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Acrab

Akrab, Elacrab, Graffias, Beta1,2 Scorpii, 8 Scorpii

Acrab (also commonly called Graffias) lies at the base of Scorpius' northern pincer, near the point where the Ecliptic enters the constellation of the Scorpion from neighbouring Libra. In combination with two stars to the south - Dschubba and Fang - it forms a small asterism known as the Crown of the Scorpion, falling somewhat to the northwest of the bright star Antares in the sky.

The name Acrab is formally recognised for this star, or more specifically the primary member of this multiple star system, Beta1 Scorpii Aa. That traditional name derives from Arabic el-'Aqrab, 'the Scorpion' (and variations on this source are sometimes seen, such as Akrab or Elacrab). The star is also sometimes called Graffias, 'the Claws', a name that is also sometimes used of Xi Scorpii to the north.

Acrab is a complex multiple system, a group of young hot blue stars probably no more than twelve million years old (and quite possibly rather younger)*. These stars form two triple systems, respectively designated Beta1 and Beta2 Scorpii. Each of these systems has a similar underlying structure: a close binary with a third star in orbit around it. Beta1 contains the primary pair of massive blue stars, designated Aa and Ab, with Beta Scorpii B in a distant 610-year orbit around them. At the heart of Beta2 is the binary pair Ea and Eb with Beta Scorpii C in relatively much closer orbit that takes just 39 years to complete. The entire system lies some 400 light years from the Sun.

The omission of 'Beta Scorpii D' from the list of components is not an error, and in fact there is a star with this designation. That star, eighth magnitude HD 144273, lies somewhat to the north of the main system in the sky, and some thirty light years beyond it in space, and is not considered part of the main Beta Scorpii complex of stars.


* Twelve million years is little more than an eyeblink on cosmic timescales. To illustrate, imagine the history of the Milky Way Galaxy, from its formation to the present time, represented as a single calendar year. On this model, the Galaxy formed on 1 January, and the Sun would have appeared in late August. By the same reckoning, the stars of the Beta Scorpii system would have formed no earlier than four o'clock in the afternoon on 30 December.

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