Alpha Sagittarii (α Sagittarii, abbreviated Alpha Sgr, α Sgr), also named Rukbat , is a

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

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 ...

Sagittarius Sagittarius ( ) may refer to: *Sagittarius (constellation) *Sagittarius (astrology), a sign of the Zodiac * Sagittarius of Gap, a 6th century bishop *Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy Ships *'' S ...

Properties

Alpha Sagittarii is a blue, class B

dwarf star A dwarf star is a star of relatively small size and low luminosity. Most main sequence stars are dwarf stars. The meaning of the word "dwarf" was later extended to some star-sized objects that are not stars, and compact stellar remnants that are ...

. It does not appear particularly bright in the sky to the naked eye, with a visual

apparent magnitude Apparent magnitude () is a measure of the Irradiance, 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 (astronomy), ...

of +3.97. The star has an

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 ...

about twice that of 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 ...

and is nearly three times as massive, with a luminosity about 130 times that of the Sun. Based on an excess emission of infrared radiation, it may have a

debris disk A debris disk (American English), or debris disc ( Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris ...

, much like

Vega Vega is the brightest star in the northern constellation of Lyra. It has the Bayer designation α Lyrae, which is Latinised to Alpha Lyrae and abbreviated Alpha Lyr or α Lyr. This star is relatively close at only from the Sun, and ...

. It is a single-lined spectroscopic binary system. The ROSAT All Sky Survey discovered that Alpha Sagittarii is emitting an excess flux of X-rays, which is not expected to originate from a star of this spectral class. The most likely explanation is that the companion is an active pre-main sequence star or else a star that has just reached the main sequence.

Nomenclature

''α Sagittarii'' ( Latinised to ''Alpha Sagittarii'') is the star's

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 ...

. It is unclear why

Bayer Bayer AG (English: , commonly pronounced ; ) is a German multinational pharmaceutical and biotechnology company and is one of the largest pharmaceutical companies and biomedical companies in the world. Headquartered in Leverkusen, Bayer' ...

designated this star as the

alpha Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ...

in his ''

Uranometria is a star atlas produced by Johann Bayer. It was published in Augsburg in 1603 by Christoph Mang (''Christophorus Mangus'') under the full title (from Latin: ''Uranometria, containing charts of all the constellations, drawn by a new method an ...

'' star atlas (placing it in the second magnitude class), rather than Kaus Australis or Nunki, which are at least five times brighter than α Sgr. This led some old star charts to occasionally depict Alpha and Beta Sagittarii as much brighter than they are in reality, as they are invisible from northern Europe, being too far south to see there. The star bore the traditional names ''Rukbat'' and ''Alrami'', derived from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

''rukbat al-rāmī'' 'the knee of the archer'. The star

Delta Cassiopeiae Delta Cassiopeiae (δ Cassiopeiae, abbreviated Delta Cas, δ Cas) is an eclipsing binary star system in the northern circumpolar constellation of Cassiopeia. Based on parallax measurements taken during the Hipparcos mission, it is approxi ...

also bore the traditional names ''Ruchbah'' or ''Rukbat'', from the Arabic word ركبة ''rukbah'' meaning "knee". 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 ...

(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 ''Rukbat'' for this star (Delta Cassiopeiae was later given the name ''Ruchbah''). In

Chinese 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-ethnic ...

, (), meaning '' Celestial Spring'', refers to an asterism consisting of Alpha Sagittarii, Beta¹ Sagittarii and Beta² Sagittarii. 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 Alpha Sagittarii itself is (, .)AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 2 日
This star, together with Beta¹ Sagittarii and Beta² Sagittarii, were ''Al Ṣuradain'' (ألسردين), the two Surad, desert birds.

In popular culture

A fictionalized version of the Rukbat system is the setting for

Anne McCaffrey Anne Inez McCaffrey (1 April 1926 – 21 November 2011) was an American writer known for the ''Dragonriders of Pern'' science fiction series. She was the first woman to win a Hugo Award for fiction (Best Novella, ''Weyr Search'', 1968) an ...

Dragonriders of Pern ''Dragonriders of Pern'' is a science fantasy series written primarily by American-Irish author Anne McCaffrey, who initiated it in 1967. Beginning in 2003, her middle child Todd McCaffrey has written Pern novels, both solo and jointly with A ...

series of novels. In the canon of this series, the system has five planets in standard orbits, two asteroid belts, an

Oort cloud The Oort cloud (pronounced or ), sometimes called the Öpik–Oort cloud, is scientific theory, theorized to be a cloud of billions of Volatile (astrogeology), icy planetesimals surrounding the Sun at distances ranging from 2,000 to 200,000 A ...

, and has also captured a

rogue planet A rogue planet, also termed a free-floating planet (FFP) or an isolated planetary-mass object (iPMO), is an interstellar object of planetary mass which is not gravitationally bound to any star or brown dwarf. Rogue planets may originate from ...

into a highly eccentric orbit which passes both through the Oort cloud and through the inner solar system on a 250 year orbit.

Notes

References

External links

* {{DEFAULTSORT:Alpha Sagittarii Sagittarius (constellation) Sagittarii, Alpha B-type main-sequence stars Rukbat 095347 181869 PD-40 08955 7348