Big Dipper's Naked Eye Double Star Mizar and Alcor Are Six Stars

Summary: The Big Dipper’s naked eye double star Mizar and Alcor, which form the Horse and Rider asterism in the dipper’s handle, really are six stars.

Mizar and Alcor (upper right); bowl and part of handle of Big Dipper asterism are captured in photograph taken by astronaut Donald R. Pettit, Expedition Six NASA, ISS science officer, on board the International Space Station, March 2003: Donald R. Pettit/NASA, Public Domain, via NASA Human Spaceflight

The Big Dipper’s naked eye double star Mizar and Alcor, which form the Horse and Rider asterism in the middle of the asterismal dipper’s handle, really are six stars and qualify as the second known closest sextuple, or six-part, system.

The Horse and Rider, an asterism within one of North America’s most well-known asterisms, the Big Dipper, is associated with the constellation of Ursa Major (Latin: ursa, “female bear” + maior, “greater, larger”). Ursa Major is a familiar fixture in the northern celestial hemisphere as a circumpolar constellation with year-round visibility. The ursine configuration is the third largest of the 88 modern constellations adopted by the International Astronomical Union (IAU) in 1922.

The asterismal Big Dipper comprises the seven brightest stars in the Great Bear constellation. Four (Dubhe, Megrez, Merak, Phecda) outline the dipper’s bowl. Three (Alkaid, Alioth, Mizar) shape the dipper’s handle.

Mizar and its companion star, Alcor, stand out as the bright, naked eye double star at the bend of the Big Dipper’s handle. Mizar’s astronomical designation is Zeta Ursae Majoris (ζ Ursae Majoris; Zeta UMa; ζ UMa). The traditional name for the asterism’s horse derives from mi’zar, Arabic for “cloak, veil.” The Book of Fixed Stars (Arabic: Kitab al-Kawākib al-Thābita), a mid-10th century astronomical text written in Arabic by Persian astronomer ‘Abd al-Rahman al-Sufi (Dec. 7, 903-May 25, 986), identifies al-anâk (“the little she-goat”) as another traditional Arabic name for Zeta Ursae Majoris.

Alcor’s astronomical designation is 80 Ursae Majoris (80 UMa). English naval officer and astronomer William Henry Smyth (Jan. 21, 1788-Sept. 8, 1865) explains the traditional name of Alcor as a corruption of al-jaún, Arabic for “courser," which is a swift horse.” The Book of Fixed Stars lists four additional traditional Arabic names for 80 Ursae Majoris: as al-nuaïsch (“little stretcher”), al-saïdak (“steadfast, trustworthy”), al-schitâ (“winter”) and al-suhâ (“forgotten, neglected”).

The traditional name of al-suhâ appears in an Arabic proverb that emphasizes the star’s fabled role as a tester of keen eyesight: “I show him al-suhâ, and he shows me the moon.” Dr. George M. Bohigian, an ophthalmologist at Washington University School of Medicine in St. Louis, Missouri, has correlated the naked eye resolvability of Mizar and Alcor with the 20/20 line on the Snellen eye chart. Dr. Bohigian notes that standardization and easy reproduction account for the status of the Snellen eye chart, developed by Dutch ophthalmologist Herman Snellen (Feb. 19, 1834-Jan. 18, 1908) in 1862, as a commonly used, worldwide test of visual acuity.

The naked eye double star Mizar and Alcor form, in fact, a sextuple, or six-part, system of three pairs. The Mizar-Alcol system consists of primary star Mizar’s quadruple stellar system and companion star Alcor’s binary star system.

As a quadruple stellar system, Mizar comprises four stars paired in two orbiting sets. Mizar A’s two components have nearly identical sizes as spectral class A stars. Mizar B’s pair are slightly cooler and fainter class A stars than Mizar A’s duo. Mizar A’s companion and Mizar B’s companion are so close to them that the companions are not observable as separate stars by even the largest telescopes. Spectroscopic observations, which show periodic Doppler shifts in light wavelengths, have revealed the companions’ existence.

In February and March 2010, two separate research teams published their independent discovery of that Alcor consists of a binary star system. Alcor A’s companion, Alcor B, is a faint red dwarf.

“Red dwarfs are not commonly reported around the brighter higher mass type of star that Alcor is, but we have a hunch that they are actually fairly common,” B. Rebecca Oppenheimer, American Museum of Natural History curator and professor, says of her team’s finding, published Feb. 1, 2010, in The Astrophysical Journal. “This discovery shows that even the brightest and most familiar stars in the sky hold secrets we have yet to reveal.”

Erik E. Mamajek, associate professor in the University of Rochester’s Department of Physics and Astronomy, and his team’s three co-authors explain the significance of their finding in the March 2010 issue of The Astronomical Journal: “In comparing the Mizar-Alcor sextuplet to the known multiple star population, it appears that Mizar-Alcor is the second known closest multiple system with six (or more) components after Castor.”

The takeaway for the asterismal Big Dipper’s naked eye double star Mizar and Alcor as really numbering six stars is that naked eye viewers whose keen eyesight resolves both Alcor as the Rider and Mizar as the Horse now know that there is more to the Mizar-Alcor system than meets the eye.

(left) Mizar binary star; Mizar A's companion star detected by U.S. Naval Observatory and U.S. Naval Research Laboratory astronomers via high-resolution optical interferometer; APOD (Astronomy Picture of the Day), Wednesday, Feb. 19, 1997: J. Benson et al., NPOI, USNO, NRL, Public Domain, via NASA APOD
(right) Alcor A and Alcor B: Project 1640/AMNH (American Museum of Natural History) and Digital Universe Atlas, No usage restrictions, via EurekAlert!

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

Mizar and Alcor (upper right); bowl and part of handle of Big Dipper asterism are captured in photograph taken by astronaut Donald R. Pettit, Expedition Six NASA, ISS science officer, on board the International Space Station, March 2003: Donald R. Pettit/NASA, Public Domain, via NASA Human Spaceflight @ http://spaceflight.nasa.gov/gallery/images/station/crew-6/html/iss006e40545.html

(left) Mizar binary star; Mizar A’s companion star detected by U.S. Naval Observatory and U.S. Naval Research Laboratory astronomers via a high-resolution optical interferometer; APOD (Astronomy Picture of the Day), Wednesday, Feb. 19, 1997: J. Benson et al., NPOI (Navy Precision Optical Interferometer) Group, USNO (U.S. Naval Observatory’s Naval Oceanography Portal), NRL (U.S. Naval Research Laboratory), Public Domain, via NASA APOD @ https://apod.nasa.gov/apod/ap970219.html
(right) Alcor A and Alcor B: Project 1640/AMNH and Digital Universe Atlas, No usage restrictions, via EurekAlert! @ https://www.eurekalert.org/news-releases/893353; (image URL @ https://www.eurekalert.org/multimedia/924234)

For further information:

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