Albireo

Template:Infobox Double Star/Maintenance/Single Coordinates

Albireo or β Cygni (Beta Cygni, short β Cyg) is a double star in the constellation of Swan.

Albireo is a well-known double star, separable already in small telescopes from 5 cm aperture. It is considered one of the most beautiful double stars of the northern sky and is a popular observation and demonstration object for amateur astronomers, because the components show a visually clearly visible color contrast - the main star appears yellow-orange, the companion bluish. The main star Albireo A (β1 Cygni) is 3.1 mag bright and has spectral class K3 II. The companion Albireo B (β2 Cygni) has a brightness of 5.1 mag and has spectral class B8 Ve. The stars have an angular separation of 34.6″, the companion takes a position angle of 54° to the main star (Ep. 2020).

However, Albireo A is also a close physical binary star and consists of the components Albireo Aa (V = 3.18 ± 0.03 mag) and Albireo Ac (V = 5.82 ± 0.19 mag). Albireo Ac was discovered in 1976 during speckle interferometry observations with the 2.1-m telescope at Kitt Peak National Observatory and lies 0.3″ (position angle 52°, Ep. 2020) from Aa. Orbital calculations give widely varying orbital periods, such as 214 years (Scardia and others 2008) or 69 years (Roberts & Mason 2018). In 1978, the 1.93-m telescope at the Observatoire de Haute-Provence used speckle interferometry to discover another component at a distance of only 0.125″ from Aa, which was named Albireo Ab. Albireo Aa and Ab have only been successfully resolved twice (as of 2018). Although the component Ac was discovered before the component Ab, its discovery was published after that of Ab, so the designations "Ab" and "Ac" appear to be reversed.

Albireo Aa is a bright giant of spectral type K2 or K3 with an effective temperature of about 4270 K. The exact spectral type of Albireo Ac is difficult to measure because of its closeness to Aa. Spectral type determinations such as B0 V (ten Brummelaar et al. 2000) or B8:p (Ginestet & Carquillat 2002) are available. The effective temperatures would differ accordingly, namely 30 000 K in the former case or about 12 500 K in the latter case. Based on available orbital calculations and astrometric data (Hipparcos, Gaia DR2), the mass of Albireo Aa was calculated to be 0.7 M⊙. Such a small value is surprising and seems doubtful. The typical value (according to the evolutionary path in the HR diagram) is over 5 M⊙. Albireo B is a Be main-sequence star with a mass of 4.0 ± 0.8 M⊙ and an effective temperature of 13 200 ± 600 K.

In medieval Arabic sources, Albireo is mentioned as منقار الدجاجة / minqār ad-daǧāǧa, meaning "beak of the hen". However, the word "albireo" is likely to derive from the Greek word ornis ("bird") for the constellation of the swan in ancient Greece, and is the result of a series of misunderstandings and mistranslations. On July 20, 2016, the IAU adopted the proper name Albireo as the standardized proper name for the star β1 Cygni only. Accordingly, the star β2 Cygni has no proper name.

Optical or physical double star?

Regarding the question whether Albireo A and B form an optical or physical double star, the data situation shows an inconsistent picture. In the past, it was assumed that Albireo A and B are a physical double star, since it is very unlikely that two such bright stars (3.1 mag and 5.1 mag) would lie so close together (0.01°) in the sky by pure chance.

Parallaxes of 8.46 ± 0.58 mas (Albireo A) and 8.67 ± 0.65 (Albireo B) mas were measured with Hipparcos (Hipparcos Catalogue, 1997), corresponding to distances of 386 ± 29 Lj and 376 ± 31 Lj, respectively. Thus, within the error bars, a physical double star is conceivable. On the other hand, quite different values for the proper motions of Albireo A (µα = -7.09 ± 0.52 mas/year, µδ = -5.63 ± 0.66 mas/year) and Albireo B (µα = -1.95 ± 0.52 mas/year, µδ = -0.98 ± 0.66 mas/year) were measured. According to modern Gaia measurements (Gaia DR2, 2018), the parallaxes are 9.95 ± 0.60 mas (Albireo A) and 8.38 ± 0.17 mas (Albireo B), resulting in distances of 328 ± 21 Lj and 389 ± 8 Lj, respectively. Consequently, an optical double star would be present. However, the actual parallax inaccuracy is probably higher than the formal error, since Gaia is not designed to measure very bright stars (G < 6) and the close binary nature of component A is not taken into account.

A 2018 short study, after analyzing Hipparcos and Gaia-DR2 data, concludes that Albireo is "most likely" (most probably) an optical binary. However, this does not argue against a common origin of the components, e.g. as members of a disintegrating motion cluster.