It is estimated that up to 10% of all visible stars represent binary star systems. Binaries are star systems where two or more stars revolve around each other. When these revolutions of one component star around the other leads to a change in magnitude, we have an example of an eclipsing binary. Sirius, one of the brightest stars in the sky, is a binary system involving one dominant bluish component and a less dominant secondary component. A number of binaries have their component stars being of different colors which makes for very attractive photos (see below).

Note: Optical doubles represent unrelated stars coincidentally aligned when viewed from earth and which are dramatically different from true doubles (binaries).

Note: For an excellent observation list involving double stars, see The Astronomical League's Double Star List and which formed the basis for the samples below.