EPS CRU (Epsilon Crucis). Here is one of the most viewed of all stars, while at the same time being one of the most obscure. How can that be? At fourth magnitude (3.59, just fainter than third), Epsilon Crucis has a prominent place as the "fifth star" of one of the most famed of all constellations, Crux, the Southern Cross, lying almost on a line between brighter Delta Crucis (the Cross's most northerly star) and first magnitude Acrux (Alpha Crucis). The Southern Cross, an icon of the southern hemisphere, appears prominently on the flags of several nations, states, and territories, notably those of Australia, New Zealand, and Brazil (as well as on the banners of Papua New Guinea, Samoa, and some others). Epsilon takes its place among the brighter quartet on all of them except for the flag of New Zealand, where it is curiously absent. So anytime anyone in these five-star lands salutes or admires their national symbol, they at the same time must view our Eps Cru. With the exception of one flag, we see the star placed as it is viewed in the sky. On Brazil's flag, however, the Cross (with Epsilon) is reversed, following an ancient tradition where the stars appear as they do on a celestial globe, as in a God's-eye view from outside the sky, and as the stars also do on the ceiling of Grand Central Station in New York City. That said, nobody else seems to pay much attention to this otherwise conspicuous star. Certainly not the astronomers. It's hardly examined at all. Too bad, since while appearing as just one more class K (K3.5) giant, the star does have differences to it. At a distance of 228 light years, Epsilon Crucis shines with a luminosity of 330 Suns from a coolish 4150 Kelvin surface. A significant giant, the star has swelled to a radius 35 times that of the Sun, nearly half the size of the orbit of Mercury. Luminosity, temperature, and theory then tell of a star with a mass 1.7 times that of the Sun and reveal that the star is not just sitting there quietly fusing helium into carbon and oxygen, but is in a state of flux, most likely still brightening and swelling with a dead helium core, though it may have passed helium ignition and be fading -- it's impossible to tell. (One source suggests a lower mass of 1.4 Suns). With a mass of 1.7, the star began life some two billion years ago as a class A dwarf. The metal content is normal, about 80 percent solar. As a star in evolutionary transit, Eps Cru is also an irregular variable that changes erratically (or so we surmise, since no variation period is known) between magnitude 3.4 and 4.0. In spite of its variable nature, the star does, however, serve as a calibrator for other studies. It seems though that it will remain best known as it waves in the breezes of southern skies. (Thanks to Steven Raine, who suggested this star, and to Andre Bordeleau of the Montreal Planetarium, who researched the flags and wrote about them in the October 2008 issue of the "Planetarian," from which the above discussion was taken.)
Written by Jim Kaler 4/10/09. Return to STARS.