There are a few standout systems that contain three or more stars visible through small telescopes. It is quite a challenge for the naked eye to separate this into two stars. A good eye combined with a telescope and a steady atmosphere can split each of these stars into two. On some nights, I have managed to see all four stars in a 2.5 inch telescope with 80x magnification. On nights when the atmosphere is not as steady, one can fail to split them with an 8 inch scope.
Many double stars appear single through any telescope.
A few of these are of interest to visual astronomers because the stars eclipse each other. When the fainter star eclipses the brighter one as seen from Earth, the light we receive from the star system drops and the star appears fainter. These eclipsing binaries are one of many classes of variable stars . Algol , a fairly bright star in the constellation Perseus, is the best-known eclipsing binary. It has a period of about three days.