Star system

Stars can live in small groups.

They pull on each other. This pull keeps them together. They move in circles around each other.

Some groups have two stars. These are called binary stars. Other groups have three stars. Some even have many more stars.

Sometimes stars just look close. They are not actually a group. They do not pull on each other. These are just optical stars.

Most stars in our galaxy are single. They live all by themselves. But many stars like to have friends.

Groups can look like a mobile.

A mobile hangs from a ceiling. These stars hang in the sky. It is a beautiful sight.

A star system is a group of stars. These stars orbit each other. They stay together because of gravity. Gravity is the pull that holds things together.

Some systems have only two stars. We call these binary stars. Other systems have more. A system with three stars is a triple star. Systems can have four, five, or even nine stars. Most stars in our galaxy are single. They live alone. About one third of stars in the Milky Way are in binary systems.

Some groups are stable. We call these hierarchical systems. They work like a mobile hanging from a ceiling. In these systems, stars move in organized layers. Small groups of stars orbit each other. Then, those groups orbit a larger center.

Other groups are not stable. These are called trapezia. These stars have messy orbits. They compete for space. Often, one star gets pushed out. It flies away into space at high speeds. This can happen in bright areas called nebulae.

A star system is a group of stars that orbit one another. They stay together because of gravitational attraction, which is the pull between objects. While some people use this term for just one star, it usually means a small group. A star system is different from a planetary system, which includes planets and comets. It is also smaller than a star cluster or a galaxy. Clusters are much larger and can have 100 to 1,000 stars.

These systems work in different ways depending on how many stars are inside. A system with two stars is called a binary star. If there are three stars, it is a triple or ternary system. Systems can have up to nine stars, which are called nonuple systems. Some systems are hierarchical, meaning they are very organized. In these, small groups of stars orbit each other in nested layers. This is like a mobile hanging from a ceiling. Each layer moves in a stable way around a center point.

Other systems are not as stable and are called trapezia. These stars have orbits that interact strongly and can be chaotic. This often happens in stellar nurseries, which are bright areas called nebulae. Because the orbits are messy, the stars compete for space. Often, one star gets pushed out of the group. This star then flies away into space at a very high speed. This can happen in the Orion Nebula, where stars like AE Aurigae were likely ejected.

Scientists use many tools to find these systems in the sky. They might watch for changes in brightness caused by eclipses. They can also look for a varying Doppler shift. Some researchers look at how stars reflect light off each other. In the Milky Way galaxy, about one-third of star systems are binary. Most stars are actually single stars. In a 1999 catalog by Tokovinin, 551 out of 728 systems were triple stars.

Naming these stars can be a hard job for astronomers. They often add letters like A, B, or C to the name. For example, the star Sirius has a companion called Sirius B. Some scientists use numbers to show how the stars are grouped. This helps them map out the hierarchy of the system. One famous example is the star Castor. It looks like two stars, but it is actually a sextuple system. This means it has six stars working together in layers.

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.