What defines a binary star system?

A binary star system is defined as two stars that revolve around each other due to gravitational attraction. This definition is crucial, as it highlights the interaction between the two stars—specifically, how their mutual gravitational forces cause them to orbit around a common center of mass.

In a binary system, the gravitational pull is a fundamental aspect that not only keeps both stars bound together but also influences their motions, brightness, and evolution over time. These pairs can take various forms, such as visual binaries (which can be seen separately through a telescope), eclipsing binaries (where one star passes in front of the other from our viewpoint), or spectroscopic binaries (detected through shifts in their spectral lines due to their motion).

The other choices do not accurately describe the nature of binary star systems. While two stars being at the same distance from Earth doesn't imply they interact gravitationally, the concept of a single star with two observing points misrepresents the nature of binary systems, as does the idea of a binary system with no gravitational pull, which would not constitute a binary system at all.