Binary systems in which both components can be seen are called visual binary stars :若双星中所有成员皆可以看得到,我们便称之为目视双星。
Sometimes , in a binary star system , one star will pass in front of the other producing what we call eclipsing binary . it can be treated as a special kind of visual binary . eclipsing binary systems are variable stars see below :有些双星系统,其中一颗星会在另一颗星前经过,产生周期性的光度变化,我们称这种双星为食双星,基本上我们可以把食双星看成为目视双星的一种。