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AcruxAcrux (α Cru / α Crucis / Alpha Crucis) is the brightest star in constellation Crux (the Southern Cross) and the thirteeth brightest star in the nighttime sky, at visual magnitude 0.77. Since Acrux is at roughly −60° declination, it is only visible south of the Tropic of Cancer and therefore didn't receive an ancient traditional name; "Acrux" is simply a combination of the A in Alpha plus Crux. Acrux is the southernmost first magnitude star, just a bit more southerly than Alpha Centauri. Acrux is a trinary star located 320 light years from the solar system. Only two components are visually distinguishable, α1 and α2, separated by 4 arcseconds. α1 is magnitude 1.33 and α2 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 2,500 and 1,600 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 1500 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 one astronomical unit. 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 lying over twice as far away from the solar system as Acrux. Crucis, Alpha
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