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What is the difference between double star and binary star?

babaonet wrote:What is the difference between double star and binary star?

A binary star is a pair of stars which physically orbit each other. "Double star" simply refers to a pair of stars seen in the sky as closely spaced. Most double stars are true binaries, but some are simply chance alignments of otherwise unrelated stars (called visual doubles or optical doubles).

IC. When two stars physically orbit each other its called a Binary star system. On the other hand while two stars being on the same line of sight to be virtually appears as one from earth is called a double star. That means one can call a binary star as double star because they also appear as one star from the earth, right? Is there any star system where more then two stars orbits each other as binary star system do?

babaonet wrote:IC. When two stars physically orbit each other its called a Binary star system. On the other hand while two stars being on the same line of sight to be virtually appears as one from earth is called a double star. That means one can call a binary star as double star because they also appear as one star from the earth, right? Is there any star system where more then two stars orbits each other as binary star system do?

Yes, most binary stars are also (observationally) double stars. Not all, however. There are binary stars that are only known to be so because of spectroscopic measurements or eclipse detections; they can't be optically resolved as a pair. Calling such a system "double" would be misleading in most contexts.

There are many multiple star systems with more than two stars. Alpha Centauri, for instance, is a triple star system. Mizar, in the Big Dipper, consists of four stars in mutual orbit.

Thank you very much for your valuable information which through away my confusions on the subject.