Mizar and Alcor form the famous naked-eye pair in the bend of the Big Dipper’s handle. Located approximately 83 light-years from Earth, they appear to be two closely spaced stars but are actually a remarkable six-star system: Mizar contains four stars, while Alcor contains two.
Easy to Find in the Big Dipper’s Handle
Mizar and Alcor are located together at the bend in the Big Dipper’s handle, making them among the easiest stellar pairs to find in the northern sky. Mizar is the brighter star, with an apparent magnitude of approximately 2.2, while Alcor shines at magnitude 4.0 just above it. They are separated by about 12 arcminutes—roughly two-fifths the apparent width of the full Moon—and can be distinguished without optical aid under reasonably clear skies.
The Horse and Rider Once Tested Eyesight
Mizar and Alcor are traditionally known as the Horse and Rider, with brighter Mizar representing the horse and fainter Alcor its rider. Distinguishing them without optical aid was used as a test of sharp eyesight, particularly among the ancient Persians and Bedouin Arabs. Many people can separate the pair under clear, reasonably dark skies, although light pollution and poor atmospheric conditions make Alcor harder to see.
Together They Form a Six-Star System
Mizar’s four stars and Alcor’s two stars together form a remarkable six-star system. The Mizar and Alcor subsystems are separated by approximately 17,800 astronomical units, equivalent to about 0.28 light-years. Despite this enormous distance, measurements of their positions and motions indicate that they are gravitationally bound, completing a shared orbit that may take roughly 750,000 years. The stars producing nearly all the system’s visible light are hot A-type main-sequence stars with surface temperatures of roughly 8,000–9,000 kelvins, while the faint companions are much cooler, lower-mass stars.
A Telescope Splits Mizar in Two
Although Mizar appears as one star to the naked eye, a small telescope separates it into Mizar A and Mizar B, two blue-white stars about 14 arcseconds apart. This makes Mizar one of the night sky’s easiest and most attractive telescopic double stars. The apparent pair is physically connected, but the stars lie several hundred times farther apart than Earth and the Sun and probably take thousands of years to orbit each other.
Mizar Is Actually Four Stars
Mizar A and Mizar B are each binary systems in their own right, making Mizar a four-star system. The two stars forming Mizar A orbit each other every 20.5 days, while the components of Mizar B complete an orbit in approximately 176 days. These close pairs cannot be separated through an ordinary telescope and were instead discovered from the shifting spectral lines produced as their stars move around one another.
Alcor Has a Hidden Red-Dwarf Companion
Alcor also appears to be a single star but consists of two stars. Its dominant member, Alcor A, is a white A-type main-sequence star, while Alcor B is a much fainter red dwarf discovered through adaptive-optics observations in 2009. The two are separated by roughly 25–30 times the average distance between Earth and the Sun, but Alcor B is overwhelmed by the glare of its brighter companion and cannot be seen through an ordinary telescope.
Mizar Changed the Study of Binary Stars
Mizar has played an exceptional role in the history of astronomy. Benedetto Castelli recorded its telescopic companion in 1617, making it one of the first double stars identified through a telescope. Mizar A and Mizar B became the first double star photographed in 1857, while observations of Mizar A in 1889 revealed the first known spectroscopic binary—a pair detected through regularly shifting lines in its spectrum. In 1996, astronomers using optical interferometry directly resolved Mizar A’s two close stars, which previously had been detectable only through their combined light.
Mizar and Alcor Were Born with Other Big Dipper Stars
Mizar and Alcor belong to the Ursa Major Moving Group, a collection of stars that formed from the same cloud of gas and continue traveling through space in a similar direction. Five of the Big Dipper’s seven principal stars belong to this group; only Dubhe and Alkaid are unrelated stars moving along different paths. The group is now widely dispersed rather than held together as a compact star cluster, but its shared motion preserves evidence of its common origin approximately 400 million years ago.