Object: beta Cygni (Albireo)

Constellation
RA / DEC
Magnitude A / B
Separation
Position angle
Spectral class
Colour A / B
: Cygnus
: 19:30:43 / +27.57
: 3.4 / 4.7
: 34.7"
: 55°
: K3II / B0V
: bright deep yellow / blue-white
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Telescope
Eye-piece
Magnification / Field of View '
: 23/08/08 / 22.50
: Landgraaf
: 4 / 4
: Orion Optics UK 300 mm
: 17 mm Nagler type 4
: 94 / 52
Stacks Image 608
Observing report
This double is the most beautiful in the Summer skies. The bright deep yellow and the soft blue-white glow jump out at you, even at the lowest magnification. I noticed that when I stopped down my 300mm telescope to 140mm, the colours became even more obvious. With brighter stars you definitely see more colour using a small aperture telescope, at lowest possible magnification.

Beta Cygni is well separated and because of that easy to split. Albireo was best in my 17mm Nagler. At lower magnifications it seemed as if the direct surroundings of Albireo were more ore less empty. But in the 17mm Nagler I saw a lot of fainter stars around Albireo. I sketched only a few of them to get my orientation right. To the southwest of the bright yellow A-component I saw two very tiny specks of light close together, and to the east there were three evenly bright stars forming a triangle. I could not detect any interesting asterisms ore other field stars that where special.
Notes
In the case of Albireo, most observers see the same colors, however in different hues, yellow and blue. Webb sees them as “yellow and blue”, Smyth sees them as “topaz yellow and sapphire blue”, Burnham describes them as “golden yellow and sapphire blue” and Sissy Haas sees “brilliant citrus orange and vivid royal blue” I myself describe them as “bright deep yellow and blue-white. According to Kaler (The hundred greatest stars) the brighter yellow A-component is a class K giant, and like Arcturus pausing in the process of dying, fusing Helium to Carbon in its core.

The blue-white B-component is a hot class B dwarf, a star that is just like our sun burning hydrogen on the main sequence. It is not known if Albireo is a true visual double star or just an optical double, a chance alignment. The most recent measurements show that the A-component lies at 385 light years, the B-component at 376 light years. But in both distances are uncertain by 25 light years. If they make a true physical double star, their period would be 10.000 years, which makes the motion insensible. The bright yellow K giant has a spectroscopic companion, a blue class B dwarf. The blue-white B component is a B-emission star. Its spectrum shows that it is rotating so rapidly (over 250 km per second at the equator) that it has spun off a surrounding disk.