23 Ursae Majoris

Lore & Background

23 Ursae Majoris has the Bayer designation h Ursae Majoris and the Flamsteed designation 23 Ursae Majoris. The system is moving closer to Earth with a heliocentric radial velocity of −10 km/s. The primary component is a yellow-white F-type subgiant with 1.9 times the Sun's mass, 2.9 times the Sun's radius, and 15 times the Sun's luminosity at an effective temperature of 6,651 K. Orbiting at an angular separation of 22.7 arcseconds is a 9th magnitude secondary companion. There is also a magnitude +10.5 optical companion at an angular separation of 99.6 arcseconds.

Reader's Guide

23 Ursae Majoris is significant as a binary system visible to the naked eye, with a primary that is a yellow-white F-type subgiant. Its inclusion in the Arabic asterism Sarīr Banāt al-Na'sh (the Throne of the daughters of Na'sh) and the pond asterism Al-Haud gives it cultural and historical context. According to the catalogue of stars in Technical Memorandum 33-507, this star was designated Alhaud IV, part of a seven-star group including f, τ, e, θ, υ, and φ. The system's measured properties—distance, radial velocity, and stellar parameters—provide concrete data for stellar evolution studies. The presence of a wide optical companion further enriches its observational profile.

Did You Know?

• 23 Ursae Majoris is approximately 77.7 light years from the Sun.
• It has a heliocentric radial velocity of −10 km/s, moving closer to Earth.
• The primary component has 1.9 times the Sun's mass and 2.9 times the Sun's radius.
• It was designated Alhaud IV in the Arabic asterism Al-Haud.

A Pioneering Discovery in Binary Stars

Mizar occupies a distinguished place in the history of observational astronomy. In 1617, Benedetto Castelli reportedly prompted Galileo Galilei to examine the star through his telescope, and Galileo went on to produce a detailed record of its doubled appearance, making Mizar possibly the first telescopic binary recognized by European astronomers. Around 1650, Riccioli independently noted the star's dual nature. The story deepened in the late nineteenth century when Antonia Maury, through her spectral classification work, identified Mizar A as the first spectroscopic binary ever discovered, with an orbit formally published in 1890. A further surprise arrived in 1908, when Mizar B was also revealed to be a spectroscopic binary with a six-month orbital period. The system's full complexity was finally confirmed visually in 1996, when the Navy Prototype Optical Interferometer captured extremely high-resolution images of the Mizar A pair, a full 107 years after their initial spectroscopic detection.

Anatomy of a Quadruple System

What appears to the naked eye as a single second-magnitude point of light is actually a quadruple stellar system. Mizar A consists of two nearly identical A2Vp stars, chemically peculiar objects whose slowly rotating photospheres exhibit elevated strontium and silicon abundances. Each component shines at roughly 35 times the Sun's luminosity, with a surface temperature near 9,000 kelvin and a radius of about 2.4 solar radii. They orbit one another in approximately 20 days, 12 hours, and 55 minutes. Mizar B, the fainter companion, is a single-lined spectroscopic binary classified as an Am star, with a primary mass of 1.85 solar masses and a tiny secondary of only 25 percent the Sun's mass. The two visible components are separated by 14.4 arcseconds, and Mizar B swings within 380 astronomical units of Mizar A over a period of thousands of years. The entire system sits roughly 83 light-years from Earth, a distance measured by the Hipparcos satellite, and belongs to the Ursa Major Moving Group.

Names Across Cultures and Centuries

The star's traditional name, Mizar, traces back to the Arabic word miʼzar, meaning an apron or wrapper. In 2016, the International Astronomical Union's Working Group on Star Names formally approved Mizar as the proper name for component Aa, though popular usage extends it to all four stars. Arab astronomers called the Mizar-Alcor pair the horse and rider, and the two stars have long served as a classic test of minimal visual acuity. In Indian astronomy, Mizar is identified as Vashistha, one of the Saptarishi. Chinese tradition offers two names: Běi Dǒu liù, the Sixth Star of the Northern Dipper, and Kāi Yáng, the Star of the Opener of Heat, while Taoist thought personifies it as the Lu star. Among the Mi'kmaq, Mizar is Chickadee in the myth of the great bear and seven hunters, with Alcor representing his cooking pot. Its formal designations include the Bayer label Zeta Ursae Majoris and the Flamsteed number 79 Ursae Majoris.

Mizar Beyond the Sky

Mizar's cultural footprint extends well beyond the observatory. Two United States Navy vessels have carried the name: USS Mizar, a cargo and passenger liner converted for naval service, and USNS Mizar. In music, the band Steely Dan wove the star into their song Sign In Stranger, featured on the album The Royal Scam. Science fiction writer Stephen Tall set his 1971 short story The Bear With the Knot on His Tail in the Mizar system, where the Earth ship Stardust encounters a race of friendly, spherical aliens. The star also appears in numerous astronomical reference works, including Jim Kaler's Stars website and the WikiSky catalog, which aggregates data from surveys such as DSS2, SDSS, GALEX, IRAS, and Hydrogen-alpha imaging. Its enduring role as a vision test and its layered discovery history have kept Mizar a recurring subject in both popular and technical literature for centuries.

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