ACRUX (Alpha Crucis). Among the most famous constellations in the sky is a "modern" one that is quite invisible from most of the populated northern hemisphere:
Crux, the Southern Cross. Some 60 degrees below the celestial equator, Crux is well-visible only roughly south of the Tropic of Cancer (a good reason to go to Hawaii). From nearly all the temperate southern hemisphere, Crux is circumpolar (never setting). Crux's southernmost star, Acrux, is not only the Cross's brightest star (for which it received the Alpha designation), but is the twelfth brightest star in the sky (at 0.77, almost "zeroth" magnitude). It also holds the honor of being the southernmost first magnitude star (just beating out
Rigil Kentaurus, Alpha Centauri). Too far south to have received an ancient proper name, "Acrux" simply comes from the A in Alpha plus Crux. A wonderful surprise awaits the telescopic viewer, as Acrux is not one star, but near-twins separated by only 4 seconds of arc. The brighter, Alpha-1, is by itself first magnitude (1.33) and ranks 20th in the sky, while the other, Alpha-2, is bright second magnitude (1.73). Both are hot class B (almost class O) stars, the temperatures about 28,000 and 26,000 Kelvin respectively. The brighter is classed as a subgiant (implying possible beginning evolution), the dimmer as a more ordinary hydrogen-fusing dwarf. From their common distance of 320 light years, we find enormous respective luminosities 25,000 and 16,000 times that of the
Sun. And still surprises await. While Alpha-2 is a single star, Alpha-1 is again double, but one whose components are so close that they can be detected only by the effect of their orbital motion on the spectrum. The components of Alpha-1, thought to be around 14 and 10 times the mass of the Sun, orbit in only 76 days at a distance from each other of about one astroomical unit, the distance between the Earth and the Sun. Alpha-1 and Alpha-2 (which has a mass of around 13 solar) 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 would (if the separation really is 420 AU) look like a brilliant naked-eye double star, as two blue-white points separated by a bit over a tenth of a degree, together shining with the light of 10,000 full Moons. Another "companion," another class B subgiant, lies 90 seconds of arc away from triple Acrux. It shares Acrux's motion through space, and appears as if it might be gravitationally bound to Acrux. However, if at Acrux's distance, it is under-luminous for its class, and is probably just a line of sight partner over twice as far away. 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.
