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.