Acrux

Acrux (Alpha Crucis) is a star in the constellation Crux, the Southern Cross. Since the Southern Cross is roughly 60 degrees below the celestial equator, Crux is only visible south of the Tropic of Cancer and therefore didn't receive an ancient proper name; "Acrux" is simply a combination of the A in Alpha plus Crux. Acrux has a stellar magnitude of 0.77, and is the twelfth brightest star in the sky. It is the southernmost first magnitude star, just beating out Rigil Kentaurus (Alpha Centauri).

Acrux is a trinary star located 320 light years from the solar system. Only two components are visually distinguishable, Alpha-1 and Alpha-2, separated by 4 arcseconds. Alpha-1 is magnitude 1.33 and Alpha-2 is magnitude 1.73, both hot class B (almost class O) stars, with surface temperatures of about 28,000 and 26,000 Kelvin respectively; their respective luminosities are 25,000 and 16,000 times that of the Sun. Alpha-1 and Alpha-2 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 1500 years, and may be much longer.

Alpha-1 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 one astronomical unit. The masses of Alpha-2 and the brighter component of Alpha-1 suggest that the stars will someday explode as supernovae. The fainter component of Alpha-1 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 lying over twice as far away from the solar system as Acrux.