Star system

A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a star cluster or galaxy, although, broadly speaking, they are also star systems. Star systems are not to be confused with planetary systems, which include planets and similar bodies (such as comets).

Terminology A star system of two stars is known as a binary star, binary star system or physical double star. Systems with four or more components are rare, and are much less commonly found than those with 2 or 3. Multiple-star systems are called ternary if they contain three stars, quaternary if they contain four, etc. These systems are smaller than open star clusters, which have more complex dynamics and typically have from 100 to 1,000 stars.

Optical doubles and multiples Binary and multiple star systems are also known as physical multiple stars, to distinguish them from optical multiple stars, which merely look close together when viewed from Earth. Multiple stars may refer to either optical or physical, but optical multiples do not form a star system. Triple stars that are not all gravitationally bound (and thus do not form a triple star system) might comprise a physical binary and an optical companion (such as Beta Cephei) or, in rare cases, a purely optical triple star (such as Gamma Serpentis).

Abundance Research on binary and multiple stars estimates they make up about a third of the star systems in the Milky Way galaxy, with two-thirds of stars being single. Binary stars are the most common non-single stars. With multiple star systems, the number of known systems decreases exponentially with multiplicity. For example, in the 1999 revision of Tokovinin's catalog of physical multiple stars, 551 out of the 728 systems described are triple. However, because of suspected selection effects, the ability to interpret these statistics is very limited.

Detection There are various methods to detect star systems and distinguish them from optical binaries multiples. These include:

Make observations six months apart and look for differences caused by parallaxes. (Not feasible for distant stars.) Directly observe the stars orbiting each other or an apparently empty space (such as a dim star or neutron star). (Not feasible for distant stars or those with long orbital periods.) Observe a varying Doppler shift. Observe fluctuations in brightness that result from eclipses. (Relies on the Earth being in the orbital plane.) Observe fluctuations in brightness that result from stars reflecting each other's light or gravitationally deforming each other.

Orbital characteristics In systems that satisfy the assumptions of the two-body problem – including having negligible tidal effects, perturbations (from the gravity of other bodies), and transfer of mass between stars – the two stars will trace out a stable elliptical orbit around the barycenter of the system. Examples of binary systems are Sirius, Procyon and Cygnus X-1, the latter of which consists of a star and a black hole. Multiple-star systems can be divided into two main dynamical classes:

Hierarchical systems are stable and consist of nested orbits that do not interact much. Each level of the hierarchy can be treated as a two-body problem. Trapezia have unstable, strongly interacting orbits and are modelled as an n-body problem, exhibiting chaotic behavior. They can have 2, 3, or 4 stars.

Hierarchical systems

Most multiple-star systems are organized in what is called a hierarchical system: the stars in the system can be divided into two smaller groups, each of which traverses a larger orbit around the system's center of mass. Each of these smaller groups must also be hierarchical, which means that they must be divided into smaller subgroups which themselves are hierarchical, and so on. Each level of the hierarchy can be treated as a two-body problem by considering close pairs as if they were a single star. In these systems there is little interaction between the orbits and the stars' motion will continue to approximate stable Keplerian orbits around the system's center of mass. For example, stable trinary systems consist of two stars in a close binary system, with a third orbiting this pair at a distance much larger than that of the binary orbit. If the inner and outer orbits are comparable in size, the system may become dynamically unstable, leading to a star being ejected from the system. EZ Aquarii is an example of a physical hierarchical triple system, which has an outer star orbiting an inner binary composed of two more red dwarf stars.

Mobile diagrams

Hierarchical arrangements can be organized by what Evans (1968) called mobile diagrams, which look similar to ornamental mobiles hung from the ceiling. Each level of the mobile illustrates the decomposition of the system into two or more systems with smaller size. Evans calls a diagram multiplex if there is a node with more than two children, i.e. if the decomposition of some subsystem involves two or more orbits with comparable size. Because multiplexes may be unstable, multiple stars are expected to be simplex, meaning that at each level there are exactly two children. Evans calls the number of levels in the diagram its hierarchy.

… excerpt ends here. Continue reading the full article.