October is a colorful month, with autumn foliage at its peak here in New England. There’s a splash of color in the northern sky as well, and it’s epitomized by the beautiful double star beta (?) Cygni, better known as Albireo.
This stellar showpiece combines a magnitude 3.3 star of spectral class K8 with a 5.5 mag B9-type star. The differences in spectral class yield contrasting colors or yellow and blue, more poetically described as “topaz and sapphire.” A generous 34 arc-second separation makes Albireo an easy target for small-aperture telescopes. In fact, the colors seem more intense in a 4-inch telescope than in a 10-inch. Albireo is a “must” target for autumn star parties, and is sure to surprise and delight the viewer who assumes all stars are white.
Albireo was first observed by Flamsteed in 1681. In 1976, the spectroscope revealed that the brighter component (Albireo A) is an extremely close binary pair. The companion is similar to Albireo B, and lies a mere 0.4 arc-seconds away – an impossible split for all but the largest optical telescopes. On his “Stars” website, Jim Kaler notes, ‘From Albireo B, Albireo A would appear as brilliant orbiting orange and blue points about half a degree apart, the K giant shining with the light of 35 full Moons, the close class B companion at about half of that”. For an interesting “live” view of Albireo, check out the YouTube video clip.
There has been some debate as to whether Albireo A and B form a true binary system or are merely optically aligned. At a distance of over 385 light years, the two physically separated by 60 times the diameter of our solar system. Recent measures show that they are, indeed, traveling together and must have an orbital period of many thousands of years.
Albireo is the most-observed double star in the northern sky, but is it the most beautiful? Next month, we look at a serious challenger. Can you guess its identity?