Zeta Ursae Majoris


Distance (Light Years) 78.2 ± 1.1
Apparent Magnitude 2.23
Color (B-V)0.02

Names For This Star

This star is also known as Mizat or Mirza. The names derive from the Arabic Mi'zar meaning a girdle or waist-cloth. The name seems to have been applied to the star as the result of an error.

Description of the Star

Mizar is a hot, white A1VpSrSi main sequence star having a luminosity about 63 times that of the sun. The spectral type implies anomalously strong lines associated with strontium and silicon. This type of star is associated with an especially strong magnetic field.

The spectrum is peculiar for an A1V star, but the A1V spectral type implies an effective temperature of 9600 K, a diameter of more than twice that of the sun, and a mass about three times that of the sun.

The Mizar Star System

Mizar is a multiple star system. The primary Mizar A has a fainter companion Mizar B, at Apparent Magnitude 3.95. Mizar B is also a type A main sequence star separated from A by 14 arc seconds, which corresponds to a projected separation of 340 AU or more than 8 times the diameter of the orbit of planet Pluto.

Spectral analysis reveals Mizar A itself to be a binary star consisting of two main sequence A2V stars orbiting around each other with a period of 20.5 days in a rather elliptical orbit having a semimajor axis of about 27 million miles.

Spectral analysis shows Mizar B itself to be a binary star, so that there are at least four stars in the Mizar system.

The Ursa Major Cluster

The fourth magnitude star Alcor can be seen at 12 arc minutes separation from Mizar, corresponding to a projected distance of more than 0.25 ly. However, since Alcor is about 3 ly farther from the earth than the Mizar system, it is unlikely that Alcor is a part of the Mizar system.

The Ursa Major Cluster

According to Burnham, Mizar is part of a star cluster including at least 16 other stars from the Ursa Major area of the sky, as well as Alioth, Merak, Alcor, Phecda, and Megrez in the Big Dipper. The Ursa Major cluster represents an ellipsoidal volume of space about 30 ly long by 18 ly in width.

Other Designations For This Star


79 Ursae Majoris

Hipparcos Identifier (HIP Number)


Harvard Revised (HR Number)


Henry Draper Catalog (HD Number)


Bonner Durchmusterung (BD Number)

BD+55 1598
Smithsonian Astrophysical
Observatory compendium (SAO Number)

Fundamental Katalog (FK5 Number)



Copyright © 1998 - 2010 by Arnold V. Lesikar,
Professor Emeritus
Dept. of Physics, Astronomy, and Engineering Science,
St. Cloud State University,St. Cloud, MN 56301-4498

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