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.