A single star has turned out be a sextuplet system made of three pairs of binary stars, (pairs of stars that revolve around a common center).
One of the brightest stars in the Big Dipper can be seen by those with good eyesight to actually be a pair of stars. The two stars were named Mizar and Alcor in antiquity.
In 1617, using one of the first telescopes, Benedetto Castelli (a protégé of Galileo’s) discovered that Mizar was in fact a binary pair of stars, now named Mizar A and Mizar B.
In 1890, Mizar A turned out to be a binary pair itself. Almost two decades later, in 1908, Mizar B was also found to be a pair of stars. The entire Mizar/Alcor group became the first-known quintuple star system.
Alcor seemed to be left out of this process until Eric Mamajek from the University of Rochester discovered that Alcor is also really two stars. This discovery was made independently by Ben Oppenheimer of the American Museum of Natural History.
Mamajek isn’t done studying the grouping:
You see how the disk of Alcor B doesn't seem perfectly round?…Some of us have a feeling that Alcor might actually have another surprise in store for us.
Diagram of binary star system by Dantor, 2007.