Acrux

α Crucis (Latinised to Alpha 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 α Cru A and α Cru B and those of A's components—α Cru Aa and α Cru Ab—derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems,^{[dubious – discuss]} and adopted by the International Astronomical Union (IAU).^{[16][unreliable source?]}

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.^{[17][better source needed]} In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[18] 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.^[19] The WGSN approved the name Acrux for the star α Cru Aa on 20 July 2016 and it is now so entered in the IAU Catalog of Star Names.^[15]

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 (both around 25°N) and not at all from New Orleans, United States, or Cairo, Egypt (both about 30°N). 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.^[20]

In Chinese, 十字架 (Shí Zì Jià, "Cross"), refers to an asterism consisting of Acrux, Mimosa, Gamma Crucis and Delta Crucis.^[21] Consequently, Acrux itself is known as 十字架二 (Shí Zì Jià èr, "the Second Star of Cross").^[22]

This star is known as Estrela de Magalhães ("Star of Magellan") in Portuguese.^[23]

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. 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 1,300 years.^[3]

α¹ and α² are binaries themselves, with α¹ being a spectroscopic binary and α² an interferometric binary.^[6] The primary and secondary components of α¹ have masses of 17 and 6.8 times that of the Sun (M_☉), estimated radii of 6.8 and 3.2 R_☉, and effective temperatures of approximately 29,000 and 20,000 K. They take 75.747 days to complete an orbit around each other at a semi-major axis of almost exactly one astronomical unit (au), with a considerable orbital eccentricity of 0.37. The components of α² have masses estimated at 12.4 and 9.8 M_☉, radii at 5.4 and 4.4 R_☉, and effective temperatures at 26,950 and 24,350 K, respectively. Their orbit is not so well-constrained, with a likely orbital period or 405.4 days, semi-major axis of 3.0 au and eccentricity of 0.40, although an orbital period of 202.9 days coupled with a higher eccentricity of 0.64 and a semi-major axis of 1.90 au is still possible. The orbits of the α¹ and α² systems are significantly misaligned with each other, indicating that the system was initially an unstable multiple system which underwent dynamical unfolding.^[6]

The mass of the brighter component of α¹ suggest that it is going to expand into a red supergiant (similar to Betelgeuse and Antares) before exploding as supernovae.^[24] Component Ab may perform electron capture in the degenerate O+Ne+Mg core and trigger a supernova explosion,^[25][26] otherwise it will become a massive white dwarf.^[24]

Photometry with the TESS satellite has shown that one of the stars in the α Crucis system is a β Cephei variable, although α¹ and α² Crucis are too close for TESS to resolve and determine which one is the pulsator.^[5]

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.^[27] A bow shock is present around α Crucis, and is visible in the infrared spectrum, but is not aligned with α Crucis; the bow shock likely formed from large-scale motions in the interstellar matter.^[28]

The cooler, less-luminous B-class star HR 4729 (HD 108250) 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.^[29][30] It is itself a spectroscopic binary system, sometimes catalogued as component C (α Cru C) of the Acrux multiple system. Another fainter visual companion listed as component D or α Cru D. A further seven faint stars are also listed as companions out to a distance of about two arc-minutes.^[31]

Acrux is represented in the flags of Australia, New Zealand, Samoa, and Papua New Guinea as one of five stars that compose the Southern Cross. It is also featured in the flag of Brazil, along with 26 other stars, each of which represents a state; Acrux represents the state of São Paulo.^[32] As of 2015, it is also represented on the cover of the Brazilian passport.

The Brazilian oceanographic research vessel Alpha Crucis is named after the star.

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