Binary stars are systems where two stars orbit around a common center of mass. They are common in the universe, with about half of all stars being part of a binary system. These stars can be close together or far apart, and their gravitational pull affects each other's motion. There are two main types of binary stars: visual binaries, which can be seen separately through telescopes, and spectroscopic binaries, which are identified by analyzing their light spectra. Studying binary stars helps astronomers understand stellar evolution and the properties of stars, including their masses and ages.