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.

This can happen either because the pair forms a binary star, i.e. a binary system of stars in mutual orbit, gravitationally bound to each other, or because it is an optical double, a chance alignment of two stars in the sky that lie at different distances. Binary stars are important to stellar astronomers as knowledge of their motions allows direct calculation of stellar mass and other stellar parameters.

Since the beginning of the 1780s, both professional and amateur double star observers have telescopically measured the distances and angles between double stars to determine the relative motions of the pairs. If the relative motion of a pair determines a curved arc of an orbit, or if the relative motion is small compared to the common proper motion of both stars, it may be concluded that the pair is in mutual orbit as a binary star. Otherwise, the pair is optical.Multiple stars are also studied in this way, although the dynamics of multiple stellar systems are more complex than those of binary stars.

Double star may refer to:

Two stars that orbit close to each other are called binary star.

Double Star is a science fiction novel by Robert A. Heinlein, first serialized in Astounding Science Fiction (February, March, April 1956) and published in hardcover the same year. It received the 1956 Hugo Award for Best Novel (his first).

Plot summary

The story, which is told in the first person, centers on down-and-out actor Lawrence Smith (stage name Lorenzo Smythe, a.k.a. "The Great Lorenzo"). A brilliant actor and mimic (or so we are told, by Smith himself), he is down to his last coin when a spaceman hires him to double for an unspecified public figure. It is only when he is on his way to Mars that he finds out he will have to impersonate one of the most prominent politicians in the solar system (and one with whose views Smith deeply disagrees): John Joseph Bonforte. Bonforte is the leader of the Expansionist coalition, currently out of office but with a good chance of changing that at the next general election. Bonforte has been kidnapped by his political opponents, and his aides want Smith to impersonate Bonforte while they try to find him.