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.