UPS CET (Upsilon Ceti). The English language being awash with
puns, it's hard to avoid pointing out that bang-on fourth magnitude
(4.00) "Ups Cet" may be so for being the "lowest" (most southerly,
and therefore ignored) Greek-lettered star in the constellation of
Cetus, the Whale or Sea Monster. It
might be, too, that the star is annoyed by having an ill-defined
spectral class and temperature (not, as pointed out by the poet W.
H. Auden, that it really cares a lot). Upsilon Ceti (the star's
full name) was originally considered to be a class M (M0.5) giant, which would imply a
temperature of 3900 Kelvin. An estimate of 4200 Kelvin, however,
is much more consistent with class K4, while a later classification
suggests K7 (with no argument ever about its giant status). One
thing the star has going for it, however, is an estimate of angular
diameter, which, coupled to an accurate distance of 293 (plus or
minus 5) light years, gives a radius of 51 times that of the Sun. The high temperature limit, though,
suggests a luminosity of 360 Suns and a radius of 36 solar, while
the low limit gives 630 solar luminosities and 58 solar radii (the
range largely due to the contribution of infrared radiation to the
total). An intermediate temperature of 3900 Kelvin (suggesting a
K9 class) gives 525 solar luminosities and (in good agreement) 50
solar radii, showing the power of such angular measures, for which
Upsilon Ceti is used as a calibration star for the measures of
others. A projected equatorial rotation speed of 7.5 kilometers
per second then gives a ponderous rotation period that could be as
long as 333 days. Defining the mass and evolutionary condition of
Ups Cet is difficult because of the similarity in appearance for a
range of masses. The best estimate is around 1.7 Suns, but with a
large error. The star could be brightening with a dead helium
core, fading some having already set off its helium to fuse to
carbon, or brightening for the second time with a dead carbon core.
But since the second option is fast, and since if it is in the
third of the trio of states, the star would probably be variable
(which it isn't), the best guess is option 1. If so, Upsilon Ceti,
with an age of 1.9 billion years, gave up core hydrogen fusion
"just" 100 million years ago. Upsilon Ceti was also originally
listed as a very mild "barium star," which implies chemical
enrichment by mass transfer from a giant companion that is now a white dwarf. But the
chemical subclassification probably spurious. The bottom line is
that in spite of all the stellar research, there are still stars,
even modestly bright ones, that are understudied and have uncertain
parameters.
Written by Jim Kaler 11/04/11. Return to STARS.