A binary star is a system where two stars orbit around a common center of mass. These stars can be of similar or different sizes and brightness. Binary stars are important for astronomers because they allow for the measurement of stellar masses, which helps in understanding the life cycles of stars. There are two main types of binary stars: visual binaries, which can be seen separately through telescopes, and spectroscopic binaries, which are identified by their spectral lines. Studying binary stars contributes to our knowledge of stellar evolution and the dynamics of star systems in the universe.