Star system

A star system or stellar system is a small group of stars that move around each other because of gravitational attraction. Gravity holds these stars together, just like the 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. In this system, a bright star, Algol A, is hidden or dimmed by a dimmer star, Algol B, every 2.87 days. This makes it look like a blinking star in the night sky. This 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 with three stars orbiting each other.

Star systems are different from larger groups of stars, such as a star cluster or a galaxy. These larger groups can have thousands or even millions of stars. Star systems focus on just a few stars, while planetary systems include not just stars but also planets, comets, and other objects orbiting them. Learning about star systems helps us understand how stars are born, live, and eventually die.

Terminology

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

Optical doubles and multiples

Binary and multiple star systems are called physical multiples because the stars are held together by gravity. Sometimes stars look close together in the night sky, but they are not really part of the same system. These are called optical multiples.

For example, a group of three stars might look like they belong together. But only two of them might actually be pulled together by gravity, with the third star just happening to appear nearby — 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

About a third of the star systems in our Milky Way galaxy have more than one star. The rest are single stars. Binary stars—two stars orbiting each other—are the most common type of multiple star system.

Detection

Scientists have several ways to find and study star systems. One way is to watch stars from Earth at different times and see small changes in where they appear. This is called parallax. Another way is to watch stars move around each other, but this can be hard if the stars are very far away. Scientists can also find star systems by noticing changes in the color or brightness of stars. These changes happen when stars move toward or away from us, or when one star passes in front of another. These methods help us learn about stars and how they move in space.

Main article: Doppler spectroscopy

Orbital characteristics

In star systems where two stars move around each other, they follow a steady, oval path around a common middle point called the barycenter. Examples include famous pairs like Sirius and Procyon. These systems can be simple or very complex.

Multiple-star systems can be steady and organized or unsteady and mixed up. Steady systems have stars set in layers, where each pair works like one star. Unsteady systems, often called trapezia, have stars that move in tricky ways and may later form simpler patterns.

Designations and nomenclature

In a star system with 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 are close together 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.