Acrux

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 much of the temperate southern hemisphere, however, the constellation is circumpolar (never setting). Crux's southernmost star, Acrux, 325 light years away (second Hipparcos reduction), 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" rather obviously comes from the "A" in Alpha plus "Crux." An icon of the southern hemisphere, Crux and its prominent stars (which include another of first magnitude, Mimosa) appear on the flags of several southern-hemisphere nations, including those of Australia, New Zealand, and Brazil. A wonderful surprise awaits the telescopic viewer, as Acrux is not one star, but near-twins separated by only 4 seconds of arc. And therein lies a huge problem, as the estimates of the magnitudes of the individuals, hence that of the sum of the two (that of the single star as viewed with the unaided eye), are scattered all over the place, as each star affects the measure of the other. Here we adopt something in the middle, magnitude 1.3 for Alpha-1 (westernmost and brighter) and 1.8 for the fainter, the two summing to 0.8. The brighter, Alpha-1, is thus by itself first magnitude (ranking 20th in the sky), while together the two shine at zeroth, ranking 13th (though the uncertainties could change things quite a bit). Both are hot class B (almost class O) stars, Alpha-1 classed as a B0.5 subgiant, Alpha-2 as a B1 dwarf. But there is yet another surprise. While Alpha-2 is a single star, Alpha-1 is again binary, 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, estimated in the extreme to have masses around 14 and 10 (a rough estimate again adopted here) times that of the Sun, orbit in only 75.78 days. With these masses, they would be separated on the average by only about one Astronomical Unit (the distance between the Earth and the Sun), a rather high eccentricity taking them between about 1.5 and 0.5 AU. The dominant component must have a temperature close to 30,000 Kelvin. All these estimates then lead to more estimates of luminosities of 25,000 and 7000 Suns. With these approximate parameters, both would be hydrogen-fusing dwarfs. Alpha-2 presents a simpler problem. With a temperature estimated from its class of 27,000 Kelvin (needed to account for ultraviolet light) and a very uncertain correction for a bit of interstellar dust absorption, it shines with a luminosity of 20,000 Suns, which makes its mass about 13 times solar. Separated by at least 400 AU, Alpha-2 and the Alpha-1 pair would take at least 1300 years to make a full orbit. From Alpha-2, Alpha-1 would (if the separation really is 400 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 nearly 10,000 full Moons. Another "companion," a fifth magnitude (4.9) star classed as a B4 subgiant, lies 90 seconds of arc away from triple Acrux. It more or less shares Acrux's motion through space, and appears as if it might be gravitationally bound to the inner trio. However, if at Acrux's distance, it's under-luminous for its class, seeming as a young dwarf; it can't possibly belong to the system and be an evolving star. It's also moving a bit too much, so it may well be just a line-of-sight coincidence. If it is really bound to the trio, it would be at least 9000 AU away from Acrux proper and take more than 130,000 years to go around. The masses of all three, the two of Alpha-1 and single Alpha-2, suggest that the stars will someday explode as supernovae, though the fainter component of Alpha-1 may escape to become a massive white dwarf. It's so close to its more massive companion, which will be the first to blow, that it may be ejected right out of the system. All this said, we must be very aware of all the uncertainties involved, which for this star are legion indeed; few of the data and conclusions above should be taken as absolute or even to be reasonably accurate.

Written by Jim Kaler 4/07/00; revised 7/03/09. Return to STARS.