Beautiful Albireo AB

Explanation: Beta Cygni is a single bright star to the naked eye. About 420 light-years away it marks the foot of the Northern Cross, famous asterism in the constellation Cygnus. But a view through the eyepiece of a small telescope will transform it into a beautiful double star, a treasure of the night sky in blue and gold. Beta Cygni is also known as Albireo , designated Albireo AB to indicate its two bright component stars. Their visually striking color difference is illustrated in this telescopic snapshot , along with their associated visible spectrum of starlight shown in insets to the right. Albireo A, top inset, shows the spectrum of a K-type giant star, cooler than the Sun and emitting most of its energy at yellow and red wavelengths. Below, Albireo B has the spectrum of a main sequence star much hotter than the Sun, emitting more energy in blue and violet. Albireo A is known to be a binary star, two stars together orbiting a common center of mass, though the two stars are too close together to be seen separately with a small telescope. Well-separated Albireo A and B most likely represent an optical double star and not a physical binary system because the two components have clearly different measured motions through space.