Acrux

Lore & Background

The name Acrux for α Crucis was coined in the 19th century by American astronomer as a contraction of its Bayer designation. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) 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 α Cru 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 (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. In Chinese, it refers to an asterism consisting of Acrux, Mimosa, Gamma Crucis and Delta Crucis. Consequently, Acrux itself is known as 'the Second Star of Cross'. This star is known as Estrela de Magalhães ('Star of Magellan') in Portuguese.

Reader's Guide

The two innermost visual components of Acrux are α Crucis A (α1 Crucis) and α Crucis B (α2 Crucis), separated by 3.9". α1 is magnitude 1.33 whereas α2 is magnitude 1.75. Spectral types of B0.5 IV and B1 V have been assigned for the components, showing that they are hot B-type stars. They orbit over such a long period that motion is only barely seen. From their projected separation of 430 astronomical units, the period is estimated to be around 1,300 years. α1 and α2 are binaries themselves, with α1 being a single-lined spectroscopic binary and α2 an interferometric binary. The components of α1 have masses of 17 and 6.8 times that of the Sun, estimated radii of , and effective temperatures of approximately 29,000 and . 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 mass of α1 Crucis A suggest that it is going to expand into a red supergiant (similar to Betelgeuse and Antares) before exploding as supernovae. Component Ab may perform electron capture in the degenerate O+Ne+Mg core and trigger a supernova explosion, otherwise it will become a massive white dwarf. The components of α2 have masses estimated at , radii at , and effective temperatures at 26,950 and , respectively. Their orbit is not so well-constrained, with a likely orbital period of 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. 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. 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. 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

Did You Know?

• Acrux is the southernmost first-magnitude star, 2.3 degrees more southerly than Alpha Centauri.
• The system was the second ever to be recognized as a binary, in 1685 by a Jesuit priest.
• 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.
• The Brazilian oceanographic research vessel Alpha Crucis is named after the star.