Epoch J2000.0 Equinox J2000.0
|Right ascension||16h 05m 26.23198s|
|Declination||–19° 48′ 19.6300″|
|Apparent magnitude (V)||2.62|
|Right ascension||16h 05m 26.57128s|
|Declination||–19° 48′ 06.8556″|
|Apparent magnitude (V)||4.92|
|Spectral type||B1V + B2V|
|U−B color index||–0.08 / –0.70|
|B−V color index||–0.08 / –0.02|
|Radial velocity (Rv)||–1.0 km/s|
|Proper motion (μ)||RA: –5.20 mas/yr
Dec.: –24.04 mas/yr
|Parallax (π)||8.07 ± 0.78 mas|
|Distance||400 ± 40 ly
(120 ± 10 pc)
|Radial velocity (Rv)||–3.6 km/s|
|Proper motion (μ)||RA: –5.07 mas/yr
Dec.: –25.87 mas/yr
|Parallax (π)||8.19 ± 1.17 mas|
|Distance||approx. 400 ly
(approx. 120 pc)
|Rotational velocity (v sin i)||100 km/s|
|Rotational velocity (v sin i)||65 km/s|
Observed through a small telescope, Beta Scorpii appears as a binary star 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 the hierarchy of orbiting components in this 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 3.9 arcseconds with an orbital period of 610 years. β Scorpii A is itself a spectroscopic binary with the two components, designated β Scorpii Aa (also named Acrab) and β Scorpii Ab, separated by 1.42 milliarcseconds and 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.
This brings the total number of stars in the system to six. (There is no D component; this is now an artifact of earlier system models.)
β Scorpii (Latinised to Beta Scorpii) is the star's Bayer designation; 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 (Arabic: العقرب) al-'Aqrab 'the Scorpion' for the whole constellation, as well as Graffias, a name it shared with Xi Scorpii.
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness. The WGSN approved the name Acrab for β Scorpii Aa on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.
It was known as 房宿四 (the Fourth star of the Room) in Chinese.
The two most massive members of the system have the spectrum of B-type main sequence stars. Component C has a stellar classification of B2V. They are both hot stars at least 10 times as massive as the Sun, and will have short lives. Both are expected to end their stellar evolution with massive Type II supernova explosions.
The Beta Scorpii system is a kinematic member of the Upper Scorpius subgroup of the Scorpius-Centaurus Association, a group of thousands of young stars with mean age 11 million years at distance 470 light years (145 parsecs). A recent analysis of the HR diagram position for the most massive star Beta-1 Scorpii A estimates its effective temperature to be 26,240 Kelvin with a luminosity of 19,500 Suns, consistent with an isochronal age of 11 million years and an estimated mass of 12 solar masses.
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