Acrux, designated α Crucis is a multiple star system 321 light-years from the Sun in the constellation of Crux and part of the asterism known as the Southern Cross. With a combined visual magnitude of +0.76, it is the brightest object in Crux and the 13th brightest star in the night sky. It is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri. To the naked eye Acrux appears as a single star, but it is actually a multiple star system containing six 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 Acrux A and Acrux 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. α1 Crucis is itself a spectroscopic binary with components designated α Crucis Aa and α Crucis Ab. Its two component stars orbit every 76 days at a separation of about 1 astronomical unit. Acrux C is a more distant companion star, forming a triple star through small telescopes.
Nomenclature
α Crucis is the system's Bayer designation; α1 and α2 Crucis, those of its two main components stars. The designations of these two constituents as Acrux A and Acrux B and those of A's components - Acrux Aa and Acrux Ab - derive from the convention used by the Washington Multiplicity Catalog for multiple star systems, and adopted by the International Astronomical Union. The historical name Acrux for α1 Crucis is an "Americanism" coined in the 19th century, but entering common use only by the mid 20th century. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalog 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 Acrux for the star Acrux Aa on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names. Since Acrux is at −63° declination, making it the southernmost first-magnitude star, it is only visible south of latitude 27° North. It barely rises from cities such as Miami, United States, or Karachi, Pakistan and not at all from New Orleans, United States, or Cairo, Egypt. Because of Earth's axial precession, the star was visible to ancient Hindu astronomers in India who named it Tri-shanku. It was also visible to the ancient Romans and Greeks, who regarded it as part of the constellation of Centaurus. In Chinese, 十字架, refers to an asterism consisting of Acrux, Mimosa, Gamma Crucis and Delta Crucis. Consequently, Acrux itself is known as 十字架二. This star is known as Estrela de Magalhães in Portuguese.
Stellar properties
The two components, α1 and α2 Crucis, are separated by 4 arcseconds. α1 is magnitude 1.40 and α2 is magnitude 2.09, both early class B stars, with surface temperatures of about 28,000 and 26,000 K respectively. Their luminosities are 25,000 and 16,000 times that of the Sun. α1 and α2 orbit over such a long period that motion is only barely seen. From their minimum separation of 430 astronomical units, the period is estimated to be around 1,500 years. α1 is itself a spectroscopic binary star, with its components thought to be around 14 and 10 times the mass of the Sun and orbiting in only 76 days at a separation of about 1 AU. The masses of α2 and the brighter component α1 suggest that the stars will someday explode as supernovae. Component Ab may survive to become a massive white dwarf. Rizzuto and colleagues determined in 2011 that the α Crucis system was 66% likely to be a member of the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus Association. It was not previously seen to be a member of the group. The cooler less luminous B class star HR 4729 lies 90 arcseconds away from triple star system α Crucis and shares its motion through space, suggesting it may be gravitationally bound to it, and it is therefore generally assumed to be physically associated. It is itself a spectroscopic binary system, sometimes catalogued as component C of the Acrux multiple system. Another fainter visual companion listed as component D or Acrux D. A further seven faint stars are also listed as companions out to a distance of about two arc-minutes. On 2008 October 2, the Cassini–Huygens spacecraft resolved three of the components of the multiple star system as Saturn's disk occulted it.
