Binary stars are among the commonest stellar systems in our galaxy, accounting for about 50% of all stars. They consist of two stars in orbit around their common centre of gravity. Those which can be resolved by telescope are referred to as visual binaries. Spectroscopic binaries comprise star pairs too close to be resolved visually, but which exhibit Doppler shifts in spectral lines because of varying velocities of the two stars in the line of sight. Other stars reveal the presence of an invisible companion by changes in their proper motion. Double stars appear to be visually close to each other, but may be at vastly different distances from the observer.