The clearest evidence for the existence of binary stars is provided by visual binary
stars. We look at these stars in a telescope and see a pair of stars rather than just one.
Look here for a picture of one
Some prominent examples in the sky are the star Castor ( 
Geminorum) and
Cygni. Figure 21.1 of your textbook shows the observations of Castor. Typically,
the distance between the components of a visual binary star are many astronomical units
of tens of astronomical units, and the periods are decades or even centuries.
