CHI ERI (Chi Eridani). Eridanus, the
celestial River, classically ended at Acamar, Theta Eridani, but in "modern" times
was pushed by southern astronomical explorers farther south to have a much brighter finale at Achernar, Alpha Eri. Just before the
river flows into the sea, we encounter fourth magnitude Chi
Eridani. Its most significant attribute is as a guide star, as
with Phi Eri (just east of Chi), it makes a "pointer" that leads
the eye to the east toward Iota
Horologii (in Horologium, the Clock),
a star with an orbiting planet. The star does, however, also
illustrate the speed with which stellar evolution can take place.
Chi Eri is a class G (G8) giant 57 light years away that has more
recently been called a G9 subgiant. With a temperature of 5060
Kelvin, it shines with the light of just 10.8 times that of the Sun, not much for a giant, but in fact
appropriate for a lower-mass subgiant. From temperature and
luminosity we derive a modest radius 4.2 times solar. The rotation
velocity, hence rotation period, is unknown. Chi Eri's mass
depends on what one assumes its internal conditions might be, that
is, as to whether it is a true giant with a helium-fusing core or
a star that has just given up hydrogen fusion. Given its
independent classification as a subgiant, we assume the latter, in
which case the mass comes in at 1.7 times that of the Sun.
Structure and evolution theory give an age of 1.86 billion years,
and reveal that Chi began life as a class A3 dwarf just a bit less
massive than the classic A stars Sirius
and Vega. It then spent 1.83 billion years
fusing its internal hydrogen. After the hydrogen was gone, the
star cooled to its present state in a mere 30 million years, just
1.6 percent of its overall lifetime. It will now expand and
brighten hundreds of times until it initiates the fusion of its
core helium into carbon and oxygen, after which it will settle down
as a true giant. At first Chi
Eri appears to be a double
star with an 11th magnitude companion 5 seconds of arc distant.
In 1834, when the separation was first measured, however, the
separation was much greater, 12 seconds of arc. The difference is
vastly too large to be caused by orbital motion, making the so-
called "companion" a mere line-of-sight coincidence.
Written by Jim Kaler 1/18/08. Return to STARS.