Acrux (Alpha Crucis)
Acrus 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 arc-seconds. Alpha1 is a B subgiant (visual magnitude 1.4, luminosity 25,000 Lsun, surface temperature 28,000 K). Alpha2, a B dwarf (visual magnitude 2.09, luminosity 16,000 Lsun, temperature 26,000 K).
Alpha2 is a 13-Msun single star but Alpha1 is a spectroscopic binary whose 14- and 10-Msun components are separated by about one 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|
|spectral type||B0.5IV + B1V|
|distance||320 light-years (98 pc)|
|position||R.A. 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.