Acrux — Double Star in Crux

Acrux is the brightest star in the constellation Crux (the Southern Cross) at a combined magnitude of 0.76. It is a multiple star system located about 320 light-years from Earth. The two brightest components, Alpha1 Crucis (magnitude 1.33, spectral type B0.5IV) and Alpha2 Crucis (magnitude 1.73, spectral type B1V), are separated by about 4 arcseconds. Both are hot, massive blue stars. Alpha1 is itself a spectroscopic binary, making Acrux at least a triple system.

Acrux marks the foot (southern point) of the Southern Cross and is visible only from latitudes south of about 27°N. A small telescope easily splits the two main components, which present a beautiful pair of nearly matched blue-white stars — one of the finest double stars in the sky. The color contrast with the reddish Gacrux at the top of the Cross adds to the spectacle. Acrux, together with Gacrux, points roughly toward the south celestial pole. Best observed from March through July from southern latitudes.

The name Acrux is a modern contraction of 'Alpha Crucis,' coined in the 19th century. The Southern Cross was known to ancient Greeks (it was visible from the Mediterranean due to precession) but gradually sank below their horizon over millennia. European explorers rediscovered it in the Age of Exploration. The Portuguese navigator João Faras made the first known European observation of the Cross in 1500.