Acrux (α Cru / α Crucis / Alpha Crucis) is the brightest star in constellation Crux, the Southern Cross and, at visual magnitude 0.77, is the twelfth brightest star in the night time sky. Acrux is the southernmost first-magnitude star, just a bit more southerly than Alpha Centauri.
Acrux is a multiple star located 321 light years from the solar system. Only two components are visually distinguishable, α1 and α2, separated by 4 arcseconds. α1 is magnitude 1.40 and α2 is magnitude 2.09, both hot class B (almost class O) stars, with surface temperatures of about 28,000 and 26,000 kelvins 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 at least 1,500 years, and may be much longer.
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 of α1 suggest that the stars will someday explode as supernovae. The fainter component of α1 may survive to become a massive white dwarf.
Another class-B subgiant lies 90 arcseconds away from triple Acrux and shares Acrux's motion through space, suggesting it may be gravitationally bound to Acrux. However, if it is indeed located near Acrux, it is under-luminous for its class. It is probably just an optical double star, most likely several hundred light years beyond Acrux.
Since Acrux is at roughly −60° declination, it is only visible south of the Tropic of Cancer. Because of Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it “Tri-shanku”.
It is known as 十字架二 (the Second Star of the Cross) in Chinese.
Acrux is represented in the flags of Australia and New Zealand as one of five stars which comprise 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.