Beta Scorpii (β Scorpii, abbreviated Beta Sco, β Sco) is a multiple star system in the southern zodiac

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 ...

of Scorpius. It bore the traditional proper name of Acrab , though the International Astronomical Union now regards that name as applying only to the β Scorpii Aa component.

Components

Observed through a small telescope, Beta Scorpii appears as 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 ...

with a separation between the two components of 13.5 arcseconds and a combined apparent magnitude of 2.50. This pair, designated β¹ Scorpii and β² Scorpii, form the top branches of a hierarchy of six orbiting components.

Hierarchy of orbits in the β Scorpii system

β¹ Scorpii, the brighter of the pair, consists of two sub-components, designated β Scorpii A and β Scorpii B, orbiting at an angular separation of 0.3 arcseconds with an orbital period of 610 years. β Scorpii A is itself a

spectroscopic binary A binary star is a system of two star, stars that are gravity, 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 separa ...

, with the two components designated β Scorpii Aa (also named Acrab) and β Scorpii Ab. They are separated by 1.42 milliarcseconds and have an orbital period of 6.82 days. β² Scorpii also has two sub-components, designated β Scorpii C and β Scorpii E, orbiting at an angular separation of 0.1328 arcseconds with an orbital period of 39 years. β Scorpii E in turn is a spectroscopic binary with components designated β Scorpii Ea and β Scorpii Eb and having an orbital period of 10.7 days. Component β Scorpii D is the unrelated seventh magnitude star HD 144273, 520" away. Some authors have also referred to component Ab as D. A companion to component B, β Scorpii G, has been proposed to account for missing mass in the system, but no further evidence of its existence has been found. β Scorpii F refers to a theorised companion to component E.

Nomenclature

''β Scorpii'' ( Latinised to ''Beta Scorpii'') is the star'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. ...

; ''β¹'' and ''β² Scorpii'', those of its two components. The designations of the sub-components - ''β Scorpii A'', ''Aa'', ''Ab'', ''B'', ''C'', ''E'', ''Ea'' and ''Eb'' - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU). Beta Scorpii bore the traditional names ''Acrab'', ''Akrab'' or ''Elacrab'', all deriving from the Arabic name ( ar, العقرب) ''al-'Aqrab'' 'the Scorpion' for the whole constellation, as well as Graffias , Italian for "the claws", a name it shared with

Xi Scorpii Xi Scorpii (ξ Sco) is a quintuple star system in the constellation Scorpius. It was assigned this designation by Bayer, although Ptolemy had catalogued the star in Libra. Flamsteed assigned it the designation 51 Librae, but this has fal ...

. In 2016, the International Astronomical Union organized a Working Group on Star Names (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 ''Acrab'' for the component ''β Scorpii Aa'' on 21 August 2016 and it is now so included in the List of IAU-approved Star Names. In Chinese, (), meaning '' Room'', refers to an asterism consisting of both of β¹ Scorpii and β² Scorpii, π Scorpii, ρ Scorpii and δ Scorpii, . Consequently, the Chinese name for both of β¹ Scorpii and β² Scorpii is (), "the Fourth Star of Room".

Namesake

USS Graffias (AF-29) was once a United States navy ship named after the star.

Properties

The β Scorpii system is a kinematic member of 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–Crux) ...

, a group of thousands of young stars with mean age 11 million years at distance 470 light years (145 parsecs). Analysis of β¹ Scorpii as a single star derived an evolutionary age between 9 and 12 million years, but analysis of the β Scorpii system as a whole suggest an age closer to 6 million years. The two components of β Scorpii A are the most massive members of the system, and respectively. The combined spectral type is B1 V. The individual spectral types cannot be clearly measured, but are estimated to be B0.5 and B1.5. Component Aa is evolving slightly away from the

zero age 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 He ...

and its luminosity class is estimated to be intermediate between subgiant (IV) and

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 ...

(V). Component Ab has a main sequence luminosity class, a temperature of 26,400 K, and a luminosity of . Component B is over 20 times fainter than the combined component A stars and a clear spectral type has not been measured. Its mass is estimated to be approximately . Component C has a stellar classification of B2 V and a mass of . It has an effective surface temperature of 24,000 K, a radius of and a bolometric luminosity of . Component E is determined to have a temperature of 13,000 K, radius of , and luminosity of . It is

chemically peculiar In astrophysics, chemically peculiar stars (CP stars) are stars with distinctly unusual metal abundances, at least in their surface layers. Classification Chemically peculiar stars are common among hot main-sequence (hydrogen-burning) stars. Thes ...

, with high abundances of manganese and

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...

. It is possibly a mercury-manganese (HgMn) star, but abundances of other metals are unexpectedly low. Beta Scorpii is 1.01 degree from the ecliptic and can be occulted by the Moon and, very rarely, by planets. On December 9, 1906, it was occulted by Venus. The last occultation by a planet took place on 13 May 1971, by Jupiter.

In culture

Beta Scorpii appears on the flag of Brazil, symbolising the state of Maranhão.

References

External links

Beta Scorpii by Jim Kaler
* {{Stars of Scorpius Scorpii, Beta Spectroscopic binaries 6 B-type main-sequence stars Scorpius (constellation)

Acrab Beta Scorpii (β Scorpii, abbreviated Beta Sco, β Sco) is a multiple star system in the southern zodiac constellation of Scorpius. It bore the traditional proper name of Acrab , though the International Astronomical Union now regards t ...

Upper Scorpius Scorpii, 08 5984 5 144217 8 078820 1 Durchmusterung objects Mercury-manganese stars