Are Mizar and Alcor Binary Stars?

Summary: Alcor and Mizar, were the first binary stars -- a pair of stars that orbit each other -- ever known. Now, astronomers have made the surprise discovery that Alcor is also actually two stars, and is apparently gravitationally bound to the Mizar system, making the whole group a sextuplet.

What type of star is Mizar?

Mizar /ˈmaɪzɑːr/ is a second-magnitude star in the handle of the Big Dipper asterism in the constellation of Ursa Major. It has the Bayer designation ζ Ursae Majoris (Latinised as Zeta Ursae Majoris). It forms a well-known naked eye double star with the fainter star Alcor, and is itself a quadruple star system.

Why are Mizar and Alcor called binary stars?

Astronomers now believe that the Alcor binary system is gravitationally bound to the Mizar quadruple system – making six stars in all, where we see only two with the eye. ... Mizar is really four stars, and Alcor is really two stars. So what we see as two stars are really six in one!