In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope.

Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies, or between two stars as in double star astrometry.

Image Credit: Friendlystar at en.wikipedia / CC BY 3.0

Present estimates of the percentage of double versus single stars point toward a ratio of 60% or higher.

Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters.

Image Credit: PlanetStar at English Wikipedia / CC BY-SA 3.0

Studying of double stars have significant important:

1. To refine the theories of Formation and Evolution of Single Stars.
2. To provide constraints on the possible scenarios of Binary Star Formation.

Image Credit: ESO/L. Calçada (http://www.eso.org/public/images/eso1046a/)