Some multiple-star systems contain more than two stars. One famous example is the Trapezium, a nexus offour prominent stars at the heart of the Great Nebula in Orion.
When the orbit of the fainter member of a binary system takes it in front of the brighter member, the system fluctuates in brightness. Such a system, called an eclipsing binary, is of interest to variable-star observers.
Some binary stars are so close together that they cannot be visually separated. However, a spectroscope will detect tiny, periodic shifts in the motion of the brighter star. These shifts are caused by the pull of the invisible companion's gravity as it orbits. The A component of the visual double star Mizar in Ursa Major is an example of a spectioscopic binary.