Acrux

Template:Infobox Double Star/Maintenance/Single Coordinates

Acrux is the brightest star in the constellation Southern Cross (Crux) and the twelfth brightest in the sky. Because it lies very far south, it was not given a proper name in European cultures; the name Acrux is simply formed from Alpha and Crux. In astronomy it is systematically called α Crucis or α Cru for short, followed accordingly by Becrux and Gacrux.

With an apparent magnitude of 0.77 mag it is the southernmost first magnitude star.

In fact, it is a multiple star system located about 320 light-years from the solar system. The two optically separable components, α¹ Crucis and α² Crucis, have an angular separation of 4.29". α¹ Cru has an apparent magnitude of 1.3 mag, α² Cru of 1.6 mag. Both are hot stars of spectral class B, with surface temperatures of 28,000 K and 26,000 K, respectively, and luminosities of about 25000 and 16000 times that of the Sun. An orbit takes so long that the orbital motion is almost impossible to detect. From its minimum distance of 430 AU it results in at least 1500 years, but is probably much longer.
α¹ Cru is itself a spectroscopic binary star, whose components are thought to be 14 and 10 times the mass of the Sun, respectively. They orbit each other in only 76 days at a distance of about 1 AU.

Another main sequence star of spectral class B5 V, designated HR 4729 (or HD 108250), which has an apparent magnitude of 4.8 mag, is located about 90" away from Acrux at a position angle of 202 degrees and exhibits the same spatial direction of motion, so it may be gravitationally bound to the Acrux system. According to measurements by the Gaia spacecraft, it is about 309 light-years (corresponding to a parallax of 10.5623 milliarcseconds) from Earth.

The masses of the brighter components of α¹ Cru and α² Cru suggest that they will explode as supernovae at the end of their life cycle. The fainter component of α¹ Cru, on the other hand, could end up as a white dwarf.

On 20 July 2016, the IAU adopted the proper name Acrux as the standardized proper name for α1. However, it is also pointed out that this proper name is only valid for its visually brighter component. The fainter component of α1 and the star α2 therefore have no proper names.

See also

• List of the brightest stars