A report on Double star, Binary star, Mizar and Star system

This occurs because the pair either forms a binary star (i.e. a binary system of stars in mutual orbit, gravitationally bound to each other) or is an optical double, a chance line-of-sight alignment of two stars at different distances from the observer.

It forms a well-known naked eye double star with the fainter star Alcor, and is itself a quadruple star system.

A star system of two stars is known as a binary star, binary star system or physical double star.

Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.

Mizar is a visual double with a separation of 14.4 arcseconds, each of which is a spectroscopic binary.

Mizar, in Ursa Major, was observed to be double by Benedetto Castelli and Galileo.

The more general term double star is used for pairs of stars which are seen to be close together in the sky.

Early examples include Mizar and Acrux.

Mizar is often said to have been the first binary star discovered when it was observed in 1650 by Giovanni Battista Riccioli, p. 1 but it was probably observed earlier, by Benedetto Castelli and Galileo. Later, spectroscopy of its components Mizar A and B revealed that they are both binary stars themselves.

It is estimated that approximately one third of the star systems in the Milky Way are binary or multiple, with the remaining two thirds being single stars.