Definition from Wiktionary, the free dictionary
- (astronomy) Two stars which form a stellar system, such that they orbit the point of equilibrium of their gravitational fields; a binary star.
- (astronomy) Two stars that appear to be one when seen with the naked eye, either because they orbit one another (binary stars) or happen to be in the same line of sight even though they are separated by a great distance (optical binaries).
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See also 
- "double star" in the Dictionary.com Unabridged, v1.0.1, Lexico Publishing Group, 2006.
- Random House Webster's Unabridged Electronic Dictionary, 1987-1996.