When two stars are so nearly in the same direction as seen from Earth that they appear to be a single star to the naked eye but may be separated by the use of telescopes, they are referred to as a double star. The Pleiades star cluster A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. ... Earth (often referred to as The Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. ... 50 cm refracting telescope at Nice Observatory. ...

There are two different kinds of double star. In the case where two stars are only apparently close to each other, but which are in fact separated by a great distance along Earth's viewing axis are known as optical doubles or optical binaries. In the vast majority of cases^{[citation needed]}, however, the two stars are actually gravitationally bound and orbit each other; this is known as a binary star and is considered separately. In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Artists impression of a binary star system consisting of a black hole, with an accretion disc around it, and a main sequence star. ...

Optical doubles are distinguished from binary stars by observing them for a long period of time, usually years. If the relative motion looks linear it may be safely assumed that the motion is due to proper motion alone and that they are an optical double. Position angle changes progressively and distance oscillates between a maximum and minimum in the case of a binary. The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real...

The first recorded discovery of a double star was by Giovanni Battista Riccioli in 1650 when ζ Ursae Majoris (Mizar) was announced to be a double star. Since that time the search for double stars has been carried out very thoroughly and every star down to the 10th stellar magnitude has been carefully examined. At least 1 in every 18 stars in the northern half of the sky which are as bright as 9.0 magnitude is a close double star visible with a 36-inch refracting telescope^{[citation needed]}. Giovanni Battista Riccioli (b. ... // Events June 23 - Claimant King Charles II of England, Scotland and Ireland arrives in Scotland, the only of the three Kingdoms that has accepted him as ruler. ... Mizar (ζ UMa / ζ Ursae Majoris / Zeta Ursae Majoris) is a star in the constellation Ursa Major, lying just at the corner of the Big Dippers handle. ... The apparent magnitude (m) of a star, planet or other heavenly body is a measure of its apparent brightness; that is, the amount of light received from the object. ... A refracting telescope: 1 - tube; 2 - eyepiece; 3 - mountingl; 4 - tripod; 5 - counterweights A refracting or refractor telescope is a type of optical telescope that refracts or bends light at each end using lenses. ...