Star Facts: Mizar and Alcor

Alcor & Mizar
Image Credit: Gary Fildes

Mizar (Zeta Ursae Majoris) and Alcor (80 Ursae Majoris) form a true binary that can be seen without optical aid as a stellar pair in the handle of the Big Dipper asterism of Ursa Major. Both are blue main sequence dwarf stars that are loosely bound to one another, despite being separated by a hefty one light year distance. Mizar is in fact a four-star system, while Alcor is a binary system in its own right, which means that the entire system consists of six stars, in which a two-star system orbits a four-star system once every 750,000 years or so.

The Mizar/Alcor pair are situated around 86 light years from Earth, and are sometimes known as the “Horse and Rider” as in ancient times they served as a useful test of visual acuity, although even people with less than 20/20 vision these days are able to see both stars. This led the British astronomer Sir Patrick Moore to suggest that the test most likely refered to another star located visually between Mizar and Alcor, called Sidum Ludovicianum, or “Ludwig’s Star.

Quick Facts

• Constellation: Ursa Major
• Coordinates: RA 13h 23m 55.5s |Dec. +54° 55′ 31″
• Distance: 86 light years
• Star Type(s): Mizar (A2V + A2V + A1V), Alcor (A5V)
• Mass: Mizar Aa + Bb (2.43 sol), Alcor (1.8 sol)
• Diameter: Mizar Aa + Bb (4.8 sol), Alcor (1.76 sol)
• Apparent Magnitude: Mizar (+2.27), Alcor (+3.99)
• Luminosity: Mizar Aa + Bb (33.3 sol) Alcor (13.4 sol)
• Surface Temperature: Mizar Aa + Bb (9,000K), Alcor (8,000K)
• Age: Undetermined

Visibility

Since the Big Dipper is north circumpolar, it is visible to most observers in the northern hemisphere throughout the year. Look for the Mizar/Alcor system in the middle of the Big Dipper’s handle, at the spot where a “kink” bends the handgrip.

Alcor Mizar
Image Credit: Jerry Lodriguss

Physical Properties

The star systems of Mizar and Alcor are separated by around 1.1 light year, with the quadruple star system of Mizar divided up as follows: Mizar Aa and Mizar Ab separated by 0.29 AU, with an orbital period of 20.454 days; and Mizar Ba and Mizar Bb separated by 3.12 AU, with an orbital period of 57 years. By contrast, the Alcor binary system consisting of the stellar classified blue dwarf Alcor A and red dwarf Alcor B, are separated from each other by between 0.5 and 1.5 light-years. Apart from the fact that the stars Mizar and Alcor form a naked-eye binary system, and that they each are the primary star in multiple-star systems that orbit each other, there is nothing to distinguish them from the other A-type stars in the Big Dipper asterism.

Both primary stars are also members of the Ursa Major Moving Group, an assembly of stars that share a common proper motion, and therefore likely have a common origin. However, even though the Moving Group share a common proper motion, there is as yet no irrefutable evidence that the Group members are gravitationally bound to each other.

Mizar-Alcor-sextuple-model
Image Credit: A model of the Mizar system by Bob King showing the three pairs which make up the sextuple system, all of which are main sequence A-class stars , with the exception of Alcor’s red dwarf companion. Model not to scale.

History

The star Mizar derives its traditional name from the Arabic term, “mi’zar”, which could mean “apron, wrapper, covering, or cover”. On the other hand, Alcor derives its traditional name from the Arabic term, “suha”, which could mean either “[the] forgotten” or “[the] neglected’ one”, presumably as reference to the fact that Mizar’s light overpowers, or “covers” the much feebler light of Alcor.

In some Japanese mythological traditions, the star Alcor is known as “jumyoboshi”, which roughly translates into “[the] lifespan star”, since it was believed that persons who could not observe the star would die before the end of the current year.

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