Albireo

albireo star,beta cygni

Albireo, Beta Cygni (β Cyg) is a double star located in the constellation Cygnus. With an apparent magnitude of 2.90, it is the fifth brightest star in Cygnus and the faintest of the stars that form the Northern Cross. Albireo lies at an approximate distance of 415 light years from Earth. It is one of the best known contrasting double stars in the sky, best seen in a small telescope.

albireo star,beta cygni
Albireo A, image: Henryk Kowalewski (CC BY-SA 2.5)

Star system 

Albireo appears as a single star to the naked eye, but is in fact a binary system with two components that can be seen in a small telescope. The brighter of the two stars – designated Beta Cygni A or Beta1 Cygni – appears yellow, while the fainter one – Beta Cygni B or Beta2 Cygni – is blue. The two make one of the most striking contrasting pairs in the sky.

Albireo, image: Wikisky

The two stars are separated by 34.3 arcseconds, but whether or not they form a physical pair or merely lie in the same line of sight is still uncertain. If they are a binary system, they take at least 75,000 years to complete an orbit. However, as the stars do not share a common proper motion across the sky, they are not likely to be gravitationally bound.

albireo a,albireo b
Albireo, image: Wikimedia Commons/Hewholooks (CC BY-SA 3.0)

The angular separation between the two stars has not changed in over 260 years and the position angle of Albireo B has only changed slightly. However, the stars lie at different distances. The Hipparcos mission placed Albireo A at 434 ± 20 light years from Earth and Albireo B at 401 ± 13 light years. The data from the Gaia mission indicates distances between 330 and 390 light years for both stars, but is insufficient to determine whether the stars are gravitationally bound to each other.

The Washington Double Star Catalog (WDS) lists 10 more faint companions, none of them brighter than magnitude 10. Only one of the companions is closer to Albireo A than Albireo B, while others are scattered across an area up to 142’’ away from the primary.

Albireo A

Albireo A is a suspected triple star system consisting of components designated Albireo Aa, Albireo Ab, and Albireo Ac. Albireo Aa and Albireo Ac are separated by only 0.4 arcseconds and can only be resolved in 20-inch or larger telescopes. Albireo Ab is even closer to Albireo Aa. When it was discovered, the separation between the two stars was 0.1’’ and now it is 0.0’’, making it impossible to resolve the pair.

The primary component, Albireo Aa, has an apparent magnitude of 3.18 and the stellar classification K2II, indicating an orange bright giant. The star has a mass 14.52 times that of the Sun, which makes it a supernova candidate. With an effective temperature of 4,270 K, it shines with 1,200 solar luminosities. It is a slow spinner, with a projected rotational velocity of only 1.4 km/s. Interferometric measurements have yielded a diameter of 4.834 milliarcseconds, which translates into a radius 69 times that of the Sun.

The status and properties of the two possible companions are still not entirely clear. Albireo Ac is a blue-white star of the spectral type B8:p. It has an apparent magnitude of 5.82. The star is 3.84 times more massive than the Sun and about 950 times more luminous.

Albireo Aa and Albireo Ac are separated by 40 astronomical units on average and take almost 100 years to complete an orbit.

Albireo B

Albireo B is a blue-white main sequence star of the spectral type B8Ve. It has an apparent magnitude of 5.11. The star has a radius 2.59 times that of the Sun and a mass of 3.7 solar masses. With a temperature of 13,200 K, it is 230 times more luminous than the Sun. The star’s estimated age is 100 million years. Unlike Albireo Aa, it is a very fast spinner, a Be star with a projected rotational velocity of at least 250 km/s at the equator. As a result, it is losing material and is surrounded by a circumstellar disk of gas.

Facts

Even though it has the Bayer designation Beta Cygni, Albireo is fainter than Sadr (Gamma Cygni), Fawaris (Delta Cygni), and Aljanah (Epsilon Cygni).

Albireo will be the brightest star in the sky around the year 3,870,000, when it comes within 80 light years of the solar system. At its peak (around the year 4,610,000), it will shine at magnitude -0.52, which is a little fainter than Canopus is today.

The spectrum of the primary star was discovered to be composite in the late 19th century, when the star was photographed with the 11-inch Draper Telescope as part of the Henry Draper Memorial project. Observations between 1898 and 1918 confirmed that it was a double star, revealing variations in the star’s radial velocity. The two components were listed as HD 183912 and HD 183913 in the Henry Draper Catalogue in 1923.

A companion was resolved at a separation of 0.125’’ in 1978 using speckle interferometry with the 1.93m telescope at the Haute-Provence Observatory (OHP) in southeast France. The companion is designated Beta Cygni Ab in the Washington Double Star Catalog.

Observations published in 1982 revealed a companion at an angular separation of 0.44’’. The star was resolved through speckle interferometry with the 2.1m telescope at the Kitt Peak National Observatory (KPNO) in Arizona in 1976. It was listed as Beta Cygni Ac in the Washington Double Star Catalog (WDS) and as Beta Cygni C in the Catalog of Components of Double and Multiple Stars (CCDM). (This is not the same component C as the one listed in the WDS; the latter is only a visual companion.) The estimated orbital period for the pair is about 214 years.

Name

The name Albireo (pronunciation: /ælˈbɪrioʊ/) comes from a misunderstanding of the words ab ireo in the description of the constellation Cygnus in the 1515 Almagest. The name is believed to have originated from the Greek word ornis, which was the name for the Cygnus constellation and later became urnis in Arabic. When the name was translated into Latin, it was mistakenly believed to refer to Erysimon, the Greek name for hedge mustard, and became ireo (Latin for hedge mustard). The words ab ireo were eventually viewed as a miscopy and changed into al-bireo.

The name was officially approved by the International Astronomical Union’s (IAU) Working Group on Star Names (WGSN) on July 20, 2016. It formally applies only to the component Beta Cygni Aa, but is informally used for the entire system and its other individual components.

In medieval Arabic astronomy, Albireo was known as Al Minhar al Dajajah, meaning “the hen’s beak.” The name referred to the star’s position in the constellation. (Albireo is still sometimes known as the “beak star,” as it marks the beak of the celestial Swan.) This name appeared in the Egyptian astronomer Al Achsasi Al Mouakket’s Calendarium in the 17th century. It was translated into Latin as Rostrum Gallinae.

Location

Albireo is very easy to find because it is relatively bright and part of a familiar summer asterism, the Northern Cross. It lies at the base of the cross, on the opposite side of the constellation to the bright Deneb, the star that marks the top of the cross and the celestial Swan’s tail. Deneb (Alpha Cygni), Sadr (Gamma Cygni) and Albireo outline the pole of the cross, while Aljanah (Epsilon Cygni) and Fawaris (Delta Cygni) form the crossbeam.

The Northern Cross, image: Wikisky

The Northern Cross is quite prominent on a clear night and lies within the borders of the Summer Triangle, a larger summer asterism formed by Deneb with Vega in the constellation Lyra and Altair in Aquila.

Albireo can be used to find Messier 56, a bright (mag. 8.3) globular cluster located in Lyra. The cluster lies about halfway between Albireo and Sulafat, slightly closer to Albireo. Sulafat is the second brightest star in Lyra and lies on the opposite side of the constellation to Vega.

Albireo, Sulafat and Messier 56, image: Wikisky

Constellation

Albireo is located in the constellation Cygnus, the Swan. Easily recognizable for the Northern Cross asterism, Cygnus is the 16th largest constellation in the sky. It is one of the Greek constellations, which first appeared in Ptolemy’s Almagest in the 2nd century CE.

Cygnus constellation, image: Roberto Mura

Cygnus is home to a number of popular telescope targets, including the double stars Fawaris and Mu Cygni, the open clusters Messier 29, Messier 39 and NGC 6910, and the bright spiral galaxy NGC 6946, also known as the Fireworks Galaxy. Bright nebulae in the constellation include the North America Nebula (NGC 7000) and the Pelican Nebula (IC 5070) near Deneb, the Crescent Nebula (NGC 6888) and the Gamma Cygni Nebula (IC 1318) near Sadr, the Veil Nebula (NGC 6960, 6962, 6979, 6992 and 6995) near Aljanah, and the Blinking Planetary Nebula (NGC 6826).

The best time of year to see the stars and deep sky objects of Cygnus is during the month of September, when the constellation is high in the evening sky. For northern hemisphere observers, Cygnus is conspicuous in the night sky from June to December.

The 10 brightest stars in Cygnus are Deneb (Alpha Cyg, mag. 1.25), Sadr (Gamma Cyg, mag. 2.23), Aljanah (Epsilon Cyg, mag. 2.48), Fawaris (Delta Cyg, mag. 2.87), Albireo (Beta Cyg, mag. 2.90), Zeta Cygni (mag. 3.21), Xi Cygni (mag. 3.73), Tau Cygni (mag. 3.65 – 3.75), Iota2 Cygni (mag. 3.77), and Kappa Cygni (mag. 3.814).

Albireo – Beta Cygni

Spectral classK2II (Albireo Aa), B8:p (Albireo Ac), B8Ve (Albireo B)
Apparent magnitude2.90
Distance (Albireo A)430 ± 20 light years (133 ± 6 parsecs)
Parallax (Albireo A)7.51 ± 0.33 mas
Radial velocity (Albireo A)-24.07 km/s
Proper motion (Albireo A)RA: -7.17 mas/yr
Dec.: -6.15 mas/yr
ConstellationCygnus
DesignationsAlbireo, Beta Cygni, β Cyg, 6 Cygni, CCDM J19307+2758, ADS 12540, WDS 19307+2758
Beta Cygni AHR 7417, HIP 95947, FK5 732, BD+27 3410, GC 26953, GCRV 11939, IRAS 19286+2751, 2MASS J19304330+2757347, PPM 109139, SAO 87301
Beta Cygni AaHD 183912, TYC 2133-2964-1
Beta Cygni AbHD 183913, TYC 2133-2964-2
Beta Cygni BHD 183914, HIP 95951, HR 7418, BD+27 3411, GC 26956, GCRV 11941, PPM 109141, SAO 87302, 2MASS J19304540+2757549, TYC 2133-2963-1, Gaia DR1 2026113335413644928, Gaia DR2 2026113339752723456

Albireo Aa

Spectral classK2II
B-V colour index+1.13
V-R colour index+0.92
Apparent magnitude3.18
Absolute magnitude-2.45
Mass14.52 M
Radius69 R
Luminosity1,200 ± 200 L
Temperature4,270 K
Metallicity-0.1 dex
Rotational velocity1.4 km/s
Surface gravity2.0 cgs
Right ascension19h 30m 43.286s
Declination+27° 57′ 34.84″

Albireo Ac

Spectral classB8:p
B-V colour index+0.09
V-R colour index+0.09
Apparent magnitude5.82
Absolute magnitude-0.25
Mass3.84 M
Luminosity950 ± 250 L
Temperature12,000 – 30,000 ± 100 K
Right ascension19h 30m 43.295s
Declination+27° 57′ 34.62″

Albireo B

Spectral classB8Ve
U-B colour index-0.32
B-V colour index-0.10
Apparent magnitude5.11
Distance400 ± 10 light years (123 ± 4 parsecs)
Parallax8.16 ± 0.25
Radial velocity-18.80 km/s
Proper motionRA: -0.953 mas/yr
Dec.: -1.624 mas/yr
Mass3.7 ± 0.8 M
Luminosity230 ± 90 L
Radius2.59 R
Temperature13,200 ± 600 K
Age100 million years
Surface gravity4.00 ± 0.15 cgs
Right ascension19h 30m 45.3961s
Declination+27° 57′ 54.990″