What defines a binary star system?

A binary star system is defined by the presence of two stars that are gravitationally bound to each other, orbiting around a common center of mass. This configuration allows both stars to influence each other's motions due to their mutual gravitational attraction. The study of binary star systems is essential in astronomy because they provide valuable information about stellar masses, compositions, and the stages of stellar evolution.

In binary systems, the characteristics of the stars can be analyzed through their orbits, and we can apply Kepler's laws of motion to understand their behavior. This interaction is fundamental to many areas of astrophysics, including studying how stars evolve and ultimately end their life cycles.

Other options describe different astronomical phenomena. A star colliding with a black hole involves dynamics beyond the binary star interaction. A single star with multiple planets does not relate to binary systems, as it describes a star-planet relationship instead. Similarly, two nebulae interacting in space refers to the interactions of gas clouds and not to stars orbiting each other. The unique nature of the binary system is what distinguishes it from these other configurations.