Acrux (Alpha Crucis)

Alpha Crucis system

Acrux is the twelfth brightest star in the sky, the southernmost first magnitude star, and the brightest and southernmost star in Crux. In fact, Acrux is a multiple star system.


A moderate telescope shows two similar B stars separated by 4 arcseconds. Alpha1 is a B subgiant (visual magnitude 1.4, luminosity 25,000 times that of the Sun, surface temperature 28,000 K). Alpha2, a B dwarf (visual magnitude 2.09, luminosity 16,000 times that of the Sun, temperature 26,000 K).


Alpha2 is a 13-solar-mass single star but Alpha1 is a spectroscopic binary whose 14- and 10-solar-mass components are separated by about one astronomical unit (AU) and complete an orbit every 76 days. Alpha1 and Alpha2, with a minimum separation of 430 AU, take at least 1,500 years to circle around each other. Another B subgiant lies 90" away from the triplet but, despite its similar velocity through space, is probably a more distant star that happens to lie along the same line of sight.


visual magnitude 0.77 (combined), 1.40/2.09
absolute magnitude -4.19/-3.79
spectral type B0.5IV + B1V
distance 320 light-years (98 pc)
position RA 12h 26m 35.9s,
Dec -63° 05' 57"
other designations HR 4730/4731, CD -62° 2745,
HD 108248/108249, SAO 251904,
FK5 462, HIP 60718.