A double star is when two stars appear close to each other as seen from Earth. There are two kinds of double star: visual binaries and optical binaries. Visual binaries are considered to be a true binary star system and lie close enough together in space to interact gravitationally such that the stars orbit each other. Optical binaries (aka optical doubles), on the other hand, are two stars that only appear to be close together, and are actually separated by a great distance in space and are not gravitationally bound to each other. 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. In the case of a true binary, the position angle changes progressively and the distance between the two stars oscillates between a maximum and minimum. The first recorded discovery of a true binary star system was by Giovanni Battista Riccioli in 1650, when he found Mizar (ζ Ursae Majoris) was a double star. Since that time, the search for double stars has been carried out thoroughly and every star down to the 10th stellar magnitude has been examined. At least 1 in 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 telescope[citation needed]. Examples * Horse and Rider—a double star (unknown if interacting) * Sirius A and B—a visual binary system * Winnecke 4—an optical binary system Retrieved from "http://en.wikipedia.org/"
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