In a binary star system, what is the brighter star called?

In a binary star system, the brighter star is referred to as the primary star. This designation is based on its luminosity and mass compared to its companion. The primary star is usually the one that is more massive and emits more light, which is why it appears brighter in the system. The term “primary” implies it plays a leading role in the system's dynamics, often influencing the gravitational relationship and orbital characteristics of the fainter companion star. Understanding this terminology is essential, as it helps astronomers categorize and study star systems based on their characteristics and behaviors.

While the companion star can have its own significance in terms of structure and behavior, it is the primary star that stands out in terms of brightness and mass within the binary system.