XI-1 CET (Xi-1 Ceti). Dim Cetus, the Whale or Sea Monster, has not one, but two stars called by the hardest of the Greek letters to write, "Xi," the eastern one Xi-1 Ceti, the western Xi-2, the latter making part of the Whale's ragged head. Xi-1, on the other hand, would perhaps make the Whale's ear, if whales actually had ears. In spite of their common name, the two have nothing to do with each other, Xi-2 a class B (B9) giant 175 light years away, Xi-1 a fourth magnitude (4.37) class G giant twice as distant, 362 light years. Xi-1's exact class is contended, and is given either as G6 "bright giant-giant" or G8 supergiant. We'll go with the more traditional first of these, and call it G6. Xi-1 Ceti is a close spectroscopic double star. Near the ecliptic, it is occasionally occulted (crossed over by) the Moon, which gives a brightness ratio for the pair of 6:1 as well as a separation. The brighter (Xi-1 A) (the class G giant), with a temperature of 5100 Kelvin, shines 190 times more brightly than the Sun, while Xi-1 B comes in at about 30 Suns. From stellar structure theory, Xi-1 is a "clump giant" (so called because of the many stars in its situation) that is quietly fusing its core helium. With an age of some 300 million years, it is not all that far along its evolutionary path, so from its luminosity Xi-1 B should be a class A2 (or so) hydrogen-fusing dwarf of about two solar masses about a quarter of the way through its "dwarfhood." Masses and the orbital period of 4.50 years give an orbital radius of 4.8 AU, a bit short of Jupiter's distance from the Sun, consistent with the measured angular separation of 0.012 seconds of arc. Intriguingly, Xi-1 A Ceti is also a mild "barium star", one enhanced with that chemical element (mild enough that the designation is uncertain). Barium stars (the prime example Alphard, Alpha Hydrae) are thought to have been contaminated by once more massive companions that during their death stages sent enriched matter to the star that is now the giant and that are now much lower-mass white dwarfs. Xi-1 is then perhaps a triple star. Though the white dwarf is not seen, it may be making itself known through enhanced ultraviolet radiation from the star. No one really knows.
Written by Jim Kaler 12/01/06. Return to STARS.