Beta Scorpii

Beta ScorpiiSco, β Scorpii) is a multiple star system in the southern zodiac constellation of Scorpius. It has the traditional names Acrab, Akrab or Elacrab, all come from (Arabic: العقرب‎) al-'Aqrab, the Scorpion, for the whole constellation, as well as Graffias, a name it shares with Xi Scorpii. It was known as 房宿四 (the Fourth Star of the Room) in Chinese.

Observed through a small telescope, Beta Scorpii appears as a binary star with a separation between the two components of 13.5 arcseconds. This pair, β1 and β2, 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, β Sco A and β Sco B, orbiting at an angular separation of 3.9 arcseconds with an orbital period of 610 years. β Sco A is itself a spectroscopic binary with the components β Sco Aa and β Sco Ab separated by 1.42 milliarcseconds and an orbital period of 6.82 days.

The second visual component, β2 Scorpii, has two sub-components, β Sco C and β Sco E, orbiting at an angular separation of 0.1328 arcseconds with an orbital period of 39 years. β Sco E is a spectroscopic binary with the components β Sco Aa and β Sco Ab 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.

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 our own 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.

Because it is close to the ecliptic, Beta Scorpii can be occulted by the Moon and, very rarely, by planets. The last occultation by a planet took place on 13 May 1971, by Jupiter.

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