Surface temperature 4,030 K Mass 4.376 × 10^30 kg (2.2 M☉) Apparent magnitude (V) 2.08 | Magnitude 2.08 Constellation Ursa Minor | |
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Radius 29.27 million km (42.06 R☉) Similar Gamma Ursae Minoris, Thuban, Delta Ursae Minoris, Polaris, Alpha Cephei | ||
Beta Ursae Minoris (β Ursae Minoris, abbreviated Beta UMi, β UMi), also named Kochab, is the brightest star in the bowl of the Little Dipper asterism (which is part of the constellation of Ursa Minor), and only slightly fainter than Polaris, the northern pole star and brightest star in Ursa Minor. Kochab is 16 degrees from Polaris and has an apparent visual magnitude of 2.08. The distance to this star from the Sun can be deduced from the parallax measurements made during the Hipparcos mission, yielding a value of 130.9 light-years (40.1 parsecs).
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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.
Nomenclature
β Ursae Minoris (Latinised to Beta Ursae Minoris) is the star's Bayer designation.
It bore the traditional name Kochab which appeared in the Renaissance and its meaning is uncertain. It may be from Arabic الكوكب al-kawkab or Hebrew כוכב kōkhāv, both of which mean 'star', or more likely derived from Alrucaba or Rucaba, a name applied to Theta Ursae Majoris. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included Kochab for this star.
In Chinese, 北極 (Běi Jí), meaning North Pole, refers to an asterism consisting of Beta Ursae Minoris, Gamma Ursae Minoris, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694. Consequently, Beta Ursae Minoris itself is known as 北極二 (Běi Jí èr, English: the Second Star of North Pole.), representing 帝 (Dì), meaning Emperor.
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.
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". 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.
Planetary companion
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.