Alpha1 Crucis, Alpha2 Crucis - Acrux

Acrux is the brightest star in the constellation Crux, the Southern Cross, and at visual magnitude 0.81, is also the twelfth brightest star in the night sky. It lies at the southern end of the long axis of the cross, which points toward the South Celestial Pole. The name "Acrux" is simply a contraction of the A in Alpha plus the name of its constellation, Crux.

Naming and Cultural Significance

Since Acrux is at roughly declination -63°, it is only visible south of the Tropic of Cancer. Because of the Earth's axial precession, however, the star was visible to ancient Hindu astronomers in India who named it "Tri-shanku". It is known as 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 Sao Paulo.

Physical Properties

Acrux is a multiple star located 320 light years from the solar system. Only two components are visually distinguishable, α1 and α2 Cru, separated by 4.4". This pair can be resolved easily in a small telescope.

α1 Cru is magnitude 1.40 and α2 Cru is magnitude 2.09, both hot class B1 V main sequence stars, with surface temperatures of about 28,000 and 26,000 K respectively. Their luminosities are 25,000 and 16,000 times the Sun's. α1 and α2 Cru orbit over such a long period that motion is only barely seen. From their minimum separation of 430 AU, their period is at least 1,500 years, and may be much longer.

α1 Cru is itself a spectroscopic binary, with components thought to be around 14 and 10 times the mass of the Sun, orbiting in only 76 days at a separation of about 1 AU. The masses of α2 Cru 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" away from triple Acrux and shares its motion through space, suggesting that it may be gravitationally bound to the system. 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.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]