-body problem. Because the orbits are so unstable, stars are often ejected from the system. These stars can become high-velocity runaway stars. For instance, stars like AE Aurigae were likely ejected from the Trapezium Cluster in the Orion Nebula about two million years ago.

Naming these components is a complex task for astronomers. Usually, suffixes like A, B, or C are added to the system's name. In some catalogs, components are assigned letters like Aa or Ba to show they are part of a closer pair. Because different researchers use different naming methods, confusion can occur. To fix this, several schemes have been proposed. These include the KoMa scheme, the Urban/Corbin Designation Method, the Sequential Designation Method, and the Washington Multiplicity Catalog. These systems aim to better identify the hierarchy and the specific subgroups within a star system.