Alcor (/ˈælkɔːr/)[9] is a binary star system in the constellation of Ursa Major. It is the fainter companion of Mizar, the two stars forming a naked eye double in the handle of the Big Dipper (or Plough) asterism in Ursa Major. The two both lie about 83 light-years away from the Sun, as measured by the Hipparcos astrometry satellite.
Star in the constellation of Ursa Major
Alcor
Location of Alcor (Mizar is circled, Alcor is invisible beside it at this scale)
Alcor has the Flamsteed designation80 Ursae Majoris. Alcor was originally Arabicالسهاal-Suhā, meaning either the 'forgotten' or 'neglected' one; notable as a faintly perceptible companion of Mizar.[10]
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[11] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[12] included a table of the first two batches of names approved by the WGSN; which included Alcor for 80 UMa.[13]
With normal eyesight Alcor appears at about 12 minutes of arc from the second-magnitude star Mizar. Alcor is of magnitude 3.99 and spectral class A5V.
Mizar's and Alcor's proper motions show they move together, along with most of the other stars of the Big Dipper except Dubhe and Alkaid, as members of the Ursa Major Moving Group, 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 17,800AU (0.281 light-years).[2]
Alcor B
The Big Dipper's bowl and part of the handle photographed from the International Space Station. Mizar and Alcor are at the upper right.
In 2009, Alcor was discovered to have a companion star Alcor B, a magnitude 8.8 red dwarf.[14]
Alcor B was discovered independently by two groups. One group led by Eric Mamajek (University of Rochester) and colleagues at Steward Observatory University of Arizona used adaptive optics on the 6.5-meter telescope at MMT Observatory. Another led by Neil Zimmerman, a graduate student at Columbia University and member of Project 1640, an international collaborative team that includes astrophysicists at the American Museum of Natural History, the University of Cambridge's Institute of Astronomy, the California Institute of Technology, and NASA's Jet Propulsion Laboratory, used the 5-meter Hale Telescope at Palomar Observatory.[4]
Alcor B is one second of arc away from Alcor A.[4] Its spectral type is M3-4 and it is a main-sequence 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-known stellar sextuplet—only Castor is closer.[15] The Mizar–Alcor stellar sextuple system belongs to the Ursa Major Moving Group, a stellar group of stars of similar ages and velocities,[2] and the closest cluster-like object to Earth.
Other names
In traditional Indian astronomy,[16] Alcor was known as Arundhati, wife of one of the Saptarishi.
In Arabic, Al-Sahja was the rhythmical form of the usual al-Suhā. It appears as الخوّارal-Khawwar, 'the Faint One'.[17]
In the Miꞌkmaq myth of the great bear and the seven hunters,[18] 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.
Mamajek, Eric E.; Kenworthy, Matthew A.; Hinz, Philip M.; Meyer, Michael R. (2010). "Discovery of a Faint Companion to Alcor Using MMT/AO 5 μm Imaging". The Astronomical Journal. 139 (3): 919–925. arXiv:0911.5028. Bibcode:2010AJ....139..919M. doi:10.1088/0004-6256/139/3/919. S2CID51834159.
Gray, R. O; Garrison, R. F (1989). "The late A-type stars - Refined MK classification, confrontation with Stromgren photometry, and the effects of rotation". Astrophysical Journal Supplement Series. 70: 623. Bibcode:1989ApJS...70..623G. doi:10.1086/191349.
Zimmerman, Neil; Oppenheimer, Ben R; Hinkley, Sasha; Brenner, Douglas; Parry, Ian R; Sivaramakrishnan, Anand; Hillenbrand, Lynne; Beichman, Charles; Crepp, Justin R; Vasisht, Gautam; Roberts, Lewis C; Burruss, Rick; King, David L; Soummer, Rémi; Dekany, Richard; Shao, Michael; Bouchez, Antonin; Roberts, Jennifer E; Hunt, Stephanie (2010). "Parallactic Motion for Companion Discovery: An M-Dwarf Orbiting Alcor". The Astrophysical Journal. 709 (2): 733–740. arXiv:0912.1597. Bibcode:2010ApJ...709..733Z. doi:10.1088/0004-637X/709/2/733. S2CID6052794.
Jones, Jeremy; White, R. J; Boyajian, T; Schaefer, G; Baines, E; Ireland, M; Patience, J; Ten Brummelaar, T; McAlister, H; Ridgway, S. T; Sturmann, J; Sturmann, L; Turner, N; Farrington, C; Goldfinger, P. J (2015). "The Ages of A-Stars. I. Interferometric Observations and Age Estimates for Stars in the Ursa Major Moving Group". The Astrophysical Journal. 813 (1): 58. arXiv:1508.05643. Bibcode:2015ApJ...813...58J. doi:10.1088/0004-637X/813/1/58. S2CID16600591.
David, Trevor J; Hillenbrand, Lynne A (2015). "The Ages of Early-type Stars: Strömgren Photometric Methods Calibrated, Validated, Tested, and Applied to Hosts and Prospective Hosts of Directly Imaged Exoplanets". The Astrophysical Journal. 804 (2): 146. arXiv:1501.03154. Bibcode:2015ApJ...804..146D. doi:10.1088/0004-637X/804/2/146. S2CID33401607.
Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev.ed.). Cambridge, Massachusetts: Sky Pub. ISBN978-1-931559-44-7.
V.Chandran (1993-01-01). Astronomy Quiz Book. Pustak Mahal, 1993. ISBN978-81-223-0366-7. ... the seven rishis in the constellation Saptarishi (Ursa Major) ... In Vasishta (Zeta), its tiny companion star is named after Arundhati, the wife of Vasishta ... today known by their Arabic names Dubhe (Kratu), Merak (Pulaha), Phekda (Pulastya), Megrez (Atri), Benetnash (Marichi) and Mizar (Vasishta) ...
list of Arabic star names, published in Popular Astronomy, January 1895, by Professor Robert H. West, of the Syrian Protestant College at Beirut.
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