CHI CET (Chi Ceti, plus HD 11131). Half a degree southwest of fourth
magnitude Baten Kaitos (Zeta Ceti),
one of the stars that outlines the Sea Monster of the Andromeda myth, lies fifth magnitude
(4.67, almost fourth itself) Chi Ceti. One immediately thinks,
aha!, a naked-eye double. But no, Chi Ceti is fairly close to us,
75.2 light years (give or take just a half), whereas Zeta is three
times as far, the alignment just a coincidence, as so often seen.
A closer look, though, shows another, fainter star of seventh
magnitude (6.75), HD 11131 of the Henry Draper spectral
catalogue, just 3.2 minutes of arc to the west that is 74 light years
away (with an uncertainty of 5). Also called Chi Ceti B, could it
belong to the brighter star? Chi proper (HD 11171) is a
mid-temperature (an uncertain 6665 Kelvin) class F (F3) "giant" (but see below). With
most of its light in the optical spectrum, Chi radiates at a rate
of 5.7 Suns, which gives it a radius of 1.8
solar radii. Rotating at an equatorial speed of at least 61
kilometers per second, it makes a full turn in under 1.5 days. Theory
gives it a mass of 1.4 Suns and shows it to be not a giant but a dwarf perhaps three-fourths of
the way toward the end of its hydrogen-fusing lifetime of 2.8 billion
years. The "companion," HD 11131, is an intriguing class G1 dwarf
that bears a lot resemblance to the Sun. With a secure temperature
of 5768 Kelvin, it radiates at a rate of just 0.87 Suns. Temperature
and luminosity then give a radius of 0.94 solar. With a mass of
perhaps 0.9 Suns, it appears to be a fairly young dwarf, and
consistently, it's magnetically active. Are they a pair? Similar
distances would make it seem so, and they are commonly listed as a
"common proper motion" double, meaning that their
movements across the sky relative to the Sun are identical, or nearly
so. Were they at the same distance, they would be 4500 AU apart and
take close to 200,000 years to make a full orbit. From Chi, its solar
type neighbor would be some 20 times brighter than Venus appears in
our skies, while Chi proper from the little one would be another seven
times brighter. The error limits on the distance measures show that
they could also be much farther apart and not a binary at all.
Moreover, during the past 124 years, the stars' separation has
increased by 9 seconds of arc, which is far too much for orbital
movement even at their closest. So they are not a true binary. But
that is not the end of the story. Chi Ceti is part of the "Ursa Major moving group" or "stream" that is
related to Ursa Major Cluster, which consists of the five middle stars
of the Big Dipper and then some. A moving
group is not made of gravitationally bound stars, but is composed
of stars that have a memory of their origins and move similarly, but
are gradually separating as they orbit the center of the Galaxy. We might then speculate
that Chi Ceti's nearby "sun" belongs to the UMa group as well and
therefore keeps a minimal pace with Chi proper. Was our Sun part
of one in the distant past? Quite likely, but at an age of nearly
5 billion years and having orbited the Galaxy more than 20 times,
its mates are surely long lost.
Written byJim Kaler 1/09/15. Return to STARS.