Acrux

Acrux — a contraction of its Bayer designation Alpha Crucis — is the brightest star in Crux, the Southern Cross, and the 13th-brightest star in the entire night sky at magnitude 0.77. Embedded in Southern Hemisphere culture for millennia, the Southern Cross appears on the flags of Australia, New Zealand, Brazil, Papua New Guinea, and Samoa, with Acrux marking its foot. Indigenous Australian peoples used it as a seasonal and directional guide for tens of thousands of years.

To the naked eye Acrux appears as a single brilliant blue point, but a modest telescope reveals two blue B-type stars, Alpha-1 and Alpha-2, separated by about 4 arcseconds. Alpha-1 (the brighter) is itself a spectroscopic binary: its components of 17.2 and 6.8 solar masses orbit each other every 76 days at roughly 1 AU. Alpha-2 is likely a second close pair, making Acrux at least a quadruple system. All four stars blaze at temperatures between 20,000 and 29,000 K, making the combined system roughly 31,000 times more luminous than the Sun in visible light alone (and far more in ultraviolet).

At 322 light-years, Acrux lies at the southern tip of the Coalsack Nebula region, amid the rich star-forming complex of Centaurus-Crux. Its near neighbour in the field, the Jewel Box cluster (NGC 4755), contains another family of hot young stars at a similar distance, making this patch of the southern sky one of the most spectacular in the Milky Way band.