About half to two-thirds of all stars are multiple stars, in which two or more stars orbit around a common center. The numbers are still uncertain since there are also optical binary stars in addition to these physical double stars – they are two separate, distant stars coincidentally seen from Earth in the same line of sight.
You can tell whether it is an optical or a physical double star either by measuring the distances of the two stars or by measuring their position to each other over a long period of time – physical double stars orbit each other, but a round may take a few millennia, and one has to rely on a stable telescope in order to make accurate measurements. Also, the distance between two stars, which changes during a circling, can be determined only with a micrometer or photographically. However, the distance can vary so much that the stars at their closest position can no longer be separated, while they are well separated from each other when their distance is at the maximum. Double stars with known distances can also be used to test the resolution of your scope. For this, the stars should be about the same brightness. Stars with very different levels of brightness are more difficult to separate, because one star can easily outshine the other.
It is worthwhile to keep an eye not only on the brightness difference between the members of a pair, but also on the color differences.
Spectroscopic binary stars are very close pairs of stars that cannot be observed directly. They look like ordinary stars and can be detected only with special equipment which measures changes in the stellar spectra. These stars have very high orbital velocities because they are very close to each other.