GACRUX (Gamma Crucis). In our "western" star lore, the stars with proper names come from the set that could be seen from the lands of the ancient middle east and Arabia, and do not include those of the far southern hemisphere, which is largely invisible below the horizon from northern lands. Yet those who needed the stars to find their way, the old navigators, also needed quick names for some of the stars and just made them up. "Gacrux" is a prime example of such a star, its name deriving strictly from its Greek letter name, Gamma Crucis, the third-brightest star in the constellation Crux, the Southern Cross. Acrux, Alpha Crucis, was named in the same way. Tied with Shaula in Scorpius for third-place in the second magnitude rank (1.63), Gacrux is (with Shaula) the 24th brightest star in the sky. For a star of its brilliance, however, it has not received much individual attention, probably again because it is not observable throughout much of the world where we find most of the telescopes. And a pity too, as it is rather unusual among naked-eye stars. Most of the brighter stars in the sky are white class A or blue class B dwarfs, or even orange giants. Gacrux, however, is a cool red class M (M 3.5) giant star, one that shines nicely in contrast to the other three, blue, stars of the Cross. And a true giant it is. From its rather nearby distance of 88 light years, and its temperature of 3400 Kelvin (from which we can estimate the amount of invisible infrared radiation shining from its cool surface), we calculate a luminosity of 1500 Suns, which leads to a radius 113 solar. If placed at the Sun it would extend over halfway to Earth. At first, Gacrux looks double, with a faint, nearly seventh magnitude class A "companion" about two minutes of arc away from it, but it is a mere line-of- sight coincidence: the white class A star is actually about 4 times farther away, so there is no physical relation between the two. (From the A star, it would look as if Gacrux and the Sun were double.) Though usually taken as a single star, however, Gacrux may in fact be double. It is a mild "barium star" whose surface might have been contaminated by the winds from a companion that evolved first and is now an invisible white dwarf (rather like Alphard in Hydra. More certain, Gacrux is variable by a few tenths of a magnitude. Though little is known of the nature of the variability, it has been classified as a "semi- regular variable," one that is not quite predictable. It is also blowing a fairly strong, and quite noticeably variable, wind. Gacrux's evolutionary -- ageing -- status is uncertain. With a mass perhaps three times solar or less, it may well have given up not just hydrogen fusion in its core (which it must do to become a giant in the first place), but may also have gone through its core helium-fusion stage. If that is the case, Gacrux is in the process of becoming a "second-ascent" giant, one brightening into the ethereal realm of the giant stars for the second time, a speculation reinforced by its variability. Perhaps one day it will appear to us as a full-blown pulsating variable like Mira in Cetus.
Written by Jim Kaler. Return to STARS.