Acrux

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Acrux is a multiple star system and the brightest star in the southern constellation of Crux. It has the Bayer designation α Crucis, which is Latinised to Alpha Crucis and abbreviated Alpha Cru or α Cru. With a combined visual magnitude of +0.76, it is the 13th-brightest star in the night sky. It is the most southerly star of the asterism known as the Southern Cross and is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri. This system is located at a distance of 348 light-years (106.7 parsecs) from the Sun. To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing seven components. Through optical telescopes, Acrux appears as a triple star, whose two brightest components are visually separated by about 4 arcseconds and are known as α Cru A and α Cru B, α1 Crucis and α2 Crucis, or α Crucis A and α Crucis B. Both components are B-type stars, and are many times more massive and luminous than the Sun. This system was the second ever to be recognized as a binary, in 1685 by a Jesuit priest. α1 and α2 Crucis are close binaries themselves, with components designated α Crucis Aa (officially named Acrux, historically the name of the entire system), α Crucis Ab, α Crucis Ba, and α Crucis Bb. α Cru C, also known as HR 4729, is a more distant companion, forming a triple star through small telescopes. C is a spectroscopic binary and has a faint companion, which brings the total number of stars in the system to seven.