3 CEN (3 Centauri). While the name, neither proper nor Greek letter, carries no panache, fourth magnitude (4.28) 3 Cen (less often k Cen) is one of the more unusual, indeed remarkable, stars of the sky. In far northern Centaurus, just below the border with Hydra and part of a small triangle made of 1, 2, and 3 Cen, the star is just barely far enough north to carry a Flamsteed number. But that is not what makes it so prominent among stellar astronomers. Its fame rests far more on its very odd chemical composition. But first, some particulars. Seen at a substantial distance of 347 light years (give or take 26), the star is binary, made of two blue-white class B stars separated by about 8 seconds of arc. The brighter, fifth magnitude (4.53) 3 Cen A is listed as a B5 giant (but see below), while 3 Cen B is a lesser sixth magnitude (6.02) B8 dwarf. The two are dimmed by about 15 percent by interstellar dust. The focus of attention is always on 3 Cen A. The adopted temperature of 17,500 Kelvin, high for its class, is more like that of a class B4 star, the discrepancy the result of a weird metallic composition. After accounting for a fair bit of ultraviolet radiation, the star is seen to have a luminosity of 700 times that of the Sun, which leads to a radius 2.9 solar and a mass of 5.1 Suns. Temperature and luminosity also show that the star is not a giant at all, but a young dwarf only a few tens of millions of years old and far short of its dwarf-lifetime of 85 million years. Three Centauri A is best known as a "helium-weak" star, its atmospheric helium content only a quarter that of normal. As a result of low stirring by rotation (19 km/s, under 8 days), the helium (heavier than dominant hydrogen) seems to have settled downward. The "He-weak" stars are a higher-temperature extension of the "mercury-manganese stars" (like nearby Chi Lupi) in which some of the heavier metals are instead lofted upward by the pressure of radiation. And 3 Cen A fits right in, its mercury content (relative to hydrogen) measured to be as high as 30,000 times above normal. Even odder are greatly elevated abundances of phosphorus, gallium, and the noble gas krypton. Still, such stars are not all that unusual. What makes 3 Cen A really stand out is that its surface helium is almost all made of the light isotope helium-3. Helium comes in two flavors, helium-4 with two protons and two neutrons in its nucleus, and He-3 with just one neutron. Throughout the Universe, the vast majority of the stuff, more than 99 percent, is He-4. Because Earth's helium comes from the radioactive decay of uranium, ALL of ours is He-4. But in the outer layers of 3 Cen A, the ratio is tipped upside down, He-3 nearly three times as abundant as He-4, one of only two stars known that can make that statement. The cause again is almost certainly diffusion, the heavier He-4 sinking downward more than the lighter He-3. (Note again that these strange abundances are surface effects.) Much faster rotating 3 Cen B (165 km/s), however, is normal. With a luminosity of 82 Suns, a radius 2.1 times solar, and a mass of 3.0 Suns, it takes under half a day to make a rotation. Consistent with 3 Cen A, 3 Cen B is very young, showing little change since birth. Three Cen A and B, at least 800 Astronomical Units apart, show little if any orbital motion. From Kepler's Laws they would take at least 8200 years to make a circuit of each other. From 3 Cen B, 3 Cen A would shine about as brightly as a full Moon, though nothing whatever is known about planetary systems in such stars. In any case, such stars do not live long enough to make life possible -- or so we think. (Thanks to Jerry Diekmann, who suggested this star.)
Written by Jim Kaler 5/14/10. Return to STARS.