Mizar and Alcor

Mizar

Mizar and Alcor in constellation Ursa Major
Observation data Epoch J2000      Equinox J2000
Constellation	Ursa Major
Right ascension	13h 23m 55.5s
Declination	+54° 55′ 31″
Apparent magnitude (V)	2.23
Characteristics
Spectral type	A2Vp/A2Vp/A1m/A5V[1]
U−B color index	0.09
B−V color index	0.13
Astrometry
Radial velocity (Rv)	−9 km/s
Proper motion (μ)	RA: 121.23 mas/yr Dec.: −22.01 mas/yr
Parallax (π)	39.4 ± 0.3 mas
Distance	82.8 ± 0.6 ly (25.4 ± 0.2 pc)
Absolute magnitude (MV)	0.33
Other designations
Mizat, Mirza, Mitsar, Vasistha, ζ Ursae Majoris, ζ UMa, Zeta UMa, 79 Ursae Majoris, BD+55 1598A, CCDM J13240+5456AB, FK5 497, GC 18133, HD 116656, HIP 65378, HR 5054, PPM 34007, SAO 28737, WDS J13239+5456Aa,Ab.

The Mizar–Alcor stellar sextuple system consists of the quadruple system Mizar and the binary system Alcor.

Description

The Big Dipper's bowl and part of the handle photographed from the International Space Station. Mizar and Alcor are at the upper right.

Mizar (Zeta Ursae Majoris, Zeta UMa, ζ Ursae Majoris, ζ UMa) in Arabic known as "سها" is a quadruple system of two binary stars in the constellation Ursa Major and is the second star from the end of the Big Dipper's handle. Its apparent magnitude is 2.23 and its spectral class is A1V. Mizar's name comes from the Arabic مئزر mīzar, meaning a waistband or girdle.

With normal eyesight one can make out a faint companion just to the east, about 12 minutes of arc from Mizar, named Alcor or 80 Ursae Majoris. Alcor is of magnitude 3.99 and spectral class A5V.

Mizar and Alcor together are sometimes called the "Horse and Rider", and the ability to resolve the two stars with the naked eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. Arabic literature says that only those with the sharpest eyesight can see the companion of Mizar. Astronomer Sir Patrick Moore has suggested that this in fact refers to another star which lies visually between Mizar and Alcor. The name the Arabs used for Alcor was سها (suha), meaning either the ‘forgotten’ or ‘neglected’ one.^[2]

Mizar and Alcor's proper motions show they move together (they are both members of the Ursa Major Moving Group), raising the question of whether they are gravitationally bound. In 2009, it was independently reported by two groups of astronomers (Eric Mamajek et al., and Zimmerman et al.) that Alcor actually is itself a binary, consisting of Alcor A and Alcor B (a red dwarf star), and that this binary system is most likely gravitationally bound to Mizar, bringing the full count of stars in this complex system to six.^[3][4] These studies also demonstrated that the Alcor binary and Mizar quadruple are somewhat closer together than previously thought: approximately 74,000 ± 39,000 astronomical units or 0.5–1.5 light years.^[5]

The whole six-star system lies about 83 light-years away from Earth, as measured by the Hipparcos astrometry satellite.^[6][7][8] The components are all members of the Ursa Major moving group, a mostly dispersed group of stars sharing a common birth, as determined by proper motion. The other stars of the Big Dipper, except Dubhe and Alkaid, belong to this group as well.

History

Big Dipper map

Mizar is known as Vasistha and Alcor is known as Arundhati in traditional Indian astronomy.^[9] The pair is considered to symbolize marriage (Vashishtha and Arundhati were a married couple) and, in some Hindu communities, priests conducting a wedding ceremony allude to or point out the constellation as a symbol of the closeness marriage brings to a couple.^[10]

In Japanese mythology, Alcor is known as the lifespan star or "jumyouboshi" (寿命星) as it was believed that one who could not see this star would pass away by year's end. Of incidental note, the popular Japanese manga, Fist of the North Star, used this legend as a model for its death omen star (死兆星), in which it was said that people who saw the star would die later in the year.

"The Arabs in the desert regarded it as a test of penetrating vision; and they were accustomed to oppose "Suhel" to "Suha" (Canopus to Alcor) as occupying respectively the highest and lowest posts in the celestial hierarchy. So that "Vidit Alcor, at non lunam plenam", (Latin for "he saw Alcor, but not the full moon") came to be a proverbial description of one keenly alive to trifles, but dull of apprehension for broad facts."

Al Sahja was the rhythmical form of the usual Suha; and it appears as Al "Khawwar," the Faint One, in an interesting list of Arabic star-names, published in Popular Astronomy for January, 1895, by Professor Robert H. West, of the Syrian Protestant College at Beirut.

The 14th century Arabian lexicographer Al Firuzabadi called it Our Riddle, and Al Sadak, the Test,—correctly Saidak, True; while the 13th century Persian astronomical writer Al Kazwini said that "people tested their eyesight by this star." Humboldt wrote of it as being seen with difficulty, and Arago similarly alluded to it; but some now consider it brighter than formerly {p. 446} and no longer the difficult object that it was, even in the clear sky of the Desert; or as having increased in angular distance from Mizar.

Although the statement has been made that Alcor was not known to the Greeks, there is an old story that it was the Lost Pleiad Electra, which had wandered here from her companions and became Alopex, the Fox; a Latin title was Eques Stellula, the Little Starry Horseman; Eques, the Cavalier, is from the 17th-century German astronomer Bayer; while the Horse and his Rider, and, popularly, in England, Jack on the Middle Horse, are well known, Mizar being the horse. The Persian astronomer Al Biruni (973–1048 A.D.) mentioned its importance in the family life of the Arabs on the 18th day of the Syrian month Adar, the March equinox; and a modern story of that same people makes it the infant of the walidan of the three Banat.

More components of the Mizar system were discovered with the advent of the telescope and spectroscopy; a fine, easily split visual target, Mizar was the first telescopic binary discovered—most probably by Benedetto Castelli who in 1617 asked Galileo Galilei to observe it. Galileo then produced a detailed record of the double star. Later, around 1650, Riccioli wrote of Mizar appearing as a double. The secondary star, Mizar B, has magnitude 4.0 and spectral class A7, and comes within 380 AU of the primary; Mizar A and Mizar B take thousands of years to revolve around each other.

Mizar A was the first spectroscopic binary to be discovered, by Pickering in 1889. Some spectroscopic binaries cannot be visually resolved and are discovered by studying the spectral lines of the suspect system over a long period of time. The two components of Mizar A are both about 35 times as bright as the Sun, and revolve around each other in about 20 days 12 hours and 55 minutes. Mizar B was later found to be a spectroscopic binary as well, its components completing an orbital period every six months. In 1996, 107 years after their discovery, the components of the Mizar A binary system were imaged in extremely high resolution using the Navy Prototype Optical Interferometer.

Other names

Chinese Taoism personifies Ursa Majoris as the Lu star.

Mizar is Chickadee and Alcor is his cooking pot in the Mi'kmaq myth of the great bear and the seven hunters.^[11]

Mizar and Alcor in military

• USS Mizar (AF-12) and USNS Mizar (T-AGOR-11), both of United States navy ships.
• USS Alcor (AD-34) and USS Alcor (AK-259), both of United States navy ships.

See also

• Mizar in fiction

References

External links

• Mizar and Alcor articles at Jim Kaler's Stars website
• First very high resolution imaging of Mizar A (using aperture synthesis)
• A New View Of Mizar (a comprehensive article about the system)
• Mizar and Alcor on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images

Coordinates: 13^h 23^m 55.5^s, +54° 55′ 31″