Sigma Scorpii (or σ Scorpii, abbreviated Sigma Sco or σ Sco), is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking ...

in the constellation of

Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the G ...

, located near the red supergiant

Antares Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ S ...

, which outshines it. This system has a combined

apparent visual 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 lig ...

of +2.88, making it one of the brighter members of the constellation. Based upon

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 object ...

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 of celestial obj ...

mission, the distance to Sigma Scorpii is roughly 696

light-year 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 101 ...

s (214

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, a ...

s). North ''et al.'' (2007) computed a more accurate estimate of light years ( parsecs). The system consists of a spectroscopic binary with components designated Sigma Scorpii Aa1 (officially named Alniyat , the traditional name for the entire star system) and a

Beta Cephei variable Beta Cephei variables, also known as Beta Canis Majoris stars, are variable stars that exhibit small rapid variations in their brightness due to pulsations of the stars' surfaces, thought due to the unusual properties of iron at temperatures of 200, ...

) and Aa2; a third component (designated Sigma Scorpii Ab) at 0.4

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 spectroscopic pair, and a fourth component (Sigma Scorpii B) at about 20 arcseconds.

Nomenclature

''σ Scorpii'' ( Latinised to ''Sigma Scorpii'') is the star system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. T ...

. The designations of the four components as ''Sigma Scorpii Aa1'', ''Aa2'', ''Ab'' and ''B'' derive from the convention used by the Washington Multiplicity Catalog (WMC) for

s, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

(IAU). Sigma Scorpii and Tau Scorpii together bore the traditional name ''Al Niyat'' (or ''Alniyat'') derived from the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walte ...

النياط ''al-niyāţ'' "the arteries" and referring to their position flanking the star

, the scorpion's heart, with Sigma Scorpii just to the north. In 2016, the

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 List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...

(WGSN) to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name ''Alniyat'' for the component Sigma Scorpii Aa1 on February 1, 2017 and it is now so included in the List of IAU-approved Star Names. In Chinese, (), meaning ''

Heart The heart is a muscular organ found in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon diox ...

'', refers to an asterism consisting of Sigma Scorpii, Antares and Tau Scorpii. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are oft ...

for Sigma Scorpii itself is (), "the First Star of Heart". The indigenous Boorong people of northwestern Victoria in Australia saw this star and Tau Scorpii as wives of ''Djuit'' (Antares).

Properties

The brightest component of the system, Sigma Scorpii Aa, is a double-lined spectroscopic binary, which means that the pair has not been resolved using a telescope. Instead, their

orbit 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 a ...

is determined by changes in their combined

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

caused by the

Doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who ...

. This indicates that the pair complete an orbit every 33.01 days and have an

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values b ...

of 0.32. The primary component of the spectroscopic binary, Sigma Scorpii Aa1, is an

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation te ...

giant star A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

with a

stellar classification 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 t ...

of B1 III. It has around 18 times the mass of the Sun and 12 times the Sun's radius. This star is radiating about times the luminosity of the Sun from its outer envelope at 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 ...

of . This is a

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as e ...

of the Beta Cephei type, causing the apparent magnitude to vary between +2.86 and +2.94 with multiple periods of , , and 8.2 days. During each pulsation cycle, the temperature of the star varies by . The other member of the core pair, Sigma Scorpii Aa2, is a

main sequence 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 Her ...

star with a classification of B1 V. Orbiting this binary at a separation of half an

, or at least 120

Astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s (AU), four times 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 rad ...

–

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 time ...

distance, is the magnitude +5.2 Sigma Scorpii Ab, which has an

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 ...

of over a hundred years. Even farther out at 20 arcseconds, or more than 4500 AU, is Sigma Scorpii B with a magnitude of +8.7. It is

classified Classified may refer to: General *Classified information, material that a government body deems to be sensitive *Classified advertising or "classifieds" Music *Classified (rapper) (born 1977), Canadian rapper *The Classified, a 1980s American roc ...

as a B9 dwarf. Given its position, youth, and space velocity, the Sigma Scorpii system is a likely member of the

Gould Belt The Gould Belt is a local, partial ring of stars in the Milky Way, about 3,000 light-years long, tilted away from the galactic plane by about 16–20 degrees. It contains many O- and B-type stars, amounting to the nearest star-forming regio ...

, and in particular the Upper Scorpius subgroup of the

Scorpius–Centaurus association The Scorpius–Centaurus association (sometimes called Sco–Cen or Sco OB2) is the nearest OB association to the Sun. This stellar association is composed of three subgroups (Upper Scorpius, Upper Centaurus–Lupus, and Lower Centaurus–Cru ...

(Sco OB2). Recent isochronal age estimates for the system yield ages of 8–10 million years through comparison of the

HR diagram HR, Hr or hr may refer to: Arts and media Film and television * ''H.R. Pufnstuf'', a children's television series from 1969 * ''HR'', a 2013 television drama starring Alicia Silverstone * HR, a criminal organisation in the American TV series ''Pe ...

positions for the stars to modern evolutionary tracks. This agrees well with the mean age for the Upper Scorpius group which is approximately 11 million years.

References

{{Stars of Scorpius Scorpius (constellation) Scorpii, Sigma Scorpii, 20 Beta Cephei variables B-type giants B-type main-sequence stars 4 Alniyat Upper Scorpius 080112 147165 Durchmusterung objects 6084