A binary star system consists of two stars that orbit around a common center of mass. These stars can be similar in size and brightness, or they can differ significantly. Binary systems are important for astronomers because they allow for the measurement of stellar masses, which helps in understanding the evolution of stars. There are two main types of binary star systems: visual binaries, which can be seen separately through telescopes, and spectroscopic binaries, which are identified through their spectral lines. Studying binary systems provides insights into stellar dynamics and the life cycles of stars in the universe.