Beta Ursae Minoris
Epoch J2000 Equinox J2000
|Right ascension||14h 50m 42.32580s|
|Declination||+74° 09′ 19.8142″|
|Apparent magnitude (V)||2.08|
|Spectral type||K4 III|
|U−B color index||+1.78|
|B−V color index||+1.47|
|Radial velocity (Rv)||+16.96 km/s|
|Proper motion (μ)||RA: −32.61 mas/yr
Dec.: +11.42 mas/yr
|Parallax (π)||24.91 ± 0.12 mas|
|Distance||130.9 ± 0.6 ly
(40.1 ± 0.2 pc)
|Mass||2.2 ± 0.3 M☉|
|Radius||42.06 ± 0.91 R☉|
|Luminosity||390 ± 25 L☉|
|Surface gravity (log g)||1.83 cgs|
|Metallicity [Fe/H]||–0.29 dex|
|Rotational velocity (v sin i)||8 km/s|
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 130.9 light-years (40.1 parsecs).
Kochab and its neighbor Pherkad are both stars that can be seen by the naked eye and are sometimes referred to as the "Guardians of the Pole" or the "Indestructibles". They served as twin pole stars, Earth's North pole stars, from 1500 BC until 500 AD. Neither star was as proximitous to the celestial north pole as Polaris is now. 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.
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.
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.
The origin of the name Kochab appeared in the Renaissance and its meaning is uncertain; it may be from Arabic الكوكب al-kawkab or Hebrew kōkhābh, both of which mean "star", or more likely derived from Alrucaba or Rucaba, a name applied to Theta Ursae Majoris.
In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of β Ursae Minoris, γ Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694. Consequently, β Ursae Minoris itself is known as 北極二 (Běi Jí èr, English: the Second Star of North Pole.), representing 帝 (Dì), meaning Emperor.
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