Albireo (β Cyg, β Cygni, Beta Cyg, Beta Cygni) is the fifth brightest star in the constellation Cygnus. Although it has the Bayer designation beta, it is fainter than Gamma Cygni, Delta Cygni, and Epsilon Cygni. Albireo appears to the naked eye to be a single star of magnitude 3 but through a telescope, even low magnification views resolve it into a double star. The brighter yellow star (actually itself a very close binary system) makes a strikingly beautiful colour contrast with its fainter blue companion star ensuring that Albireo is one of amateur astronomers' most cherished summertime observing targets.
Approximately 380 light-years (120 pc) away from the Earth, when viewed with the naked eye, Albireo appears to be a single star. However, in a telescope it readily resolves into a double star, consisting of Albireo A (amber, apparent magnitude 3.1), and Albireo B (blue-green, apparent magnitude 5.1.) Separated by 35 seconds of arc, the two components provide one of the best contrasting double stars in the sky due to their different colors. It is not known whether the two components are orbiting around each other in a physical binary system. If they are, their orbital period is probably at least 100,000 years.
In 1976, component A was itself discovered to be a binary star, using speckle interferometry and the 2.1-meter telescope at the Kitt Peak National Observatory. An orbit for the pair has since been computed using interferometric measurements, but as only approximately a quarter of the orbit has been observed, the orbital parameters must be regarded as preliminary. The current angular separation between the components of around 0.4 arcseconds is tantalizingly close to the limit which visual observations through instruments of at least 20" in size can resolve, under very rare perfect seeing conditions.
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