Sigma² Ursae Majoris (σ² Ursae Majoris, σ² UMa) is a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

in 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 origins of the e ...

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

Parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...

measurements made by 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 of celestial obj ...

spacecraft put it at a distance of about 66.5

light years A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

(20.4

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s) from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, making this a fairly nearby system. The primary component has an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

of about 4.8, meaning it can be seen with the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

''(see

Bortle scale The Bortle scale (also known as the Bottle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by lig ...

)''. This is a visual binary, meaning that the two components can be resolved, and the orbit is derived from the positions of the two stars. The primary component Sigma² Ursae Majoris A, is a white-colored F-type

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of ...

. Its radius is about 1.75 times that of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, and it is 31% more massive. The companion is an orange K-type

main-sequence star In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

that is much fainter. The two stars are separated about 4 arcseconds away, and because of their slow

orbital motion In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

the orbit is poorly known: estimates of the

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...

range from 970 years to over 1,500 years. There is a third component, designated Sigma² Ursae Majoris C. Located 205

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

s from the primary, it is thought to be a line-of-sight coincidence, and is not related to the system.

Naming

*With π¹, π², σ¹, ρ, A and d, it composed the Arabic asterism meaning ''the Gazelles''. According to the catalogue of stars in the ''Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars'', ''Al Ṭhibā'' were the title for seven stars : A as ''Althiba I'', π¹ as ''Althiba II'', π² as ''Althiba III'', ρ as ''Althiba IV'', σ¹ as ''Althiba V'', this star (σ²) as ''Althiba VI'', and d as ''Althiba VII''. *In

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of va ...

, (), meaning '' Three Top Instructors'', refers to an asterism consisting of σ² Ursae Majoris and ρ Ursae Majoris. Consequently, σ² Ursae Majoris itself is known as (, en, the Third Star of Three Top Instructors.).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 6 月 16 日
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References

{{Stars of Ursa Major F-type subgiants K-type main-sequence stars Binary stars Ursae Majoris, Sigma Ursa Major Durchmusterung objects Ursae Majoris, 13 078154 045038 3616 Althiba VI