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.

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 triple, ternary, or trinary if they contain three stars; quadruple or quaternary if they contain four stars; quintuple or quintenary with five stars; sextuple or sextenary with six stars; septuple or septenary with seven stars; and octuple or octenary with eight stars.
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 a 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 might comprise a physical binary and an optical companion or, in rare cases, a purely optical triple star.

Abundance

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.
  • Directly observe the stars orbiting each other or an apparently empty space.
  • Observe a varying Doppler shift.
  • Observe fluctuations in brightness that result from eclipses.
  • 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, 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 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.
  • A simplex diagram of hierarchy 1, as in, describes a binary system.
  • A simplex diagram of hierarchy 2 may describe a triple system, as in, or a quadruple system, as in.
  • A simplex diagram of hierarchy 3 may describe a system with anywhere from four to eight components. The mobile diagram in shows an example of a quadruple system with hierarchy 3, consisting of a single distant component orbiting a close binary system, with one of the components of the close binary being an even closer binary.
  • A real example of a system with hierarchy 3 is Castor, also known as Alpha Geminorum or α Gem. It consists of what appears to be a visual binary star which, upon closer inspection, can be seen to consist of two spectroscopic binary stars. By itself, this would be a quadruple hierarchy 2 system as in, but it is orbited by a fainter more distant component, which is also a close red dwarf binary. This forms a sextuple system of hierarchy 3.
  • The maximum hierarchy occurring in A. A. Tokovinin's Multiple Star Catalogue, as of 1999, is 4. For example, the stars Gliese 644A and Gliese 644B form what appears to be a close visual binary star; because Gliese 644B is a spectroscopic binary, this is actually a triple system. The triple system has the more distant visual companion Gliese 643 and the still more distant visual companion Gliese 644C, which, because of their common motion with Gliese 644AB, are thought to be gravitationally bound to the triple system. This forms a quintuple system whose mobile diagram would be the diagram of level 4 appearing in.
Higher hierarchies are also possible. Most of these higher hierarchies either are stable or suffer from internal perturbations. Others consider complex multiple stars will in time theoretically disintegrate into less complex multiple stars, like more common observed triples or quadruples.

Trapezia

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. They are named after the multiple star system known as the Trapezium Cluster in the heart of the Orion Nebula. Such systems are not rare, and commonly appear close to or within bright nebulae. These stars have no standard hierarchical arrangements, but compete for stable orbits. This relationship is called interplay. Such stars eventually settle down to a close binary with a distant companion, with the other star previously in the system ejected into interstellar space at high velocities. This dynamic may explain the runaway stars that might have been ejected during a collision of two binary star groups or a multiple system. This event is credited with ejecting AE Aurigae, Mu Columbae and 53 Arietis at above 200 km·s−1 and has been traced to the Trapezium Cluster in the Orion Nebula some two million years ago.

Designations and nomenclature

Multiple star designations

The components of multiple stars can be specified by appending the suffixes A, B, C, etc., to the system's designation. Suffixes such as AB may be used to denote the pair consisting of A and B. The sequence of letters B, C, etc. may be assigned in order of separation from the component A. Components discovered close to an already known component may be assigned suffixes such as Aa, Ba, and so forth.

Nomenclature in the Multiple Star Catalogue

A. A. Tokovinin's Multiple Star Catalogue uses a system in which each subsystem in a mobile diagram is encoded by a sequence of digits. In the mobile diagram above, for example, the widest system would be given the number 1, while the subsystem containing its primary component would be numbered 11 and the subsystem containing its secondary component would be numbered 12. Subsystems which would appear below this in the mobile diagram will be given numbers with three, four, or more digits. When describing a non-hierarchical system by this method, the same subsystem number will be used more than once; for example, a system with three visual components, A, B, and C, no two of which can be grouped into a subsystem, would have two subsystems numbered 1 denoting the two binaries AB and AC. In this case, if B and C were subsequently resolved into binaries, they would be given the subsystem numbers 12 and 13.