Double stars are two stars which are locked into orbit with each other due
to gravitational interaction. Probably the most beautiful double star in
the Northern Hemisphere is Albireo in Cygnus. The pair consists of a
bright golden giant star and a blue companion which is 3 times hotter,
which creates this extraordinary colour contrast.
The yellow star, Beta Cygni A, has a luminosity of 1,000 Suns and is actually in itself a binary system as it has a magnitude 5.5 companion star in a very close, fast orbit - too close to be resolved in this image.
This snapshot of Albireo was one of the first images taken with my Canon EOS 350D
DSLR camera. By placing a thin rod in front on the scope in two sub frames
(one horizontal, one vertical), the combined image shows the star's
diffraction spikes which looks quite attractive for this target. Normally
these spikes are an artefact of some telescope designs which use spider
vanes to hold the secondary mirror in place.