Upsilon Sagittarii
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Upsilon Sagittarii (Upsilon Sgr, υ Sagittarii, υ Sgr) is a
spectroscopic binary A binary star or binary star system 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 as separate stars us ...
star system A star system or stellar system is a small number of stars that orbit each other, bound by gravity, gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally calle ...
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 ...
. Upsilon Sagittarii is the prototypical hydrogen-deficient binary (HdB), and one of only four such systems known. The unusual
spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...
of hydrogen-deficient binaries has made
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 Upsilon Sagittarii difficult.


System

υ Sgr is a
binary system A binary system is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.) The most common ki ...
with an orbital period of 137.939 days and is approximately 1,800
light years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...
from
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
. The primary star dominates the visible radiation and spectrum, but the secondary is hotter and more massive. Some sources consider the "invisible" component to be the primary on the basis of its mass. There is also a disc of material being stripped from the primary and transferring material to the secondary, but no eclipses The system is classified as a single-lined
spectroscopic binary A binary star or binary star system 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 as separate stars us ...
, but high excitation lines from the secondary can be detected in the ultraviolet. Radial velocity variations were discovered in 1899, The first orbit was calculated in 1914, reasonably close to modern understanding of the system. Analysis of optical
interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...
finds the two stars to be separated by and have a difference in brightness of about 3.5 magnitudes.


Properties

The primary component appears as an A
type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...
supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram, with absolute visual magnitudes between about −3 and −8. The temperatures of supergiant stars range ...
, although published spectral types vary from F2p to B5II. Contrasting components in the spectrum may originate from disc material, polar jets, or the star itself. The low mass and unusual chemical composition are also thought to produce misleading spectral calibrations, with the star not as massive or as luminous as the Ia luminosity class would suggest. The visible component is a helium star, almost entirely deficient of hydrogen. It has also been described as a neon star, due to the very high relative levels of that element. It has been stripped of its outer hydrogen layers after it expanded away from the main sequence. It is thought to have originated as a main sequence star with around , expanded when it exhausted its core hydrogen, and now only remains, highly inflated and giving the appearance of a supergiant star. Other estimates give higher masses, as much as and at the known inclination of 50°. The supergiant component is also classified as an
PV Telescopii variable PV Telescopii variable is a type of variable star that is established in the ''General Catalogue of Variable Stars'' with the acronym PVTEL. This class of variables are defined as "helium supergiant Bp stars with weak hydrogen lines and enhanced l ...
, although it was originally catalogued as an eclipsing binary. It shows
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), ...
fluctuations between +4.51 and +4.65 with a period of approximately 20 days. The companion is more massive than the supergiant primary, but so dim at
visible frequencies The visible spectrum is the band of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optical spectrum is sometimes consider ...
as to be undetectable, although it can be seen in
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
spectra. It is thought to be a B-type main sequence star accreting mass from the primary.


Naming

υ Sagittarii has two entries in the
Henry Draper Catalogue The ''Henry Draper Catalogue'' (HD) is an astronomical star catalogue published between 1918 and 1924, giving spectroscopic classifications for 225,300 stars; it was later expanded by the ''Henry Draper Extension'' (HDE), published between 192 ...
, HD 181615 and HD 181616. 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 '' Establishment'', refers to an asterism consisting of υ Sagittarii, ξ² Sagittarii, ο Sagittarii, π Sagittarii, 43 Sagittarii and ρ¹ 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 υ Sagittarii itself is (, .)


References


Further reading

*Dudley, R. E., Jeffery, C. S., 1990. ''Mon. Not. R. astr. Soc.'' 247, 400 {{DEFAULTSORT:Upsilon Sagittarii Sagittarii, Upsilon Sagittarius (constellation) A-type supergiants Spectroscopic binaries B-type main-sequence stars Sagittarii, 46 7342 095176 181616 Durchmusterung objects Beta Lyrae variables PV Telescopii variables