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Beta Ursae Minoris (β UMi, β Ursae Minoris) is the brightest star in the bowl of the "Little Dipper" (which is part of the constellation Ursa Minor), and only slightly fainter than Polaris, the northern pole star and brightest star in Ursa Minor. It has the traditional name Kochab. Kochab is 16 degrees from Polaris and has an apparent visual magnitude of 2.08.〔 The distance to this star can be deduced from the parallax measurements made during the Hipparcos, yielding a value of .〔 Kochab and its neighbor Pherkad served as twin pole stars, circling the North Pole, from 1500 BC until 500 AD. Ancient Egyptian astronomers referred to them as "The Indestructibles".〔(article "Planet Earth: Ancient Astronomy Calendars, Navigation, Predictions" )〕 Neither star was as proximitous to the celestial north pole as Polaris is now. Today, they are sometimes referred to as the "Guardians of the Pole."〔 Due to precession of the equinoxes, the previous holder of the title was Thuban, and the next was the present-day Polaris. This succession of pole stars is a result of Earth's precessional motion. Amateur astronomers can use Kochab as a very precise guide for setting up a telescope, as the celestial north pole is located 43 arcminutes away from Polaris, very close to the line connecting Polaris with Kochab.〔 == Properties == This is a giant star with a stellar classification of K4 III.〔 It is 130 times more luminous than the Sun. Kochab has reached a state in its evolution where the outer envelope has expanded to 42〔 times the girth of the Sun. This enlarged atmosphere is radiating 390〔 times as much luminosity as the Sun from its outer atmosphere at an effective temperature of 4,030 K.〔 This heat gives the star the orange-hued glow of a K-type star.〔 By modelling this star based upon evolutionary tracks, the mass of this star can be estimated as 2.2 ± 0.3 that of the Sun. A mass estimate using the interferometrically-measured radius of this star and its spectroscopically-determined surface gravity yields 2.5 ± 0.9 solar masses.〔 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 solar is reached.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beta Ursae Minoris」の詳細全文を読む スポンサード リンク
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