It bore the traditional name Kochab, which appeared in the Renaissance and has an uncertain meaning. It may be from Arabic: الكوكبal-kawkaborHebrew: כוכבkōkhāv, both of which are broadly used to describe a celestial body and can be translated as 'planet' or 'star'. However, it is more likely derived from AlrucabaorRucaba, a name applied to Theta Ursae Majoris.[10]: 58 In 2016, the International Astronomical Union organized a Working Group on Star Names (IAU-WGSN)[13] to catalog and standardize proper names for stars. The IAU-WGSN's first bulletin, July 2016,[14] 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 K4 III.[3] Kochab has reached a state in its evolution where the outer envelope has expanded to 44 times the radius of the Sun.[8] This enlarged atmosphere is radiating 540 times as much light from its outer atmosphere as the Sun, but through a surface more than 1,470 times larger than the Sun's surface area, hence at a lower effective temperature of 4,126 K.[6] (The Sun's effective temperature is 5,772 K.[17]) This relatively low heat gives the star the typical orange-hued glow of a K-type star.[18]
By modelling this star based upon evolutionary tracks, the mass of this star can be estimated as 2.2±0.3 M☉. A mass estimate using the interferometrically-measured radius of this star and its spectroscopically-determined surface gravity yields 2.5 ± 0.9 M☉.[7] The star is known to undergo periodic variations in luminosity over roughly 4.6 days, with the astroseismic frequencies depending sensitively on the star's mass. From this, a much lower mass estimate of 1.3 ± 0.3 M☉ is reached.[7]
From around 2500 BCE, 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).[19] In fact, around the year 2467 BCE, the true north was best determined by drawing a plumb line between Mizar and Kochab, a fact with which the Ancient Egyptians were well acquainted, as they aligned the great Pyramid of Giza with it.[19] 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.[19] As precession continued, by the year 1100 BCE, 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 2100 CE.[20]
This change in the identity of the pole stars is a result of Earth's axial precession. After 2000 BCE, 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 1700 BCE until just after 300 CE. Neither star was as close to the north celestial pole as Polaris is now.[21] Today, they are sometimes referred to as the "Guardians of the Pole".[21]
Estimated to be around 2.95 billion years old, give or take 1 billion years, Kochab was announced to have a planetary companion around 6.1 times as massive as Jupiter with an orbit of 522 days.[6]
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Kunitzsch, P.; Smart, T. (2006). A Dictionary of Modern Star Names: A short guide to 254 star names and their derivations (2nd, revised ed.). Sky Publishing. ISBN1-931559-44-9.
^"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.
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