Alcor () is a

binary star A binary star or binary star system is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved as separate stars us ...

system in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Ursa Major Ursa Major, also known as the Great Bear, is a constellation in the Northern Sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear", referring to and contrasting it with nearby Ursa M ...

. It is the fainter companion of Mizar, the two

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s forming a

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

double Double, The Double or Dubble may refer to: Mathematics and computing * Multiplication by 2 * Double precision, a floating-point representation of numbers that is typically 64 bits in length * A double number of the form x+yj, where j^2=+1 * A ...

in the handle of the

Big Dipper The Big Dipper (American English, US, Canadian English, Canada) or the Plough (British English, UK, Hiberno-English, Ireland) is an asterism (astronomy), asterism consisting of seven bright stars of the constellation Ursa Major; six of them ar ...

(or Plough) asterism in Ursa Major. The two lie about 83

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s away from the

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, as measured by the ''

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

satellite.

Nomenclature

Alcor has the

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the 88 modern constellations, modern constellations visible from southern England. They are named after John Flamsteed, au ...

''80 Ursae Majoris''. ''Alcor'' derives from

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

, meaning 'faint one';List of Arabic star names, published in ''Popular Astronomy'', January 1895, by Professor Robert H. West, of the Syrian Protestant College at Beirut. notable as a faintly perceptible companion of Mizar. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Alcor'' for 80 UMa.

Mizar and Alcor

With normal eyesight magnitude 3.99 Alcor (spectral class A5V) appears at about 12

minutes of arc A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

from the second-magnitude star Mizar. Mizar's and Alcor's

proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...

s show they move together, along with most of the other stars of the Big Dipper except

Dubhe Dubhe is a multiple star system in the northern constellation of Ursa Major. It is formally designated Alpha Ursae Majoris, Latinised from α Ursae Majoris, Despite being designated "α" (alpha), it is the second-brightest object ...

and Alkaid, as members of the

Ursa Major Moving Group The Ursa Major Moving Group, also known as Collinder 285 and the Ursa Major association, is the closest Stellar kinematics#Moving groups, stellar moving group – a set of stars with common velocities in space and thought to have a common orig ...

, a mostly dispersed group of stars sharing a common birth. However, it has yet to be demonstrated conclusively that they are gravitationally bound. Recent studies indicate that Alcor and Mizar are somewhat closer together than previously thought: approximately 74,000 ± 39,000 AU, or 0.5–1.5 light-years. The uncertainty is due to our uncertainty about the exact distances from us. If they are exactly the same distance from us (somewhat unlikely) then the distance between them is only .

Alcor B

In 2009, Alcor was discovered to have a companion star Alcor B, a magnitude 8.8

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

. Alcor B was discovered independently by two groups. One group led by Eric Mamajek (

University of Rochester The University of Rochester is a private university, private research university in Rochester, New York, United States. It was founded in 1850 and moved into its current campus, next to the Genesee River in 1930. With approximately 30,000 full ...

) and colleagues at

Steward Observatory Steward Observatory is the research arm of the Department of Astronomy at the University of Arizona (UArizona). Its offices are located on the UArizona campus in Tucson, Arizona (US). Established in 1916, the first telescope and building were ...

University of Arizona The University of Arizona (Arizona, U of A, UArizona, or UA) is a Public university, public Land-grant university, land-grant research university in Tucson, Arizona, United States. Founded in 1885 by the 13th Arizona Territorial Legislature, it ...

used

adaptive optics Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in Astronomy, astronomical telescopes and laser communication systems to remove the effects of Astronomical seeing, atmo ...

on the 6.5-meter telescope at MMT Observatory. Another led by Neil Zimmerman, a graduate student at

Columbia University Columbia University in the City of New York, commonly referred to as Columbia University, is a Private university, private Ivy League research university in New York City. Established in 1754 as King's College on the grounds of Trinity Churc ...

and member of Project 1640, an international collaborative team that includes astrophysicists at the

American Museum of Natural History The American Museum of Natural History (AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. Located in Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 21 interconn ...

, the

University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...

's Institute of Astronomy, the

California Institute of Technology The California Institute of Technology (branded as Caltech) is a private research university in Pasadena, California, United States. The university is responsible for many modern scientific advancements and is among a small group of institutes ...

, and

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center (FFRDC) in La Cañada Flintridge, California, Crescenta Valley, United States. Founded in 1936 by Cali ...

, used the 5-meter

Hale Telescope The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in 1928, he orchestrated the planning, de ...

Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...

. Alcor B is one

second of arc A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

away from Alcor A. Its

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

is M3-4 and it is a

main-sequence star In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

, a red dwarf. Alcor A and B are situated 1.2 light-years away from, and are co-moving with, the Mizar quadruple system, making the system the second-closest stellar sextuplet—only Castor is closer. The Mizar–Alcor stellar sextuple system belongs to the

, a stellar group of stars of similar ages and velocities, and the closest cluster-like object to Earth.

Other names

In Arabic, Alcor is also known as Al-Sahja (the rhythmical form of the usual al-Suhā) meaning "forgotten", "lost", or "neglected". In traditional

Indian astronomy Astronomy has a long history in the Indian subcontinent, stretching from History of India, pre-historic to History of India (1947–present), modern times. Some of the earliest roots of Indian astronomy can be dated to the period of Indus Valle ...

, Alcor was known as Arundhati, wife of one of the

Saptarishi The Saptarshi ( ) are the seven seers of ancient India who are extolled in the Vedas, and other Hindu literature such as the Skanda Purana. The Vedic Samhitas never enumerate these rishis by name, although later Vedic texts such as the Br ...

. In the

Miꞌkmaq The Mi'kmaq (also ''Mi'gmaq'', ''Lnu'', ''Mi'kmaw'' or ''Mi'gmaw''; ; , and formerly Micmac) are an Indigenous group of people of the Northeastern Woodlands, native to the areas of Canada's Atlantic Provinces, primarily Nova Scotia, New Bru ...

myth of the great bear and the seven hunters, Mizar is Chickadee and Alcor is his cooking pot.

Military namesakes

USS ''Alcor'' (AD-34) and USS ''Alcor'' (AK-259) are both United States Navy ships.

References

External links

Alcor
at Jim Kaler's ''Stars'' website * {{Sky, 13, 25, 13.5, +, 54, 59, 17 A-type main-sequence stars Ursae Majoris, g Big Dipper Ursae Majoris, 80 2 Alcor Ursa Major 5062 065477 116842 BD+55 1603 M-type main-sequence stars Ursa Major moving group