A binary star system consists of two stars that orbit around a common center of mass. These stars are held together by their mutual gravitational attraction. Binary 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 by their spectral lines. Binary star systems are important for astronomers because they help determine the masses of stars. By observing the 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.