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.