Star system
Gravitationally bound groups of stars, from binaries to complex multiples.
A star system, also called a stellar system, is a small number of stars that orbit each other, bound by gravitational attraction. The term may sometimes refer to a single star. Star systems are distinct from planetary systems, which include planets and similar bodies such as comets.
- definition
- A small number of stars bound by gravity
- common types
- Binary, ternary, quaternary, and higher multiples
- abundance
- About one-third of star systems in the Milky Way are multiple; two-thirds of stars are single
- most common multiple
- Binary stars
- rare types
- Systems with four or more components
- dynamical classes
- Hierarchical systems and trapezia
Lore & Background
Star systems range from binary stars—two stars orbiting a common barycenter—to rare systems with four or more components. Multiple-star systems are called ternary (three stars), quaternary (four), and so on. These systems are smaller than open star clusters, which typically contain 100 to 1,000 stars. Optical multiple stars, which appear close together from Earth but are not gravitationally bound, do not form true star systems. For example, Beta Cephei is a physical binary with an optical companion, while Gamma Serpentis is a purely optical triple star.
Reader's Guide
Star systems are fundamental to understanding stellar dynamics and evolution. Research indicates that roughly one-third of star systems in the Milky Way are multiple, with binary stars being the most common non-single type. However, selection effects limit the interpretation of these statistics. Hierarchical systems, which are stable and consist of nested orbits, dominate, while trapezia are unstable, young systems that often fragment into stable multiples, sometimes ejecting stars at high velocities. The study of star systems informs models of stellar formation, orbital mechanics, and the evolution of galaxies.
Did You Know?
- Binary stars are the most common non-single star systems.
- Systems with four or more components are rare and much less common than those with two or three.
- Trapezia are usually very young, unstable systems that may eject stars as galactic high-velocity stars.
Frequently Asked Questions
What exactly is a star system in astronomy?
A star system is a gravitationally bound group of two or more stars that orbit one another, though the term can also apply to a lone star. It refers specifically to the stellar components and is separate from any planets or small bodies that might circle them.
What are the most common configurations of star systems?
Binary stars—two stars sharing a mutual orbit—are by far the most frequent multiple arrangement. Ternary and quaternary systems also occur, while configurations with four or more components are considerably rarer.
How prevalent are multiple star systems compared to single stars in the Milky Way?
Roughly two-thirds of stars in our galaxy exist alone, while about one-third belong to a multiple star system. This makes binary and higher-order groupings a significant but not dominant fraction of the stellar population.
How does a star system differ from a planetary system?
A star system consists solely of stars held together by mutual gravity, whereas a planetary system adds planets, comets, and other small bodies orbiting those stars. The two concepts overlap in practice but are defined by which objects they include.
What are trapezia and hierarchical systems in stellar astronomy?
Hierarchical systems are nested multiple-star arrangements where subgroups orbit within larger orbits, creating a layered structure. Trapezia are a rarer dynamical class of four-star configurations that do not collapse into simple nested pairings.
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