Acrux system

Acrux (Alpha Crucis) is a system of three stars; a solitary star Alpha-2, and a binary system of two slightly cooler stars, Alpha-1. Alphas 1 and 2 orbit each other around a common barycenter, and both are hot class B (almost class O) stars, their temperatures about 26,000 and 28,000 K respectively. The dimmer star is an ordinary hydrogen-fusing dwarf approximately 400 AU away from the inner binary.

Overview

While Alpha-2 is a single star, Alpha-1 is again double, but one whose components are so close that they orbit each other in just 76 days at a distance of about one AU. Alpha-1 and Alpha-2, on the other hand, 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. From Alpha-2, Alpha-1 looks like a brilliant naked-eye double star, as two blue-white points separated by a bit over a tenth of a degree.

Another "companion," another class B subgiant, shares the motion of the triple Acrux system through space, and appears as if it might be gravitationally bound to it. However, if at Acrux's distance, it is under-luminous for its class.

Future

The masses of Alpha-2 and the brighter component of Alpha-1 suggest that the stars will someday explode. The fainter component of Alpha-1 may escape, however, to become a massive white dwarf.