Star system

A star system or stellar system is a small group of stars that orbit each other because of the pull of gravitational attraction. These stars are held together by gravity, which is the same force that keeps Earth orbiting the Sun. Sometimes, people might use the term "star system" to talk about just one star, like our Sun, but usually, it means two or more stars moving around each other.

One famous example is the Algol three-star system, where one bright star, Algol A, is regularly hidden or dimmed by a dimmer star, Algol B, every 2.87 days. This creates what looks like a blinking star in the night sky, and it was captured in images taken in the near-infrared using the CHARA interferometer. Another interesting system is HD 188753, which is a triple star system, meaning it has three stars orbiting each other.

Star systems are different from larger groups of stars, such as a star cluster or a galaxy, which contain many more stars. While a star system focuses on just a few stars, these larger groups can have thousands or even millions of stars. It’s also important not to confuse star systems with planetary systems, which include not just stars but also planets, comets, and other objects orbiting them. Understanding star systems helps us learn about how stars are born, live, and eventually die.

Terminology

A star system with two stars is called a binary star or a binary star system. Systems with more stars exist, but they are rarer. For example, a system with three stars is called a triple, ternary, or trinary system. These star systems are smaller than open star clusters, which usually contain hundreds to thousands of stars.

Optical doubles and multiples

Binary and multiple star systems are called physical multiple stars because the stars are actually pulled together by gravity. But sometimes stars just appear close together when we look up at the night sky from Earth — these are called optical multiples, and they are not truly part of the same star system.

For example, three-star groups that look like they belong together might actually only have two stars that are pulled together by gravity, with a third star that is just happening to appear nearby from our point of view — like Beta Cephei. In very rare cases, three stars might look close together but none of them are actually pulled together by gravity — like Gamma Serpentis.

Abundance

Research suggests that about a third of the star systems in our Milky Way galaxy consist of multiple stars, while two-thirds are single stars. Binary stars—two stars orbiting each other—are the most common type of multiple star system. While there are many triple star systems known, it can be difficult to understand exactly how common they are due to certain viewing challenges.

Detection

There are several ways scientists can find and study star systems. One way is to look at the stars from Earth at different times of the year and notice small changes in their positions, called parallaxes. Another method is to watch the stars move around each other over time, though this can be hard if the stars are very far away or take a long time to orbit. Scientists can also detect star systems by noticing changes in the color or brightness of stars, which happen when the stars move toward or away from us or when one star passes in front of another, causing an eclipse. These methods help us learn about the stars and how they move together in space.

Main article: Doppler spectroscopy

Orbital characteristics

In star systems where two stars orbit each other, they follow a stable, elliptical path around a common center called the barycenter. Examples include well-known pairs like Sirius and Procyon. These systems can be simple or quite complex.

Multiple-star systems can be either stable and organized or unstable and chaotic. Stable systems have stars arranged in nested orbits, where each pair acts like a single star. Unstable systems, often called trapezia, have stars that move in complicated ways and may eventually settle into simpler arrangements.

Designations and nomenclature

In a star system made of more than one star, we give each star a letter after the system's name, like A, B, C, and so on. If two stars close together are found near a known star, we might call them Aa and Ba. There are special books that list these stars and give them codes using numbers and letters to keep track of how they move and relate to each other.

Scientists have talked about making a better way to name these stars so everyone agrees. One idea uses letters and numbers in a certain order, while another just gives new stars numbers as they are found. In the year 2000, a group of experts agreed to use a plan called WMC, which uses letters and numbers to show how stars are grouped together based on how far apart they are or how long they take to go around each other.

Examples

Further information: Category:Binary stars

Star systems come in many shapes and sizes. Some have just two stars, called binary systems. Examples include Sirius, which has a bright star and a faint companion, and Procyon, which is similar to Sirius.

Other systems have three or more stars. Alpha Centauri is a triple system with three stars orbiting each other. Capella is a quadruple system with four stars, and some systems have even more stars, like Castor, which has six stars.

These systems show how stars can group together and orbit one another due to gravity.