It bore the traditional name Kochab, which appeared in the Renaissance and has an uncertain meaning. It may be from Arabic: الكوكبal-kawkab or Hebrew: כוכבkōkhāv, both of which are broadly used to describe a celestial body and can be translated as 'planet' or 'star'. (The Hebrew term was also applied to the planet Mercury, especially due to its lack of distinguishing features in comparison to other visible planets.)[13] However, it is more likely derived from Alrucaba or Rucaba, a name applied to Theta Ursae Majoris.[10]:58 In 2016, the International Astronomical Union organized a Working Group on Star Names (IAU-WGSN)[14] to catalog and standardize proper names for stars. The IAU-WGSN's first bulletin, July2016,[15] included a table of the first two batches of names approved by the IAU-WGSN, which included Kochab for this star.
This is a red giant star with a stellar classification of K4III.[3] Kochab has reached a state in its evolution where the outer envelope has expanded to 44times the radius of the Sun.[7] This enlarged atmosphere is radiating 540times as much light from its outer atmosphere as the Sun, but through a surface more than 1,470times larger than the Sun's surface area, hence at a lower effective temperature of 4,126K.[8] (The Sun's effective temperature is 5,772K.[18]) This relatively low heat gives the star the typical orange-hued glow of a K-type star.[19] It is not known for certain if Kochab is on the red giant branch, fusing hydrogen into helium in a shell surrounding an inert helium core, or on the horizontal branch fusing helium into carbon.[20]
By modelling this star based upon evolutionary tracks, its mass can be estimated as 1.4±0.2M☉.[8] A mass estimate using the interferometrically-measured radius of this star and its spectroscopically-determined surface gravity yields 2.5 ± 0.9M☉.[6] The star is known to undergo periodic variations in luminosity over roughly 4.6days, with the astroseismic frequencies depending sensitively on the star's mass. From this, a much lower mass estimate of 1.3 ± 0.3M☉ is reached.[6]
As the pole star
From around 2500BCE, as Thuban became less and less aligned with the north celestial pole, Kochab became one "pillar" of the circumpolar stars, first with Mizar, a star in the middle of the handle of the Big Dipper (Ursa Major), and later with Pherkad (in Ursa Minor).[21] Around the year 2467BCE, the true north was well determined by drawing a plumb line between Mizar and Kochab, a fact with which the Ancient Egyptians were well acquainted.[21] This cycle of the succession of pole stars occurs due to the precession of the equinoxes. Kochab and Mizar were referred to by Ancient Egyptian astronomers as 'The Indestructibles' lighting the North.[21] As precession continued, by the year 1100BCE, Kochab was within roughly 7° of the north celestial pole, with old references over-emphasizing this near pass by referring to Beta Ursae Minoris as "Polaris",[20] relating it to the current pole star, Polaris, which is slightly brighter and will have a much closer alignment of less than 0.5° by 2100CE.[20]
This change in the identity of the pole stars is a result of Earth's axial precession. After 2000BCE, Kochab and a new star, its neighbor Pherkad, were closer to the pole and together served as twin pole stars, circling the North Pole from around 1700BCE until just after 300CE. Neither star was as close to the north celestial pole as Polaris is now.[22] Today, they are sometimes referred to as the "Guardians of the Pole".[22]
Estimated to be around 2.95billion years old, give or take 1billion years, Kochab was announced to have a planetary companion around 6.1times as massive as Jupiter with an orbit of 522days.[8]
12
Kunitzsch, P.; Smart, T. (2006). A Dictionary of Modern Star Names: A short guide to 254star names and their derivations (2nd, reviseded.). Sky Publishing. ISBN1-931559-44-9.
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zh:陳久金 [Chen, Jiujin] (December 2005). 中國星座神話 [Chinese Constellation Myths] (in Chinese). Taiwan: 台灣書房出版有限公司 [Taiwan Book Publishing Co. Ltd.] ISBN978-986-7332-25-7.[pageneeded]
↑"研究資源 – 亮星中英對照表"[Research resources – Chinese-English star name comparison table]. 香港太空館[Hong Kong Space Museum] (in Chinese and English). Archived from the original on 2010-08-10. Retrieved 23 November 2010.
