What is the defining characteristic of a binary star system?

A binary star system is characterized by the presence of two stars that are gravitationally bound to each other, allowing them to orbit around a common center of mass. This means that the defining feature of such systems is the interaction and shared motion of the two stars, which can vary in terms of their size, brightness, and distance from one another.

In many binary star systems, the stars may have significantly different masses, but it is their orbital relationship that classifies them as a binary system. This unique interaction allows astronomers to study their properties through various observations, such as changes in brightness as stars eclipse each other, or variations in their spectra.

While it is true that binary stars can be located within galaxies, this fact alone is not sufficient to define them as a binary system, as many celestial objects exist within galactic environments. Additionally, binary stars do not imply or require clusters of many stars; a binary system can consist solely of two stars regardless of their galactic or cluster setting. Thus, the essence of a binary star system lies in the orbiting relationship between the two stars.