Albireo, or Beta Cygni, is actually the fifth brightest star in the constellation Cygnus, at magnitude 3.0. Its name originates in misunderstandings and bad translations. Albireo's original Arabic name was "Minqar al-Dajajah", meaning "the hen's beak". Latin scholars misunderstood and thought that the name came from a kind of herb, and translated it into "ab ireo" ("from ireus"). Later this was treated as a misprint and transcribed as "al-bireo".
Components
Albireo is a celestial gem, a pair of stars colored indigo and gold, embedded in the glittering background of the Cygnus Star Cloud. Albireo may well be the most observed double star because of its stunning colors and easy-to-find location at the foot of the Northern Cross. When viewed with a telescope it readily resolves into a double star separated by 35", consisting of amber Albireo A at magnitude 3.1, and sapphire Albireo B at magnitude 5.1. It is not known whether the two components are orbiting each other as a binary system. If they are, their orbital period is at least 75,000 years.
In 1976, component A was itself discovered to be a binary star. An orbit for the pair has since been computed using interferometric measurements; on average they are separated by about 40 AU, and take almost 100 years to go about each other in a highly eccentric orbit. But as only approximately a third of the orbit has been observed, its parameters must be regarded as preliminary. The current angular separation between the components, around 0.4", is tantalizingly close to the limit which can be resolved visually through 20" or larger instruments under perfect seeing conditions.
Physical Properties
The Albireo system is about 380 light years away. The brighter components of Albireo A are a stable helium-fusing giant of class K3, and a hydrogen-fusing main-sequence star of class B9. The K3 giant has a temperature of around 4400 K, a luminosity 950 times the Sun's, a radius 50 times solar, and a mass of about 5 times solar. Its close companion comes in at 11,000 K, 100 solar luminosities, and 3.2 solar masses.
Albireo B is similar to Albireo A's companion. It is a class B8 main-sequence star with a temperature of 12,100 K, a luminosity of 190 suns, and a mass of 3.3 suns. It is a very rapid rotator, with an equatorial velocity of at least 250 km/sec, and a rotation period less than 0.6 days. As is often the case among such fast-spinning stars, Albireo B is a "B-emission star" that is surrounded by a gas disk of its own making. From Albireo B, Albireo A would appear as brilliant orange and blue points about half a degree apart, the K giant shining with the light of 35 full Moons, and its class B companion at about half of that.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]