Albireo (β Cyg) is a showpiece double star and one of the great small telescope night sky objects. To the unaided eye it's the fifth brightest star in Cygnus (combined mag. +2.9) and the lone star that marks the head of "the Swan" constellation. When viewed through a small telescope or even good 10x50 binoculars, Albireo is split into two components, a mag. +3.1 primary (Albireo A) and a mag. +5.1 secondary (Albireo B). These stars are separated by a generous 34.3 arc seconds but the most pleasing aspect of this grouping and what makes it such a special object is its beautiful colour contrast, a brilliant yellowish primary coupled with a soft blue secondary.
Cygnus is one of the most recognizable constellations during the Northern Hemisphere summer and autumn months. It represents a Swan flying down the plane of the Milky Way. Containing many bright stars, positioned at the heart of the constellation is a prominent asterism in the shape of a large cross. Consequently it's often referred to as the Northern Cross. At the northern end of the cross is Deneb (α Cyg); a blue-white supergiant star (mag. +1.3) and the brightest star in the constellation. Deneb marks the "tail" of the Swan and located at the opposite side of the cross, Albireo, marks the "head" of the Swan.
Albireo is easily split using low powers through any small telescope or large binoculars (20x80mm for example). The surrounding field contains numerous fainter stars that add to the beauty of the view. At higher magnifications the separation of the two stars appears large. Larger scopes show more background stars and the colour contrast of the primary and secondary stars is strong and more evident.
Albireo is located about 430 light-years from the Earth. It's not known whether Albireo A and B are actually orbiting each other in a physical binary system. If they are their orbital period is probably at least 100,000 years. In 1976, it was discovered using speckle interferometry that Albireo A itself is a binary star. In the Washington Double Star Catalog, the pair are designated as components Aa and Ac with a current separation of 0.4 arc seconds. This is generally beyond the splitting capabilities of the average size amateur telescope. Those with a scope of 500mm (20-inch) diameter or larger may be able to split the Aa/Ac pair under exceptionally stable atmospheric conditions.
Albireo is a must see object and without doubt one of the finest double stars in the night sky. Its beauty comes from the striking colour contrast of the component stars, a brilliant yellow primary and a soft blue secondary. With a wide separation, the double is easily split with small telescopes or even good binoculars, as a result, it's a favourite target for amateur astronomers and one of the most observed double stars in the night sky.
Albireo Data Table
|Components||A (Aa and Ac) and B|
|Angular separation (arc secs)||35.3 (AB), 0.4 (AaAc)|
|Position angle (degrees)||54 (AB)|
|Other Designations||Beta Cygni, β Cyg|
Albireo A Components Data Table
|Name||Albireo Aa||Albireo Ac|
|Washington Double Star catalog (WDS)||19307+2758Aa||19307+2758Ac|
|Flamsteed||6 Cygni A||6 Cygni A|
|RA (J2000)||19h 30m 43.286s||19h 30m 43.295s|
|DEC (J2000)||+27d 57m 34.84s||+27d 57m 34.62s|
|Apparent Magnitude Aa and Ac Combined||3.08||3.08|
|Other Designations||Beta Cygni A, β Cyg A||Beta Cygni A, β Cyg A|
Albireo B Component Data Table
|Washington Double Star catalog (WDS)||19307+2758B|
|Flamsteed||6 Cygni B|
|RA (J2000)||19h 30m 45.395s|
|DEC (J2000)||+27d 57m 55.00s|
|Other Designations||Beta Cygni B, β Cyg B|