A binary star system consists of two stars that orbit around a common center of mass. These systems can be classified into two main types: visual binaries, which can be seen separately through telescopes, and spectroscopic binaries, where the stars are too close to distinguish but can be identified through their spectral lines. Binary star systems are important for astronomers because they allow for the measurement of stellar masses. By observing the orbital motion of the stars, scientists can apply Kepler's laws of planetary motion to calculate their masses, providing valuable insights into stellar evolution and the dynamics of galaxies.