Albireo in the constellation Cygnus (Swan) appears as a single point of light to the naked eye, but a telescope shows it to be a binary star: two stars bound together by their mutual gravitational attraction. The stars take thousands of years to orbit around each other. More than half of all stars live in binary systems. The Sun is somewhat unusual in that it is solitary.

In Albireo one can see both stars -- one star is cool and appears orange, the other hotter and blue. In more typical binaries, the stars are so close together that only one source can be seen. Even so, it is often possible to infer the presence of a companion, as well as the properties of its orbit. This generally involves studying the stellar spectrum (the distribution of its light over many wavelengths, in the same way that a rainbow shows the wavelength distribution of sunlight). Knowing the binary orbit lets us calculate the masses of both stars.

Radio and X-ray images of Albireo show nothing, because normal stars are much brighter in visible light than at other wavelengths. We can make radio and x-ray images of the Sun at all wavelengths because it is so close. But for a more distant star, we usually cannot detect radio waves or X-rays, unless the star is somehow unusual, such as a star in orbit around a black hole. These binaries can be very bright in X-rays.