Kochab ,

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

Beta Ursae Minoris (β Ursae Minoris, abbreviated β UMi, Beta UMi), is the brightest

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the bowl of the

Little Dipper Ursa Minor (, contrasting with Ursa Major), also known as the Little Bear, is a constellation located in the far northern sky. As with the Great Bear, the tail of the Little Bear may also be seen as the handle of a ladle, hence the North Amer ...

asterism (which is part of the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Ursa Minor Ursa Minor (, contrasting with Ursa Major), also known as the Little Bear, is a constellation located in the far northern celestial hemisphere, northern sky. As with the Great Bear, the tail of the Little Bear may also be seen as the handle of ...

), and only slightly fainter than

Polaris Polaris is a star in the northern circumpolar constellation of Ursa Minor. It is designated α Ursae Minoris (Latinisation of names, Latinized to ''Alpha Ursae Minoris'') and is commonly called the North Star or Pole Star. With an ...

, the northern

pole star A pole star is a visible star that is approximately aligned with the axis of rotation of an astronomical body; that is, a star whose apparent position is close to one of the celestial poles. On Earth, a pole star would lie directly overhead when ...

and brightest star in Ursa Minor. Kochab is 16 degrees from Polaris and has an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

of 2.08. The distance to this star from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

can be deduced from the

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements made during the

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions and distances of ...

mission, yielding a value of .

Amateur astronomer Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Even though scientific research may not be their primary goal, some amateur astronomers m ...

s can use Kochab as a precise guide for

equatorial mount An equatorial mount is a mount for instruments that compensates for Earth's rotation by having one rotational axis, called ''polar axis'', parallel to the Earth's axis of rotation. This type of mount is used for astronomical telescope mount, tel ...

alignment: The celestial north pole is located 38

arcminutes A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

away from

, very close to the line connecting Polaris with Kochab.

Nomenclature

''β Ursae Minoris'' ( Latinised to ''Beta Ursae Minoris'') is the star's

. It bore the traditional name ''Kochab'', which appeared in the

Renaissance The Renaissance ( , ) is a Periodization, period of history and a European cultural movement covering the 15th and 16th centuries. It marked the transition from the Middle Ages to modernity and was characterized by an effort to revive and sur ...

and has an uncertain meaning. It may be from or , 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.) However, it is more likely derived from or , a name applied to Theta Ursae Majoris. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(IAU-WGSN) to catalog and standardize proper names for stars. The IAU-WGSN's first bulletin, July 2016, included a table of the first two batches of names approved by the IAU-WGSN, which included ''Kochab'' for this star. In

Chinese astronomy Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The Ancient China, ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categori ...

, ('

North Pole The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's rotation, Earth's axis of rotation meets its surface. It is called the True North Pole to distingu ...

') refers to an asterism consisting of Beta Ursae Minoris,

Gamma Ursae Minoris Gamma Ursae Minoris (γ Ursae Minoris, abbreviated Gamma UMi, γ UMi), also named Pherkad , is a star in the northern constellation of Ursa Minor. Together with Beta Ursae Minoris (Kochab), it forms the end of the dipper pan of the "Litt ...

, 5 Ursae Minoris, 4 Ursae Minoris and Σ 1694. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Beta Ursae Minoris itself is ('the Second Star of North Pole'), representing ('emperor').

Properties

This is a

red giant star A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The stellar atmosphere, outer atmosphere is inflated and tenuous, making the radius large and the surface t ...

with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of K4 III. Kochab has reached a state in its

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

where the outer envelope has expanded to 44 times the radius of the Sun. 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 The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 4,126 K. (The Sun's effective temperature is 5,772 K.) This relatively low heat gives the star the typical orange-hued glow of a

K-type star In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

. It is not known for certain if Kochab is on the

red giant branch The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass st ...

, fusing hydrogen into helium in a shell surrounding an inert helium core, or on the

horizontal branch The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proc ...

fusing helium into carbon. By modelling this star based upon evolutionary tracks, its mass can be estimated as . A mass estimate using the interferometrically-measured radius of this star and its spectroscopically-determined surface gravity yields 2.5 ± 0.9 . 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 is reached.

As the pole star

From around 2500 BCE, as

Thuban Thuban (), with Bayer designation Alpha Draconis or α Draconis, is a binary star system in the northern constellation of Draco. A relatively inconspicuous star in the night sky of the Northern Hemisphere, it is historically signifi ...

became less and less aligned with the north celestial pole, Kochab became one "pillar" of the

circumpolar star A circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest p ...

s, first with

Mizar Mizar is a second-magnitude star in the handle of the Big Dipper asterism in the constellation of Ursa Major. It has the Bayer designation ζ Ursae Majoris ( Latinised as Zeta Ursae Majoris). It forms a well-known naked eye d ...

, a star in the middle of the handle of the Big Dipper (Ursa Major), and later with Pherkad (in Ursa Minor). In fact, around the year 2467 BCE, the

true north True north is the direction along Earth's surface towards the place where the imaginary rotational axis of the Earth intersects the surface of the Earth on its Northern Hemisphere, northern half, the True North Pole. True south is the direction ...

was best determined by drawing a

plumb line A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical direction as a reference line, or plumb-line. It is a precursor to the spirit level and used to est ...

between Mizar and Kochab, a fact with which the

Ancient Egyptians Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower ...

were well acquainted, as they aligned the great Pyramid of Giza with it. This cycle of the succession of

s occurs due to the

precession of the equinoxes In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's Rotation around a fixed axis, rotational axis. In the absence of precession, the astronomical body's orbit would show ...

. Kochab and Mizar were referred to by Ancient Egyptian astronomers as '

The Indestructibles The Indestructibles ( – literally "the ones not knowing destruction") was the name given by ancient Egyptian astronomers to two bright stars which, at that time, could always be seen circling the North Pole. The name is directly related to Egyp ...

' lighting the North. 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", relating it to the current pole star,

, which is slightly brighter and will have a much closer alignment of less than 0.5° by 2100 CE. This change in the identity of the pole stars is a result of Earth's

axial precession In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body's rotational axis. In the absence of precession, the astronomical body's orbit would show axial parallelism. In parti ...

. 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

from around 1700 BCE until just after 300 CE. Neither star was as close to the north celestial pole as Polaris is now. Today, they are sometimes referred to as the "Guardians of the Pole".

Planetary system

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.

References

{{DEFAULTSORT:Beta Ursae Minoris Ursae Minoris, Beta K-type giants Ursa Minor Northern pole stars Kochab Ursae Minoris, 07 072607 5563 131873 Durchmusterung objects Planetary systems with one confirmed planet